Example Projects

Project Title & Abstract

Research Area

Supervisor

1D Models of Mass Ejection in Type I Photospheric Radius Expansion X-Ray bursts

In a close in binary star systems a sufficiently massive stars may explode as supernova and leave behind a neutron star, with a diameter of about the size of a city.  Later, as the system evolves, the smaller companion star may expand and transfer mass onto the neutron star, usually through an accretion disk.  When the material finally arrives at the surface of the neutron star, it first collects up in a thin and highly compressed layer, usually about the hight of a three-story house, before it ignites nuclear burning in a flash that incinerates the matter within seconds, burning it to heavy elements.  These events, recurring on time-scales of hours to days, are known as Type I X-ray bursts. Despite these bursts arise from such a thin layer, they are so energetic that we can see them all the way across the galaxy.

But whereas nuclear power and fusion on earth seems an incredibly powerful source of energy, releasing up to a few 0.1% of the rest mass of the matter, it hardly can match up with the incredible gravitational potential at the surface of a neutron star, amounting to some 20% of the rest mass!  So when such a nuclear-powered Type I burst goes off, it hardly has enough energy to blast off the surface of the neutron star - at least not in its entirety.  Some bursts, however, reach such high luminosity that they can blast off some of the material off the surface in the form of a "wind" - and may eject some of the newly synthesised material into the surrounding where it may be observable, due mostly to imprinting is signature on the light from the neutron star by absorbing some of the light, depending on its chemical composition.  We may directly learn about the freshly synthesised material of the burst - material that otherwise would disappear inside the neutron star for good without us ever being able to directly observe it.

The goal of this project is to model such high-luminosity bursts with wind and expansion of the photosphere, using a 1-dimensional hydrodynamic stellar evolution code, Kepler.  This may sound boring at first, but the code also follows an extended adaptive thermonuclear reaction network with some thousand nuclear species - as we do want to know what is being made and what is being ejected - and you can watch their evolution throughout the star as the simulation goes on.  You would run such models to explore different physical parameters of the system, such as accretion rate and composition of the accreted material, and follow each of system through several cycles of bursting, analysing the composition and amounts of material being ejected.  These would be the first such models with detailed nucleosynthesis.

References

H. Yu, N.N. Weinberg: "Super-Eddington Winds from Type I X-Ray Bursts", arXiv:1806.00164 (2018).

Astronomy & Astrophysics Professor Alexander Heger,
Associate Professor Duncan Galloway

3D imaging with in-line holography by exploiting implicit correlations in partially coherent waves

A Partial coherence of waves can be detected by the visibility of fringes in a double-slit experiment.  In a qualitative sense, a large degree of coherence, such as from mono-chromatic laser light sources, creates highly visible fringes.  On the contrary, a complete lack of coherence destroys all fringes and the visibility is accordingly zero.  Correlations between two locations at two different times can be used to quantify varying degrees of coherence in a stochastic wave field.

In-line holography typically analyses interference fringes imparted by specimens, which modulate incident wavefronts to create fine detail that develops as an exiting wave propagates in free space.  By studying this detail in recorded intensities, the wavefront deformation can be quantitatively measured to infer spatially varying phase shifts created by specimens of interest.  Since partial coherence degrades the contrast in these fringes, such effects are often considered to be detrimental to in-line holography.  In prior work, we have theoretically studied how this holographic fringe information can be generically described in partially coherent and aberrated fields [1].  We have recently found an interesting way to exploit this diminishment of fringe contrast, using the intrinsic properties of partially coherent sources.  In the context of phase contrast imaging, we realized that this variation of contrast surprisingly encodes useful depth information about the specimen, which is tomographic in nature [2].

We are looking for a talented student to study this depth-sensitive phase-contrast imaging approach further.  There are opportunities to explore this rich diffraction physics in light-optical experiments and discover new methods for probing specimen properties.  Alternatively, an interested student could further study the theoretical foundations and improve computer simulations to model these non-trivial effects.

References

[1] Mario A. Beltran, Marcus J. Kitchen, Timothy C. Petersen, and David M. Paganin, "Aberrations in shift-invariant linear optical imaging systems using partially coherent fields", Optics Communications, volume 355, 398-405 (2015).

[2] Mario A. Beltran, Timothy C. Petersen, Marcus J. Kitchen, and David M. Paganin,  "Extraction of depth moments by exploiting the partial coherence of radiation", Journal of Optics, volume 18, number 7 (2016).

Imaging Physics,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Timothy Petersen,
Dr Alexis Bishop,
Dr Marcus Kitchen

3D Simulations of Low-mass Stars

Stars which have masses < 10 times that of the Sun are the most numerous in the Universe. Because of this they are very important contributors to the electromagnetic radiation of galaxies, and also the chemical evolution of the Universe. Given their importance, it is vital that we have reliable models of low-mass stars. However one-dimensional models are now showing their weaknesses, largely due to the revolution currently occurring in observations, through large surveys like GAIA, Kepler, GALAH. It is now quite urgent to update and improve the stellar models. One way forward is to make detailed 3D models of stars, which include hydrodynamics and nuclear burning, amongst other physics. Although it is still currently impossible to simulate the whole life of a star in 3D, short phases can now be simulated using the current generation of powerful Supercomputers. We can learn about the complex interplay of various physics from these 3D models, and then translate this to the 1D full-lifetime simulations, which will still be needed for many decades to come. In this project the student will work with a 3D stellar hydrodynamics code and analyse the flow and convective boundaries in a low-mass star. The student will run the code on the largest supercomputers in Australia (Magnus and/or Raijin).

Astronomy & Astrophysics Dr Simon Campbell

A binary or a burst?

At the time of writing, there have been seven published detections of gravitational waves. All of these detections are merging binaries with waveforms described by Einstein's theory of general relativity. In this project, we will use Bayesian inference to determine if future gravitational-wave events seen by LIGO are best described by templates derived from numerical relativity, or by "something else." Something else could be difficult-to-model general relativistic effects or exotic new physics.

Astronomy & Astrophysics Dr Eric Thrane

Are the holes in the middle of protoplanetary discs made by planets?

What creates the large cavities seen in the centres of many protoplanetary discs --- photoevaporation of dust by the central star, or dynamical carving by massive planets or substellar companions? The aim of this project is to look for evidence of the latter. We will scour the GAIA DR2 dataset for evidence of unusual motions of stars in transitional discs, to see if we can distinguish between these two hypotheses. We will also use models to predict how the gas flow in the central cavities of discs might reveal the presence of hidden planets.

Astronomy & Astrophysics Associate Professor Daniel Price,
Dr Valentin Christiaens

Artificial Intelligence algorithms for real-time processing at the Large Hadron Collider

The LHCb experiment located at the Large Hadron Collider has been an incredibly successful. It has in recent years revealed hints for that the Standard Model of particle physics might not describe all phenomena. In particular there are signs that we do not have what is called lepton universality and that muons and electrons might behave differently. In order to understand this better, the LHCb experiment will go through a set of upgrades that will increase the available data by 2 orders of magnitude.

The data processing requirements for this is a challenge that does not yet have a solution. In the project you will investigate how Artificial Intelligence algorithms might be implemented in Field Programmable Gate Arrays (FPGAs) that form the boundary between software and hardware. In particular you will look at how the data coming from an upgraded calorimeter can be translated into energies and arrival times of electrons in real-time at a processing rate of 40 MHz. The programming environment will be Python.

Particle Physics Professor Ulrik Egede

Arrays on a regular lattice with optimal signal transmit and receive configurations

Recent work [1] used discrete projection theory to design families of numerical arrays where each array exhibits mathematically perfect auto-correlation, i.e. their off-peak periodic correlation is everywhere zero (or some constant). Furthermore, the cross-correlation between any pair of these family members is as low as theoretically possible. Such arrays find practical use in areas such as x-ray astronomy, digital communications and image watermarking. In [2, 3] it was shown that very large families of such arrays can be produced (in 2D, of order p 2 arrays, each of size pxp).

If we consider each active array location as a transmitter and/or receiver of signals, then these arrays can be adapted, using beam-steering methods, to produce powerful and very narrow directional beams useful for application in radar, ultrasound and microwave imaging. The transceivers can then be driven as Multiple Input Multiple Output (MIMO) arrays for intelligent sensing. The same array of N transceivers can also be reconfigured to act as a single transceiver: working in Single Input Single Output (SISO) mode.

This project considers the theoretical and practical construction of arrays with full MIMO capability by harnessing N independent ‘point-source’ transceivers configured in a pxp array to produce a single beam that has optimally narrow beamwidth and minimal side-lobes. Positive progress in this area was reported recently in [4]. Those arrays achieved side-lobes at least 3dB lower than previous efforts based on methods like simulated annealing and genetic algorithms. This project also considers the inverse problem, how to drive the same N transceivers so that the pxp array appears as a powerful single ‘point-source’ to maximise the SISO response.

Reference:

[1] Families of multi-dimensional arrays with optimal correlations between all members, A. Tirkel, B. Cavy and I. Svalbe, Electronics Letters, 51(15) 1167-1168 (2105).

[2] Extended families of 2D arrays with near optimal auto and low cross-correlation, I. Svalbe and A. Tirkel, EURASIP Journal on Advances in Signal Processing (1) 18 (2107).

[3] Large families of ‘grey’ arrays with perfect auto-correlation and optimal cross-correlation, I. Svalbe, M. Ceko and A. Tirkel, to be presented at DCGI, Vienna, Austria, September, 2017.

[4] Radar retina or millimetre wave eye? A. Tirkel and I. Svalbe, invited paper, IEEE Conference UKRCON 2017, Kiev, Ukraine, June, 2017.

Imaging Physics Dr Imants Svalbe,
Dr Andrew Kingston (Research School of Physics & Engineering ANU)

Asteroseismology of red giants

Stars are big balls of gas and they are excellent cavities for sound waves. The study of these waves in the Sun is known as selioseismology, in analogy with the seismic waves found on Earth. Recently, the Kepler satellite has been observing thousands of red-giants. This has allowed us to determine the seismic oscillations of these stars - a field of study known as "asteroseismology". The theory of these oscillations is well known but it is only now that we are getting real data for real stars. Kepler data is very accurate and it is allowing us to probe the interior of red-giants for the first time. The details of mixing during the phase of helium burning in the core are particularly poorly known. Recent work using Kepler data might allow us the possibility to determine the extent of the mixed region from observations! This project would involve gaining a thorough understanding of seismology and also the details of core helium burning in low mass stars. Then you will modify the stellar evolution code to calculate the things that observers can measure. Can we then use the existing data to make some constraints on the mixing seen in the models?

References

Christensen-Dalsgaard, J 2014, 'Stellar Oscillations', online lecture notes at: users-phys.au.dk/jcd/oscilnotes/print-chap-full.pdf

Bedding, TR et al. 2011, 'Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars' Nature, vol. 471, pp. 608-611.

Van Grootel, V et al. 2010, 'Early asteroseismic results from Kepler: structural and core parameters of the hot B subdwarf KPD 1943+4058 as inferred from g-mode oscillations' Astrophysical Journal Letters, vol. 718, no. 2, pp. L97-L101.

Astronomy & Astrophysics Professor John Lattanzio,
Dr Simon Campbell

Atom based potentials for atomtronics

Atomtronics is an emerging research area that seeks to develop devices that will exploit the unique properties of ultracold atoms to deliver benefits over conventional technologies. Potentials based on optical fields are currently used to manipulate and pattern ultracold atoms, which limits feature sizes to the wavelength of light. This project would investigate potentials for ultracold atoms based on atom-atom interactions. In this manner, we can realize corrals, interfaces, barriers and junctions for ultracold atoms that can be more than 10 times finer than what can be currently achieved. Of particular interest is the realization of an atom-based Josephson tunnel junction for use in an atomic SQUID device.

Quantum Gases Professor Kris Helmerson

Atomic-scale structural and electronic studies on light-harvesting metal-halide perovskites

Hybrid organic-inorganic perovskites are an emerging class of photovoltaic materials with the potential to outperform silicon [1]. Solar cells made of metal-halide perovskite offer material costs below $2/m2 and certified efficiencies beyond 20%. However, the underlying physical mechanisms allowing for strong light absorption and efficient electron-hole separation in metal-halide perovskites are not fully understood. In particular, very few studies have been performed on the atomic-scale properties of these materials. This project aims to combine expertise in synthesis, crystal growth, solar cell assembly and morphology control, with scanning-probe based surface analysis, in order to deepen our understanding of the structural and electronic properties of perovskite materials at the atomic scale. Metal-halide perovskite crystals will be synthesised by collaborating groups. Atomic-scale structural, electronic and optoelectronic properties of such materials will be studied by low-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS), non-contact atomic force microscopy (ncAFM), as well as synchrotron-based x-ray studies (x-ray photoelectron spectroscopy (XPS), near-edge x-ray absorption fine structure (NEXAFS)). Perovskite crystals will be cleaved in ultrahigh vacuum (UHV) and characterised in situ. These experiments will allow to correlate atomic-scale electronic structure with the materials’ light-harvesting functionality.

References

[1] Samuel D. Stranks and Henry J. Snaith, "Metal-halide perovskites for photovoltaic and light-emitting devices", Nature Nanotechnology, volume 10, 391-402 (2015).

Condensed Matter Physics Dr Agustin Schiffrin

Building the next generation of protoplanetary disc models

The new generation of astronomical instruments, the Atacama Large Millimetre Array (ALMA) and adaptive optics systems, offer revolutionary views of forming planetary systems around young stars. They reveal a multitude of unexpected structures in protoplanetary discs: rings and gaps, azimuthal asymetries, spirals. The detailed interpretation of theses new observations requires advanced disc models combining  hydrodynamics, radiative transfer and chemistry. This remains beyond current modelling capabilities.

In this project, we will explore how we can use advanced machine learning methods to signficantly speed up the calculations by predicting the disc chemical and thermal evolution from a database of existing models. Peliminary tests have been performed and look very promising. The goal of this project will be to quantify systematicaly how well machine learning perform and to use the newly developed algorithm to study the evolution of 3D disc models, where dust concentrates in "traps", where we believe the core of planets could form.

Astronomy & Astrophysics Dr Cristophe Pinte

Can we form planets quickly?

There is an old idea in planet formation called the Goldreich-Ward mechanism, where planets are made quickly by direct gravitational fragmentation of a thin dust layer. The aim of this project is to try to simulate this mechanism for the first time in 3D, to see if it might actually work in practice.

Astronomy & Astrophysics Associate Professor Daniel Price,
Dr Cristophe Pinte

Coloured X-ray Imaging at the Quantum Level

X-ray interactions with matter are highly dependent upon the energy of the incident photons. Yet in X-ray imaging it is common to ignore this effect even though most images are made using poly-energetic X-ray sources. If the energy of each photon can be recorded then it becomes possible to recover the electron density of materials in complex structures and to isolate specific chemical signatures. This information would be extremely useful for improving detection of diseases in medical imaging, and for isolating dangerous materials in border security [1]. Energy-resolving, photon counting detectors do exist, but they are extremely expensive and suffer poor spatial and/or temporal resolution. On the positive side, they produce a virtually noise free image. This project will explore the possibility of using new high-speed X-ray cameras for detecting individual X-ray photons and determining their energy. When successful, this technology will be employed to enable noise free, material specific imaging in real-time with spatial resolution on the micron scale.

This project will involve experimental studies using our high powered liquid metal jet X-ray source and the Imaging and Medical Beamline at the Australian Synchrotron adjacent to Monash University. Energy resolved images will be employed to enable high contrast, low noise imaging with both phase and absorption contrast imaging in 2D and 3D.

References

[1] Marcus J Kitchen, David M Paganin, Kentaro Uesugi, Beth J Allison, Robert A Lewis, Stuart B Hooper, and Konstantin M Pavlov, "Phase contrast image segmentation using a Laue analyser crystal", Physics in Medicine and Biology, volume 56, number 3, 515-534 (2011).

Imaging Physics Dr Marcus Kitchen,
Dr Kaye Morgan

Computational star and planet formation

How are stars born? How are solar systems formed in the discs around newborn stars? We are getting plenty of new observations from telescopes like the Atacama Large Millimetre/Submillimetre Array (ALMA) and the James Webb Space Telescope (JWST), but to understand what we’re seeing, we need to model it. Our group develops advanced numerical methods for modelling star and planet formation on modern supercomputers. A typical honours project might involve improving our modelling of dust, gas, radiation, magnetic fields or gravity to try to better explain the observations, or to apply our existing 3D code to some open problems. Feel free to swing by and have a chat to discuss possible directions.

Astronomy & Astrophysics Associate Professor Daniel Price

Computed Tomography at the Quantum Limit using Phase Contrast X-ray Imaging

The biggest problem with X-ray imaging in diagnostic medicine is the use of potentially dangerous ionizing radiation. Researchers across the globe are exploring Phase Contrast X-ray Imaging (PCXI) techniques to revolutionise X-ray imaging for applications in diagnostic imaging, materials science and security applications PCXI modalities increase the contrast of interfaces between materials by rendering gradients in the X-ray wavefield visible. Its advent has had a profound impact in many fields of science from biomedical imaging to materials characterization for industrial research. Using synchrotron radiation, our team has shown that this sensitivity can increase the signal-to-noise ratio of 3D tomographic imaging (CT) by up to two orders of magnitude. Remarkably, this enables us to reduce the radiation dose by factors in the tens of thousands [1]. Our aim is to develop this technology for use on smaller X-ray sources, with the end goal of translating it for massively reducing the radiation dose in human diagnostic imaging.

This project will investigate the use of new laboratory-based, high powered, highly coherent X-ray sources for developing an ultra-low dose CT capability. These sources include the liquid metal jet anode X-ray source at Monash University. Using phase contrast images produces on these sources in combination with phase retrieval and iterative reconstruction algorithms, we aim to produce high quality 3D images of the inner structures of objects using extremely low doses of radiation.

References

[1] Kitchen MJ, et al., "CT dose reduction factors in the thousands using X-ray phase contrast". arXive preprint, 2017, arXiv:1704.03556.

Imaging Physics Dr Marcus Kitchen,
Professor David Paganin

Confronting Theory and Experiment at the Large Hadron Collider

High-energy collisions between subatomic particles, such as those that occur at the LHC, are often too complicated to calculate by hand. At Monash, we use random numbers and a property called factorisation to simulate the quantum probability distributions of such reactions. Such computer models, called “Monte Carlo event generators", are able to provide simulated events in as much detail as the real collider events, against which therefore very detailed comparisons can be made. This is essentially how theory and experiment are compared in practice, in modern high-energy physics. However, the necessity to make approximations implies that no model is absolutely perfect. In this project, we will consider state-of-the-art physics models and subject them to trial by fire in the form of constraints imposed by very recent measurements performed at the Large Hadron Collider (LHC). The project is well suited for students with an interest in collider physics phenomenology, computing (the simulations and analyses are written in C++), and will also involve aspects of LHC data analysis.

Particle Physics Associate Professor Peter Skands

Constraining nuclear reactions with thermonuclear bursts

Many of the thousands of thermonuclear reactions taking place in X-ray bursts from neutron stars have rates that cannot be measured precisely in terrestrial laboratories. In recent years, modelling efforts have begun to identify which specific reactions have the most influence on the observational properties of the bursts. At the same time, large samples of high-quality observational data on bursts exist, but comprehensive comparisons of the model predictions with observations have not taken place. This project would involve comparisons of numerical burning model predictions with observations to attempt to constrain nuclear reaction rates. The student will take advantage of the sample of several 1000s of bursts as part of the Multi INstrument Burst ARchive (MINBAR), as well as existing collections of model runs from collaborators, and local modelling capabilities. Part of the project could involve development of online web-based tools to improve the usability of existing data. Additional work could include exploring varying stellar parameters including gravity (mass/radius) and heating rate.

Reference

Cyburt, RH et al. 2010, 'The jina reaclib database: Its recent updates and impact on type-I X-ray bursts', Astrophysical Journal, Supplement Series, vol. 189, no. 1, pp 240-252.

Astronomy & Astrophysics Associate Professor Duncan Galloway,
Professor Alexander Heger

Constraining supernovae properties by their nucleosynthesis

Most heavy elements from oxygen to iron are dominantly made by the deaths of massive stars as supernovae. Whereas fully understanding such core collapse supernovae requires multi-dimensional simulations including complicated and expensive radiation transport physics, there is some progress in developing simpler approximation formulae for these supernovae given the structure of the star at the time of its death. Depending on the explosion properties, supernovae synthesise and eject elements in different proportions, which can be used as a diagnostic of the explosion model. For this project you will use an analytic model for supernova explosions and their energies to simulate the nucleosynthesis of these stars. The result is to be compared to the abundance patterns - elemental and isotopic - that we find in the in the universe today, in the sun, and on earth. The goal of the project is to constrain the properties of the analytic supernovae model in its ability to reproduce the observed data.

References

Hans-Thomas, J 2012, 'Explosion mechanisms of core-collapse supernovae', Annual Review of Nuclear and Particle Science, vol. 62, issue 1, pp. 407-451.

Pecha, O & Thompson, TA 2013, 'The Landscape of the Neutrino Mechanism of Core-Collapse Supernovae: Neutron Star and Black Hole Mass Functions, Explosion Energies and Nickel Yields', arXiv.org > astro-ph > arXiv:1409.0540

Rauscher, T, Heger, A, Hoffman, RD & Woosley, SE 2002, 'Nucleosynthesis in Massive Stars with Improved Nuclear and Stellar Physics', The Astrophysical Journal, Volume 576, Issue 1, pp. 323-348.

Woosley, SE, Heger, A & Weaver, TA 2002, 'The evolution and explosion of massive stars', Reviews of Modern Physics, vol. 74, Issue 4, pp. 1015-1071.

Astronomy & Astrophysics Professor Alexander Heger

Defects and artificial atoms in 2D semiconductors

Atomically thin semiconductors, materials comprised of surface only, have remarkable electronic and optical properties and allow us to study basic physical concepts in a well-controlled environment. Besides this, these materials are envisioned to be used in future types of electronics, making them attractive for both basic and applied research. In 2D semiconductors, donor impurities (ions) and their bound electron, as well as excitons - electrons bound to holes - form bound states analogous to the hydrogen atom with an effective mass and dielectric constant. The dielectric environment of atomically thin semiconductors can be easily tuned, and the artificial atoms and molecules of donors, donor clusters, and excitons can be studied by scanning tunnelling microscopy - a technique utilising quantum mechanical tunnelling at different energies (voltages) to probe density of states, which allows us to access energy levels in the artificial atoms.

The aim of this project is to form artificial atoms and molecules in 2D semiconductors through creation of defects which act as donors, or by adsorption of donor species such as alkali metals. The local density of states, and thus energy levels, of the artificial atoms and molecules will be investigated via scanning tunnelling microscopy.

Condensed Matter Physics Dr Antonija Grubisic-Cabo,
Professor Michael Fuhrer

Detecting gravitational waves from supermassive black holes

Gravitational-wave astronomy is now a reality.  Advanced LIGO has detected gravitational waves from merging black holes, each with mass tens of times more than the Sun.  A complementary experiment using rapidly rotating neutron stars (pulsars) aims to detect gravitational waves from black holes with masses more than a billion times that of our Sun.  In this project, we will either work with the pulsar data to make such a detection possible, or develop the astrophysical models to understand the expected gravitational-wave signal.

Astronomy & Astrophysics Dr Paul Lasky,
Dr Xingjiang Zhu

Detecting the signatures of forming planets

Planet form in discs of gas and dust rotating around young stars. While the recent Kepler survey has shown that planet formation is ubiquitous, with more than one planet per star on average, forming planets within the disc have remained elusive so far, with only a few candidates.

In this project, we will explore how we can calculate the kinematical signatures left by a planet in a disc as well as the signature of gas accretion onto the forming planet and compare it to current observations, as well as make predictions for the next generation of instruments.

Astronomy & Astrophysics Dr Cristophe Pinte

Dirac Electronic Materials

Recently new materials have emerged in which the electron dynamics are described by the Dirac equation in two dimensions. An example is graphene, the two-dimensional honeycomb lattice of carbon atoms that is the basic building block of graphite. In graphene, the electrons obey a massless Dirac equation, with the role of the relativistic electron spin played by a spinor ("pseudospin") composed of the two ? orbitals in the unit cell. Single layer molybdenite (MoS2) has a massive Dirac equation and is a direct-bandgap semiconductor while retaining the chiral properties of the spin-1/2 pseudospin. Three-dimensional topological insulators such as Bi2Se3 are insulating in their interiors, but exhibit metallic surface states with a massless Dirac structure similar to graphene, but with the real quantum mechanical spin as the Dirac spinor. Professor Michael Fuhrer's group is studying these materials experimentally in order to understand how their unusual band structures determine their electronic and optical properties. The experimental research involves:

* Electronic transport measurements on microfabricated devices[1-3,5-10,12]. Semiconductor micro- and nano-fabrication tools (at Monash and at the Melbourne Centre for Nanofabrication) are used to create electronic devices with controlled geometry. Cryogenic electronic measurements of resistivity, Hall effect, etc. are used to understand scattering by disorder and phonons, quantum transport (weak localization or anti-localization, quantum Hall effects), etc.

* Scanning-probe microscopy[4,7,11]. Scanning-probe microscopy techniques, such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) are used to understand the atomic structure and electronic properties of two-dimensional materials. By coupling scanned-probe techniques with micro-fabricated devices, new information can be gained using techniques such as Kelvin probe microscopy (to measure local potentials in current-carrying devices) or scanned-gate microscopy (to measure the local sensitivity to a tip acting as a gate to induce charge in a device).

* Surface modification[1,3,5]. Two-dimensional Dirac materials are atomically confined at surfaces and interact strongly with their environments. Ultra-high vacuum surface science techniques are used to controllably modify the properties of two-dimensional materials, introducing charged impurities, point defects, modifying the dielectric constant, adding magnetic interactions, and changing the dopant density. Coupled with electronic transport experiments and scanned probe experiments surface modification allows insight into the relationship between atomic structure and electronic properties of these materials.

* Optical spectroscopy and optoelectronics[9]. Dirac semiconductors such as MoS2 have direct bandgaps, and chiral optical excitation can be used to excite spin and pseudospin polarizations. Additionally, two-dimensional materials have strong and tunable electron-electron interactions because the dielectric properties are determined by the surrounding media, leading to large excitonic effects. Optical spectroscopy can be used to study these effects in Dirac semiconductors.

A range of projects involving these experimental techniques are available for Honours students; the specific project can be tailored to match the skills and interests of the student.

References:

[1] J. H. Chen, C. Jang, S. Adam, M. S. Fuhrer, E. D. Williams, and M. Ishigami, "Charged Impurity Scattering in Graphene," Nature Physics 4, 377 (2008).

[2] J. H. Chen, C. Jang, S. Xiao, M. Ishigami, M. S. Fuhrer, "Intrinsic and Extrinsic Performance Limits of Graphene Devices on SiO2," Nature Nanotechnology 3, 206 - 209 (2008).

[3] C. Jang, S. Adam, J.-H. Chen, E. D. Williams, S. Das Sarma, M. S. Fuhrer, "Tuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering," Physical Review Letters 101, 146805 (2008).

[4] William G. Cullen, Mahito Yamamoto, Kristen M. Burson, Jianhao Chen, Chaun Jang, Liang Li, Michael S. Fuhrer, Ellen D. Williams, "High-fidelity conformation of graphene to SiO2 topographic features," Physical Review Letters 105, 215504 (2010).

[5] Jian-Hao Chen, W. G. Cullen, E. D. Williams, and M. S. Fuhrer, "Tunable Kondo Effect in Graphene with Defects," Nature Physics 7, 535 (2011).

[6] Sungjae Cho and Michael S. Fuhrer, "Massless and massive particle-in-a-box states in single-and bi-layer graphene," Nano Research 4, 385 (2011).

[7] A. E. Curtin, M. S. Fuhrer, J. L. Tedesco, R. L. Myers-Ward, C. R. Eddy, Jr., and D. K. Gaskill, "Kelvin probe microscopy and electronic transport in graphene on SiC(0001) in the minimum conductivity regime," Applied Physics Letters 98, 243111 (2011).

[8] Dohun Kim, Sungjae Cho, Nicholas P. Butch, Paul Syers, Kevin Kirshenbaum, Shaffique Adam, Johnpierre Paglione, Michael S. Fuhrer, "Surface conduction of topological Dirac electrons in bulk insulating Bi2Se3," Nature Physics 8, 460 (2012).

[9] J. Yan, M.-H. Kim, J.A. Elle, A.B. Sushkov, G.S. Jenkins, H.M. Milchberg, M.S. Fuhrer, and H.D. Drew, "Dual-gated bilayer graphene hot electron bolometer," Nature Nanotechnology 7, 472 (2012).

[10] Sungjae Cho, Dohun Kim, Paul Syers, Nicholas P. Butch, Johnpierre Paglione, and Michael S. Fuhrer, "Topological insulator quantum dot with tunable barriers," Nano Letters 12, 469 (2012).

[11] Mahito Yamamoto, Olivier Pierre-Louis, Jia Huang, Michael S. Fuhrer, T. L. Einstein, William G. Cullen, "Princess and the Pea at the nanoscale: Wrinkling and unbinding of graphene on nanoparticles," Physical Review X 2, 041018 (2012).

[12] Dohun Kim, Qiuzi Li, Paul Syers, Nicholas P. Butch, Johnpierre Paglione, S. Das Sarma, Michael S. Fuhrer, "Intrinsic Electron-Phonon Resistivity in Bi2Se3 in the Topological Regime," Physical Review Letters 109, 166801 (2012).

Condensed Matter Physics Professor Michael Fuhrer

Distortion elimination in acoustic transducers

Audio reproduction transducers (speakers) are have been essentially unchanged in design and performance for many decades. The performance, in terms of linearity of reproduction from a driving signal is surprisingly poor, particularly at low frequencies (<1 kHz), with significant amplitude (~10 %) and phase nonlinearity being present even in the best drivers. Distortion of the output waveform arises from departures from the idealized model (a driven damped harmonic oscillator), which generates additional harmonics, and significantly, intermodulation distortion (typically several percent for frequencies < 200 Hz), when a complex drive signal is present. Correction of the nonlinearities is possible by modifying the driving waveform to compensate for the nonlinear motion of the driver.

This experimental and computational project is to develop a practical algorithm to compensate for the nonlinearities in the driver behaviour, and hence ensure the reproduced sound intensity is proportional to the driving signal.

Imaging Physics Dr Alexis Bishop

Do stars swallow their planets?

How special is our Solar System? For years the Sun was thought to be a typical solar type star, but strikingly a few years ago the Sun was found to be chemically peculiar, with a deficit of rocky-forming elements. More intriguingly, this anomaly could be related to the less-common planet architecture (inner rocky planets, outer gaseous planets, orbits that never cross) of our System.

In fact, the apparent regularity of the motion of the planets in our Solar System suggests that they formed on orbits similar to their current ones and that nothing dramatic happened to them during their lifetime. On the other hand, most of the exoplanetary systems discovered so far have suffered from severe planet migrations and, eventually, planet engulfment events. When rocky material enters into the star, it is rapidly dissolved in the stellar envelope. Such external pollution will produce a stellar chemical pattern that mirrors the composition observed in rocky objects,  with the most refractory elements (e.g., Fe, Ti, Al, Sc) being more abundant than in the Sun, where this pollution seems to have not occurred.

The main goal of this project is to test the chemical homogeneity of stellar systems, such as open clusters and binary systems, to a level that was unthinkable only few years ago, taking advantage of a special technique that will allow us to achieve very high-precision abundance determinations. Members of these systems were formed from the same material, so they should have started as chemically identical. Any difference in the rocky-forming elements among the members of these stellar associations is likely related to the architecture of planetary systems or to planetary engulfment events.

This project will test the intriguing possibility that the chemical composition of stars can carry information relevant to the architecture and evolution of planetary systems. Unveiling chemical signatures of planet engulfment events will allow us to asses how rare our Solar System is and will yield a list of stars with the best chances of hosting stable planetary systems like ours.

Find more details on my webpage: https://research.monash.edu/en/persons/lorenzo-spina

References

[1] Melendez et al. 2009, "The Peculiar Solar Composition and Its Possible Relation to Planet Formation", ApJ, 704, 66

[2] Melendez & Ramirez 2017, "Planet signatures in the chemical composition of Sun-like stars", The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, arXiv:1611.04064

[3] Spina et al. 2018, “Chemical inhomogeneities in the Pleiades: signatures of rocky-forming material in stellar atmospheres”, eprint arXiv:1807.00941

Astronomy & Astrophysics Dr Lorenzo Spina

Dynamics of single molecules in a nanotube

The cell is a crowded environment where complex chemical reactions take place, typically involving only a few numbers of molecules. While a number of these reactions have been studied in bulk assays or even at the single molecule level, the role of crowded environment or confinement has typically not been investigated. The goal of this project is to study the dynamics of single molecules and molecular complexes under the influence of confinement. An apparatus will be developed to create nanotube extensions from vesicle by pulling on self-assembled vesicle membranes. The nanotube-vesicle structures can be made stable in the case where the membranes are formed by cross-linkable polymers. Single molecules, such as genomic length DNA, will be driven to enter the nanotube and the dynamics of the molecule will be studied by fluorescence microscopy.

Imaging Physics Professor Kris Helmerson

Electrodynamics of Weyl semimetals

Weyl semimetals are new topological solids with protected massless Dirac electronic states in their bulk and exotic Fermi arc surface states. The electrodynamics of these materials have additional "axion" terms that can manifest as in transport as in optical phenomena and have attracted recently a lot of attention.

The project is devoted to an analysis of a distribution of electric and magnetic fields induced by a point charge near Weyl semimetal surface that can be addressed within the method of image charges.  [1].

References

[1] X.L. Qi, R. Li, J. Zang, S.-C. Zhang - Science 323, 1184 (2009)

Quantum Gases,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Dmitry Efimkin

Electrons and photons as emergent phenomena

This project addresses the question: are electrons and photons fundamental particles or emergent phenomena? In ref. [1] it was demonstrated that one could construct a U(1) gauge theory (of light) based on a local bosonic model. Fundamental to this approach is the notion of a new kind of order, called topological order, in which particles arise from excitations of a string-net condensed phase. However, these strings are fundamentally different from the superstrings in high-energy particle physics. A simple lattice bosonic model will be used to explore the emergence of U(1) gauge theory in the low energy regime. Possible extensions will include an exploration of how spin-2 gravitons might arise from a lattice bosonic model.

References:

[1] M. Levin and X-G Wen, "Colloquium: Photons and electrons as emergent phenomena", Reviews of Modern Physics, Volume 77, Issue 3 (2005).


Particle Physics,
Theoretical & Computational Physics
Professor Michael Morgan

Ensemble gravitational wave detections: more than the sum of the parts

Gravitational-wave astronomy is now a reality. In February 2016, LIGO announced the first direct detection of gravitational waves from the collision of two black holes, each with mass approximately 30 times that of the Sun. From the first observing run of Advanced LIGO, two bona fide detections were made of binary black hole mergers, and one further candidate detection. These detections allow us to predict the event rate of future detections given the planned improvement in instrument sensitivity. The future is very bright with tens to hundreds of detections expected in the next two or three years. In this project, we will explore physics that can be learned from an ensemble of gravitational wave detections that cannot be learned from any given detection. For example, we recently published a paper [1] showing that gravitational-wave memory—a permanent deformation of spacetime following the emission of gravitational waves—can be detected confidently once approximately 30 loud binary black hole mergers have been detected with Advanced LIGO. Potential projects involve looking for deviations from General Relativity in ultra-strong gravitational fields or trying to understand how these stellar-mass black holes formed in the first place.

References

[1] Paul D. Lasky, Eric Thrane, Yuri Levin, Jonathan.Blackman, and Yanbei Chen, "Detecting Gravitational-Wave Memory with LIGO: Implications of GW150914", Physical Review Letters, Volume 117, 061102 (2016).

Astronomy & Astrophysics Dr Eric Thrane,
Dr Paul Lasky

Entangling an atom and a brain wave

Is there a quantum state which is optimal for detecting a brain wave? Neurons communicate by emitting pulses which are a very predictable shape. Neuroscientists believe that thinking is encoded in the timing of these pulses, but not in changes in their amplitude or shape. Rather than detecting these pulses with invasive voltage probes, we aim to measure the magnetic fields generated by the tiny neurocurrents that flow between neurons. In this project you'll explore whether there is an optimal quantum state for a spin-1 atom to detect the magnetic field of a single neural impulse. The project will have theoretical, simulation and experimental components.

Quantum Gases Dr Lincoln Turner,
Dr Russell Anderson

Excitonic superfluidity in electron-hole bilayers

The possibility of the excitonic superfluidity in double layer systems with spatially separated electrons and holes has been debated for decades. While strong Coulomb interactions between electrons and holes favors their Cooper pairing at elevated temperatures, the latter is very sensitive to disorder and electron-hole asymmetry inevitably present in real samples. The first convincing evidences of strong correlations between electrons and holes have been reported recently in bilayer graphene double layer system [1]. The further experimental and theoretical analysis of this system is needed.

The project is devoted to transport phenomena in bilayer graphene double layer system.

References

[1] G.W. Burg, N. Prasad, K. Kim, T. Taniguchi, K. Watanabe, A.H. MacDonald, L.F. Register, E. Tutuc - Phys. Rev. Lett. 120, 177702 (2018)

Quantum Gases,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Dmitry Efimkin

Exoplanets: Resonance capture during planet formation

Planets are formed in protoplanetary disks, the latter being composed of a mixture of dust and gas. The process involves accumlation of dust into planetesimals, which in turn collide to form planet "cores" which, if massive enough, go on to accrete a massive gas envelope to become gas giants. These nascent planets in turn raise a tide in the disk from which they form. Such a tide will be in the form of a spiral density wave, and since the associated density enhancement is non-axisymmetric, it will exert a torque on the planet and hence exchange angular momentum with its orbit.

The latter process is called planet MGEation. It causes the planet to move towards or away from the star, depending on the "balance of torques" (there will be perturbations to the planet's orbit coming from the interior and exterior portions of the disk), and in some cases such torques will balance and the planet will remain stationary (at least on some timescale which is probably short compared to the lifetime of the disk).

In the case that there is more than one planet (which is surely the general case), the rates of change of their semimajor axes will in general be different. As such, the ratio of the orbital periods will either increase ("divergent MGEation") or decrease ("convergent MGEation"). In the latter case, the planets would collide if it weren't for a beautiful process called resonance capture which prevents this from occurring.

This project will study the process of resonance capture with a view to understanding the period-ratio distribution of exoplanetary systems, especially those discovered by the Kepler space telescope. This is a prominent unsolved problem in exoplanet science, and its solution will give us deep insight into the process of planet formation.

Astronomy & Astrophysics Dr Rosemary Mardling

Exoplanets: Transit timing variations (TTVs) in the Kepler planet candidates

Before the Kepler space telescope was launched, the vast majority of exoplanet detections were made using the Radial Velocity method. This spectrocopic method measures the minute Doppler shifts in all available stellar spectral lines, and allows one to measure the minimum mass of the planet responsible for the motion of the star as long as one has a good estimate for the latter. In contrast, the transit method of detection is a photometric method which measures the deficit of photons when a planet passes across the face of the star being observed. This allows one to estimate the radius of the planet as long as one has a good estimate for the radius of the star. Thus the RV and transit methods are complementary, and if one detects a system both ways, one can estimate the mean density of the transiting planet. In turn, this allows one to say something about the likely internal structure of the planet, an amazing fact which has attracted geoscientists to join the burgeoning field of exoplanets.

Kepler has used the transit method to detect thousands of planet candidates, but unfortunately the Kepler field is very distant and so most of their host stars are too faint to follow up with RV measurements. [Kepler is not a "pointing telescope" (its field of view is fixed) and so a field towards the crowded Galactic centre was chosen to optimise the number of target stars.] Without RV follow-up, it is generally not possible to measure the planet's mass, and since there are "non-planet" ways to produce a planet-like signal (for example, if the star is orbited by a distant close binary), one cannot confirm the planetary nature of the detection. Fortunately, there are quite a few bright stars with planet candidates in the Kepler field which are amendable to RV analysis, and since their planet transit, their actual masses and hence their densities have been determined. However, the vast majority of candidates are not in this category.

This would have been a disaster for the Kepler mission had it not been realised that the presence of a second planet could reveal itself through the perturbations it causes to the timing of the observed transit, in particular, to the time at which the planet crosses the midline of the star even when the second planet is not detectable (by transiting itself or by RVs). By forming a time series of these transit timing variations, one is then able to use it to deduce (in favourable cases) many of the orbital parameters of the whole system including the planet masses.

TTVs have been a boon to the Kepler mission, but the vast majority of planets candidates still remain just that - candidates. This project will use new mathematical methods to reveal the planetary nature of some of these systems, thereby adding to our knowledge of the rich variety of planets and planetary system architectures revealed by Nature so far.

Astronomy & Astrophysics Dr Rosemary Mardling

Exploring the transient sky with a robotic telescope

The Gravitational-wave Optical Transient Observatory (GOTO) is Monash’s own wide-field robotic telescope, deployed since 2017 on the superb observing site on La Palma, in the Canary Islands. The telescope has the primary goal of detecting the electromagnetic counterparts of binary merger events detected with the Laser Interferometric Gravitational-wave Observatory (LIGO). However, in the normal observing mode, GOTO will survey the sky to detect many other types of violent and energetic transients, including supernovae and gamma-ray bursts, as well as more prosaic object such as asteroids and flare stars.

In 2019 the LIGO and Virgo instruments will resume operations at improved sensitivity, that will lead to discovery of many new events, including binary neutron star mergers like the spectacular  GW170817. This honours project will involve remote operation of the telescope; searches for new transient objects; and analysis of data gathered on a wide range of objects. Students will work with extensive archival data gathered over the course of the mission, and compare with detailed source models to measure the properties of distant objects.

GOTO is operated in conjunction with a collaboration of researchers from universities in the UK and Thailand; for more information see http://goto-observatory.org

Astronomy & Astrophysics Associate Professor Duncan Galloway,
Kendall Ackley

Extended supersymmetric models

Tera-scale supersymmetry is the most promising theoretical extension of the standard particle model. While supersymmetry solves numerous theoretical problems of the standard model, its breaking mechanism remains an open question. Also uncertain is the electroweak breaking (Higgs) sector in supersymmetric models.

In gaugino mediated models supersymmetry breaking is mediated by superpartners of standard gauge bosons (such as the photon) propagating in extra space dimensions. These models produce successful but somewhat fine-tuned phenomenology. Combining gaugino mediation with an extended Higgs (electroweak symmetry breaking) sector has the promise of removing the fine-tuning from the minimal model.

The aim of this project is to construct and analyze a phenomenologically viable gaugino mediated scenario in the framework of the next-to-minimal supersymmetric standard model.

References

[1] H. Baer et al., "Viable models with non-universal gaugino mediated supersymmetry breaking", arXiv:hep-ph/0204108.

[2] C. Balazs, R. Dermisek, "Yukawa coupling unification and non-universal gaugino mediation of supersymmetry breaking", arXiv:hep-ph/0303161.

[3] C. Balazs, D. Carter, "Discovery potential of the next-to-minimal supergravity motivated model", Physical Review D, Volume 78, 055001 (2008).

[4] C. Balazs, D. Carter, "Likelihood analysis of the next-to-minimal supergravity motivated model", AIP Conference Proceedings (2009).

Particle Physics Professor Csaba Balazs

Extending polytropic models

Simple polytropic models have many uses in stellar astrophysics. The simple approaches can be extended by adding an equation for the luminosity as well. In this case one can include the H-burning reactions and calculate a luminosity for the model. The idea is to try to construct polytropic models that represent the main phases of stellar evolution around the HR diagram. How can we best simulate a main sequence? Can we simply use the pp and cno cycle reactions? Will it be necessary to make some modifications? For example are all ms stars equally well approximated by polytropes with n=3? Maybe we will need to vary n? Can we simulate a red-giant somehow? Further, what is the best way to solve the Lane-Emden equation? We will investigate using a Runge-Kutta-Fehlberg scheme with a maximum specified error at each step. We can also write the Lane-Emden equation as a differential equation for &zeta as a function of &theta, so that the boundaries are now well known!
Astronomy & Astrophysics Professor John Lattanzio

Experiments on two-dimensional quantum turbulence

Two-dimensional turbulence is even more fascinating than its three-dimensional counterpart. Here, the turbulent energy is predicted to go into the formation of increasingly larger size eddies and vortices in a so-called inverse energy cascade process. Hence 2D turbulence exhibits a peculiar self-organization, giving rise to order out of chaos. This project will involve investigating two-dimensional turbulence in a superfluid atomic gas (a Bose-Einstein condensate). The turbulent behaviour and emergence of order in the superfluid gas will be studied using a number of techniques including particle imaging velocimetry and observation of Kelvin wave dynamics.

Quantum Gases Professor Kris Helmerson

Extra dimensional dark matter

String theory suggests the possibility of new space dimensions opening up at the tera-electron-volt energies. The merit of these extra dimensional models can be tested by their solutions to problems of the standard particle model: the generation of mass, dark matter, neutrino masses, and unification of forces.

Universal extra dimensional models fare well against this test. Among other virtues, they can explain dark matter by the lightest stable Kaluza-Klein particles. In the five and six dimensional models the amount of the lightest Kaluza-Klein particles is consistent with the measured amount of dark matter.

However, there exists no comprehensive analysis of how these models fare against other experimental constraints such as indirect and direct detection bounds, collider limits, precision electroweak variables, rare decays, etc. In this project we attempt this calculation and using these results we perform a viability analysis of this model. <\p>

Particle Physics,
Astronomy & Astrophysics
Professor Csaba Balazs

Extra solar planets

Observations and theory; Stability and long-term evolution of stellar and planetary systems; Tides in planets and stars; Planet formation; The three-body problem; Chaos in conservative systems.

Astronomy & Astrophysics Dr Rosemary Mardling

Femtosecond atomic-scale dynamics on a surface

The advent of scanning tunnelling microscopy more than 30 years ago has allowed for real-space imaging of single atoms and molecules on a surface. Normally, this technique is able to study the equilibrium properties of a system, and does not allow to access real-time ultrafast dynamics occurring at femtosecond timescales. Indeed, the intrinsic time resolution of scanning tunnelling microscopy is dictated by the speed of conventional electronics, which is at best on the order of hundreds of picoseconds. Many processes on surfaces however, such as charge dynamics or vibrations of molecules, can unfold at much faster timescales, below 1 picosecond.

Recent advances in scanning tunnelling microscopy combined with techniques of nonlinear optics, ultrafast photonics and pump-probe spectroscopy have allowed to access ultrafast dynamics of processes on a surface, with real-time sub-picosecond resolution, and, at the same time, real-space single atom resolution [1]. The approach relies on coupling quasi-single-cycle, ultrashort electromagnetic waveforms (e.g., a terahertz, infrared or optical pulse) to the junction of the scanning tunnelling microscope.

The project here consists of investigating structural, charge and magnetization dynamics in photo-active molecular assemblies on surfaces, with sub-picosecond time resolution, and at the scale of a single atom.

References

[1] T.L. Cocker et al., Nature 539, 263 (2016).

Condensed Matter Physics Dr Agustin Schiffrin

Few and many-body physics in ultracold atomic gases

Ultracold atomic gases have emerged as an ideal platform for investigating the physics of strongly correlated materials in a highly controllable environment. The Theory of Quantum Matter group at Monash University works primarily at the interface between condensed matter physics and the physics of ultracold atoms. We are particularly interested in systems where strong few-body correlations impact the many-body behaviour. As such, we investigate a wide range of topics, including few-body systems, impurities in degenerate quantum matter, systems out of equilibrium, and low-dimensional systems.

Quantum Gases,
Condensed Matter Physics,
Theoretical & Computational Physics
Associate Professor Meera Parish,
Dr Jesper Levinsen

Fluctuations of Quark and Gluon Jets

At centre-of-mass energies above a few billion electron-Volts (GeV), collisions between subatomic particles can produce collimated sprays of nuclear matter, called jets. Essentially, each jet starts out as a quark or gluon receiving a kick in the collision process; this  in turn generates bremsstrahlung of further quarks and gluons at successively longer wavelengths, in a patter reminiscent of fractals, until finally confinement sets in and hadrons are formed. One of the most successful computer models of jet formation, PYTHIA, is developed right here at Monash, and is widely applied e.g. to describe physics processes at the Large Hadron Collider at CERN. PYTHIA uses Markov-Chain Monte Carlo techniques to build up approximations to the bremsstrahlung and hadronisation patterns. But the intrinsic precision is limited. We will consider some of the theoretical uncertainties that arise in these approaches, and propose ways for evaluating and/or improving them. This project is well suited for students with an interest in particle physics phenomenology, computer physics (the parton-shower models are implemented as C++ algorithms), quantum field theory, statistical data analysis, and Markov-Chain / Monte Carlo algorithms.

Particle Physics Associate Professor Peter Skands

Galaxies in transition

Galaxies fall into two broad categories, those with and without star formation. These two categories are very clearly identified in WISE satellite infrared imaging, which detects the blackbody radiation produced by warm dust in star forming regions. What terminates star formation within galaxies is not clearly understood. Studies at optical wavelengths have been hampered by dust obscuration, which can make dusty star forming galaxies mimic older galaxies that don't have any star formation. We will identify galaxies that in transition using WISE infrared imaging combined with SDSS optical imaging and archival spectroscopy. We will search for evidence of what is driving the transition in galaxy properties, including galaxy mergers and proximity to galaxy groups.

References:

[1] Brown et al.., 2008, ApJ, 682, 937.

[2] Brown et al.., 2011, ApJ, 731L, 41 (arXiv : 1103.2828).

[3] Croton et al.., 2006, MNRAS, 365, 11 (arXiv:astro-ph/0407537).

[4] Toomre & Toomre., 1972, ApJ, 178, 623.

Astronomy & Astrophysics Associate Professor Michael Brown

Gravitational waves illuminate fundamental physics

Standard elementary particles acquire their masses via the mechanism of spontaneous electroweak symmetry breaking. The accompanying cosmological phase transition took place in the early Universe when its temperature fell below the tera-electron-volt scale, and its Hubble horizon was about 23 orders of magnitude smaller than today.

If the electroweak phase transition was strongly first order it proceeded via bubble-nucleation. Collision of the bubble walls containing the broken phase lead to gravitational disturbances, traces of which have been shown to be observable in future gravitational wave experiments such as eLISA.

The aim of this project is to calculate the amplitude and frequency (that is the spectrum) of the gravitational waves created during the electroweak or another phase transition that took place in the early Universe.

References

[1] "Gravitational waves at aLIGO and vacuum stability with a scalar singlet extension of the Standard Model", Csaba Balazs, Andrew Fowlie, Anupam Mazumdar, Graham White, Phys.Rev. D95 (2017) no.4, e-Print: arXiv:1611.01617

[2] "Multi-peaked signatures of primordial gravitational waves from multi-step electroweak phase transition", Thibault Vieu, Antonio P. Morais, Roman Pasechnik, e-Print: arXiv:1802.10109

[3] "Exotic Gravitational Wave Signatures from Simultaneous Phase Transitions", Djuna Croon, Graham White, JHEP 1805 (2018) 210, e-Print: arXiv:1803.05438

[4] "Primordial Anisotropies in the Gravitational Wave Background from Cosmological Phase Transitions", Michael Geller, Anson Hook, Raman Sundrum, Yuhsin Tsai e-Print: arXiv:1803.10780

[5] "A Cosmological Signature of the SM Higgs Instability: Gravitational Waves", J.R. Espinosa, D. Racco, A. Riotto e-Print: arXiv:1804.07732

[6] "Model Discrimination in Gravitational Wave spectra from Dark Phase Transitions", Djuna Croon, Veronica Sanz, Graham White e-Print: arXiv:1806.02332

Particle Physics Professor Csaba Balazs

Growth and Characterization of 2D Topological Materials

Topological materials, such as topological insulators and topological Dirac semimetals, are a new class of matter that possess new and exciting electronic properties. Allowing a wide range of new physics to be explored including Majorana fermions and the Chiral anomaly to create revolutionary new electronic devices that have the potential to transport charge through one-dimensional edge modes without dissipation.

In this project you will learn how to grow new two-dimensional topological materials via molecular beam epitaxy. A technique that allows the precise growth and control of epitaxial large area films. This will involve preparing substrates and performing growths, during growth you will utilize in-situ diffraction techniques to confirm crystallinity and sample quality.

After successfully growing these materials you will employ low-temperature scanning tunneling microscopy (STM) to study the electronic properties. This will involve studying with atomic precision the electronic structure at edges and defects with STM at Monash University.

If you have any questions, please contact Dr Mark Edmonds at mark.edmonds@monash.edu or Prof. Michael Fuhrer at michael.fuhrer@monash.edu.

Condensed Matter Physics Dr Mark Edmonds,
Professor Michael Fuhrer

"Hamiltonian learning": Machine learning for real time quantum measurements

Spinor Bose-Einstein condensates make excellent magnetometers: ultracold cold spins have vanishing thermal noise and couple strongly to magnetic fields. We use lasers to measure the BEC state and therefore measure the field. Bright beams make better measurements, but also toast the BEC faster. Too bright a beam and we have burnt off the BEC before the field has evolved it much. Too dim a beam and our measurement is swamped by noise. Do we leave the beam off and then pulse it on? How long to wait? How long a pulse? The answers to these questions depend on the strength of the magnetic field and how rapidly it is changing... and these things we know only when we start to measure! In this project you'll use ideas from machine learning and Bayesian estimation to apply adaptive and non-adaptive measurement protocols to a BEC magnetometer.

Quantum Gases Dr Lincoln Turner,
Dr Russell Anderson

Helium-rich massive stars in Globular Clusters

Globular clusters are some of the more spectacular yet mysterious components of galaxies. They are usually old objects with some 100,000 old stars. Their exact origin and formation is not known, however. Whereas it was once assumed they would form monolithically in one star formation event from just a single chemically homogeneous gas cloud, modern astronomical observations allowed us to identify at least two, sometimes more, distinct stellar populations within most of them, visible in the colour-magnitude diagram. These different populations are chemically very similar in some chemical elements, but distinctly different in others. One of the key differences is in their helium enrichment. This is what changes the location of the low-mass stars in the colour-magnitude digram, as observed. But how does it affect more massive stars, especially those that make supernovae?

The goal of this project is to model the evolution massive supernova progenitor stars, of globular cluster chemical composition, but with varying degrees of helium enrichment. The project will use a modern stellar evolution code to model evolution, nucleosynthesis, and supernova explosions of massive stars. Depending on project progress, an extension could be to compare to models of rotating massive stars and their nucleosynthesis. The results will be compared to the observational data.

Astronomy & Astrophysics Professor Alexander Heger,
Associate Professor Amanda Karakas

Higgs physics and Dark Matter in Supersymmetry

Supersymmetry is the most compelling way to extend the standard model of particle physics.  Supersymmetric models predict a new stable particle which can explain the mysterious dark matter that has been observed in astro-physical observations but is missing from the standard model.

These models also predict that at very high energies the fundamental forces unify allowing them to be embedded into a grand unified theory. Finally supersymmetric models predicts that there must be a light Higgs mass, though the minimal model predicts a Higgs mass which is slightly lighter than the one observed at the Large Hadron Collider.

The aim of this project is to investigate dark matter and Higgs physics in the next-to-minimal supersymmetric standard model, when the model satisfies unification constraints at the high energies where the model can be embedded into a grand unified theory. The results will be compared with experimental data to test if the model is allowed and how it may be discovered in future experiments.

Particle Physics,
Theoretical & Computational Physics
Dr Peter Athron

High dimensional data visualisation in high energy physics

Particle physics produces a large number of results in many different areas. One example would be Higgs physics, where the LHC experiments are releasing many measurements of its properties. Typically these measurements are compared with theoretical expectations using multiple statistical numerical tools.

This approach is understandable due to the complexity of the results, but is lacking the insight that can be developed through visualisation. In this project we will begin to implement modern statistical visualisation tools to a  data set to be selected later.

Particle Physics Professor German Valencia

How fast do old stars rotate inside?

The Kepler satellite mission became famous for finding hundreds of new planets around stars. To do so, it had to monitor them in minute detail, including all variations and oscillations of the star. Amazingly, the data was good enough to use seismology, similar to what we do on earth to determine its interior structure, or for the sun ("helioseismology "), to determine the interior structure and rotation rate of evolved - old - stars that approach the end of their life. For the Sun, we can easily observe how fast it rotates on the surface, and we know that the same rotation rate, on average, is maintained all the way to the centre. But for all other stars, the interior rotation rate was not known to date. But now we have observations. The big questions is whether our current model for transport of angular momentum and stellar evolution is good enough to explain this data. Having this data is a unique new opportunity to test physical models for the action of rotation inside stars.

The goal of this project is to model the evolution of a star like the long past its current age until the Red Giant and Horizontal Branch evolution phases. The project will use a modern stellar evolution code, and the code would also be modified to test different physics models for the action of magnetic dynamos and hydrodynamic instabilities due to rotation. The results will be compared to the observational data.

References

[1] A. Heger, S. E. Woosley and H. C. Spruit, "Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields ", The Astrophysical Journal, Volume 626, Number 1 (2005).

[2] H. C. Spruit, "Dynamo action by differential rotation in a stably stratified stellar interior ", Astronomy & Astrophysics, Volume 381, Number 3 (2002).

Astronomy & Astrophysics Professor Alexander Heger,
Paul Cally

Hyper-velocity stars - the products of tidal disruption?

We will consider the change to the stellar structure and evolution that occurs when a star wanders too close to a black hole, asking whether these features are reflected in the population of hyper-velocity stars that are thought to have had a past encounter with the supermassive black hole in the Milky Way.

Astronomy & Astrophysics Associate Professor Daniel Price,
Professor Alexander Heger

Imaging electromagnetic fields inside matter – experimental

Electromagnetic fields refract electron waves through the Aharonov Bohm effect, which shifts the electron phase.  Such shifts can be quantitatively measured from detailed fringes encoded in electron holograms, as originally envisaged by Gabor [1].  However, these phase maps typically require accurate hologram inversion through either stringent constraints of electron coherence, or carefully sculpted nano-scale specimen architecture, or the solution of differential equations, which can yield artefacts.  Such approaches were recently used to confirm the creation of electron vortices in ‘diffraction catastrophes’ arising from magnetic refraction [2], and to also discover exotic magnetic Skyrmions in specimens at room temperature [3].

Cutting edge electron microscopes are currently being equipped with new detectors to measure subtle trajectory deflections, indicative of phase gradients in electron waves, which are imprinted by refracting specimens and associated electromagnetic fields.  Electron aberration correction for ultrafine scanned probes, developed over the past decade, has enabled this information to be measured on the atomic scale [4].  The Monash Centre of Electron Microscopy have recently installed advanced electron deflection detectors on the aberration corrected Titan microscope.  We are seeking an enthusiastic student to work with our research team to measure electron phase gradients with this new technology and compare with existing electron holography approaches.  Our ultimate goal is to measure and quantify electromagnetic fields on the nanoscale to open new opportunities for discoveries in condensed matter physics.

References

[1] D. Gabor, "A new microscopic principle", Nature 161, 777-778 (1948).

[2] T. C. Petersen, M. Weyland, D. M. Paganin, T. P. Simula, S. A. Eastwood, and M. J. Morgan, "Electron Vortex Production and Control Using Aberration Induced Diffraction Catastrophes" Physical Review Letters, volume 110, 033901 (2013).

[3] Y. Tokunaga,X. Z. Yu,J. S. White,H. M. Ronnow,D. Morikawa,Y. Taguchi, and Y. Tokura, "A new class of chiral materials hosting magnetic skyrmions beyond room temperature", Nature Communications, volume 6, 7638 (2015).

[4] Naoya Shibata, Scott D. Findlay, Yuji Kohno, Hidetaka Sawada, Yukihito Kondo, and Yuichi Ikuhara, "Differential phase-contrast microscopy at atomic resolution", Nature Physics, volume 8, 611-615 (2012).

Imaging Physics,
Condensed Matter Physics
Dr Timothy Petersen,
Dr Scott Findlay,
Dr Laura Clark,
Dr Hamish Brown,
Professor David Paganin,
Professor Michael Morgan

Imaging electromagnetic fields inside matter – computational

This project aims to develop tools to map electromagnetic inside matter, on the nanoscale. Such fields are used for encoding information in data storage microelectronic devices. Since the world now generates more data than it can store, the search is on for new technologies to improve storage capacity and energy efficiency by encoding information at the smallest possible length scales. New characterisation techniques are thus needed to probe fields on this scale.  Projects exploring how advanced electron microscopy can contribute to tackling this problem are available with either a computational or experimental focus.

References

[1] Naoya Shibata, Scott D. Findlay, Yuji Kohno, Hidetaka Sawada, Yukihito Kondo, and Yuichi Ikuhara, "Differential phase-contrast microscopy at atomic resolution", Nature Physics, volume 8, 611-615 (2012).

[1] Knut Muller, Florian F. Krause, Armand Beche, Marco Schowalter, Vincent Galioit, Stefan Loffler, Johan Verbeeck, Josef Zweck, Peter Schattschneider, and Andreas Rosenauer, "Atomic electric fields revealed by a quantum mechanical approach to electron picodiffraction", Nature Communications, volume 5, 5653 (2014).

Imaging Physics,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Scott Findlay,
Dr Laura Clark,
Dr Hamish Brown,
Dr Timothy Petersen,
Professor David Paganin,
Professor Michael Morgan

Impulse-equivalent pulse trains and arrays

Functions that mimic the sharp sifting property of the discrete Dirac-delta are highly valued for their practical use in digital communication, many forms of imaging and for data encryption. A delta function has sharp localisation in the spatial domain (a perfect point spread function, or psf) and is exactly flat in the Fourier domain (a perfect modulation transform function, or MTF). The aperiodic auto-correlation for impulse-equivalent 1D sequences of finite length (or as nD arrays) have the largest possible peak value at zero relative shift, with minimal, preferably zero, correlation for all off-peak shifts.  The quality of a sequence is measured by its discrimination factor (peak to maximum off-peak ratio) and its merit factor (the ratio of peak value squared to the sum of all squared off-peak values). The sequence elements should comprise small integer values (e.g. -1 and +1 entries or binary 0, +1 entries) and should also avoid having too many zero terms to keep their energy efficiency high. The Barker sequences are the outstanding exemplars, but these are known to have a maximum length of just 13 elements,  viz. [+++++--++-+-+]. Longer sequences can be found by adopting a wider range of integer values (e.g. the Huffman sequences) or using Moharir sequences that interleave multiple arrays, each built with a ternary alphabet (-1, 0, +1). These sequences work well, but they either have lower efficiency or prove to be less robust when they are recovered in a noisy environment.

This project will build long, impulse-equivalent sequences based on Singer difference sets. Each Singer set is a binary sequence that is exactly spectrally flat and has small off-peak auto-correlation values. The length L of a Singer sequence can be quite large, L = p2 + p + 1 when p is prime. The Singer sequences are, unfortunately, quite sparse, containing just p+1 non-zero elements. However, the complement of each Singer set is also exactly spectrally flat. Pairs of Singer sequences of length L can then be combined to produce a sequence of length L that has maximal efficiency and has high quality auto-correlation. These 1D sequences, in turn, can be used to construct nD arrays with strong auto-correlation properties. The project will examine optimal ways to combine 1D Singer sequences and use these to build nD impulse-equivalent arrays.

Reference:

[1] Singer, J., 1938, A theorem in projective geometry and some applications to number theory, Trans. Amer. Math. Soc., 43, 377-385.

[2] Barker, R. H., 1953, Group synchronisation of binary digital systems, In Communication Theory, (Ed.) W. Jackson (London, Butterworths).

[3] Hunt, J. N. and Ackroyd, M. H., 1980, Some integer Huffman sequences, IEEE Trans. Information Theory, vol. 26, no. 1, 105-107, 1980.

[4] Moharir, P.S., Subba Rao, K., 1997, Non-binary sequences with superior merit factors, IETE, J. Res, 1:, 49-53.

[5] Tirkel, A., Cavy B. and Svalbe, I., 2015, Families of multi-dimensional arrays with optimal correlations between all members, Electronics Letters, 51(15) 1167-1168.

[6] Svalbe, I., Ceko M. and Tirkel, A., 2018, Large families of ‘grey’ arrays with perfect auto-correlation and optimal cross-correlation, DCGI, Vienna, Austria, September, 2017, under review for JMIV.

Imaging Physics Dr Imants Svalbe,
Andrew Tirkel

Inferring supernova physics from the production of heavy elements

Massive stars go through various burning stages during their lives and eventually develop on "onion shell" structure of layers composed of successively heaver elements towards the interior with an iron core at the centre. Eventually the iron core collapses to a neutron star or black hole, and in many cases, the envelope is ejected in a supernova explosion. The dividing line between successful explosions and black hole formation is still not completely clear. Theoretical models [1] are still beset with uncertainties. Observations of supernova progenitors suggest an upper mass limit for supernovae of about 15-18 solar masses [2], but complementary evidence is needed to substantiate this. Investigating the contribution of supernova explosions to the inventory of heavy elements in the galaxy could help further constrain the upper mass limit for supernova explosions. Interestingly, it seems that explosions of massive stars above 20 solar masses are required to produce sufficient amount of some elements like oxygen [3]. However, this conclusion is sensitive to the assumed supernova explosion energies and the "mass cut" between the neutron star and the ejecta.

In this project, we shall reinvestigate how the contribution of certain elements and radioactive isotopes (Fe, O, Al-26) depends on the fraction of massive stars that go supernovae instead of forming a black hole. We will use a simple Python-based model for the progenitor-dependence of supernova explosion properties [1] to compare the relative production of Fe to other "marker" elements for massive stars.

References

[1] Mueller, Heger, Liptai & Cameron 2016, MNRAS 460, 742

[2] Smartt 2015, PASA 32, e16

[3] Woosley & Brown 2013, ApJ 769, 99

Astronomy & Astrophysics Dr Bernhard Mueller,
Professor Alexander Heger

Interplay between Dirac electrons and magnetic textures in topological insulators

Topological insulators represent a new state of matter that has insulating bulk and exotic surface metallic states. These states are described by the ultra-relativistic massless Dirac equation and have the spin-momentum locking. The later implies directions of electronic spin and momentum to be perfectly correlated. As a result, interplay between Dirac surface states with internal or proximity induced magnetism is very rich. For example, isolated magnetic domain walls and skyrmions strongly modify the local electronic spectrum of the surface states [1].

The aim of the project is to investigate electronic and transport properties of Dirac surface states coupled with periodic lattice of magnetic textures.

References

[1] H.M. Hurst, D.K. Efimkin, J. Zang, V. Galitski - Phys. Rev. B 91, 060401 (2015)

Quantum Gases,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Dmitry Efimkin

Is dark matter superfluid light?

There are numerous observations shedding light on the properties of dark matter, however its decomposition is unknown. Based on its known properties, we examine whether dark matter can be superfluid light. Our speculation is motivated by the recent observations of Bose-Einstein condensation of photons [1] and by the successes of the bosonic star dark matter models [2].

In this project, we examine the quantitative details of the above proposal. Under what conditions do photons condense in the early Universe? Is the condensate "dark "? Is it destroyed by ordinary matter? Does the average energy density trapped in the condensate match the measurements? Do vortices in the condensate correspond to galaxies, clusters or larger observed structures? Can the mass distribution of galaxy clusters be explained? Do the galactic rotation curves agree with this model?

Based on the general properties of Bose-Einstein condensates and the standard (ΛCDM) cosmological model each of these questions can be quantitatively answered [3,4].

References

[1] J. Klaers, J. Schmitt, F. Vewinger and M. Weitz, "Bose–Einstein condensation of photons in an optical microcavity" Nature, Volume 468, Issue 7323 (2010)

[2] J.-W. Lee, "Is dark matter a BEC or scalar field? ", arXiv:0801.1442 .

[3] M. Morgan and R. Yu, "Vortices in a rotating dark matter condensate", Classical and Quantum Gravity, Volume 19, Number 17 (2002).

Particle Physics,
Quantum Gases,
Theoretical & Computational Physics
Professor Csaba Balazs,
Professor Michael Morgan

Low-dimensional organic nanostructures with topological electronic properties

Topological insulators are a novel class of materials with non-trivial electronic properties. Electrons at the boundaries of these materials can propagate without dissipating energy [1]. So far, topological phases have only been demonstrated in inorganic materials. The goal of this project is to synthesise and characterise low-dimensional organic nanostructures, in which the atomic-scale morphology and electronic structure give rise to non-trivial topological electronic states[2]. The project will exploit metal atoms and organic molecules as building units in approaches of supramolecular chemistry applied on surfaces, to achieve ultimate structural and electronic control at the single atom level [3]. The motivation of stems from the bottom-up design of advanced materials, where novel electronic signatures will be applicable for the development of dissipation-less electronics, spintronics and solid-state-based quantum information processing.

Systems of interest will be prepared in ultrahigh vacuum (UHV) and characterised in situ by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), as well as non-contact atomic force microscopy (ncAFM). X-ray-based measurements performed at the Australian Synchrotron will provide complementary chemical characterisation. Optical techniques will allow for optical probing of topological electronic phases.

References

[1] M. Z. Hasan and C. L. Kane, "Colloquium: Topological insulators", Reviews of Modern Physics, volume 82, 3045 (2010).

[2] Z. F. Wang et al, Nature Communications volume 4 (2013).

[3] Johannes V. Barth, "Molecular Architectonic on Metal Surfaces", Annual Review of Physical Chemistry, volume 58, 375-407 (2007).

Condensed Matter Physics Dr Agustin Schiffrin

Machine learning for analysis of scanning diffraction patterns in electron microscopy

Artificial intelligence and machine learning are offering new research tools across a wide range of scientific disciplines [1]. One such tool is the artificial neural network [2], which seeks to model information processing in biological neural networks (e.g. brains) and can be "trained" to accomplish certain tasks without task-specific programming. These networks are increasingly being used to solve practical problems in big-data processing, where the growth in the volume of multidimensional data has outstripped the capacity for manual interpretation by humans. Such networks have recently been applied to recognise patterns in the scattering physics experiments undertaken in atomic-resolution scanning transmission electron microscopy [3,4], and machine learning approaches are expected to become increasingly popular for intelligently guiding the reduction of big data from modern electron microscopes. Computational projects are available in developing familiarity with these tools and applying them to imaging and diffraction in electron microscopy. These novel approaches are expected to tease out new information from electron microscope specimens, such as subtle variations in nanoscale morphology, which may not be attainable by traditional analysis.

References

[1] http://www.sciencemag.org/news/2017/07/ai-changing-how-we-do-science-get-glimpse

[2] R. Rojas, "Neural Networks: A Systematic Introduction" (Springer Verlag, Berlin, 1996).

[3] R.S. Pennington, W. Van ben Broek, C. Koch, Phys. Rev. B (2014) 205409.

[4] W. Xu & J.M. LeBeau, https://arxiv.org/abs/1708.00855

Imaging Physics,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Scott Findlay,
Dr Timothy Petersen

Magnetic fields in star formation

When we model star formation in the computer, we usually chop out the star itself and replace it with a point mass, or `sink particle’. However, this is problematic in the presence of magnetic fields because it involves punching a hole in the magnetic field, thus creating magnetic monopoles which should not exist. The aim of this project is to find a better boundary condition for the magnetic field near sink particles, based on ideas from modelling magnetic fields in the solar system, in order to carry out more consistent and stable simulations of star formation.

Astronomy & Astrophysics Associate Professor Daniel Price

Magnetoresistance of semiconductors in the non-linear regime

The change of electrical resistance in a magnetic field (i.e., the magetoresistance) is typically rather small in many materials, but it can have important technological applications when it is sizeable. For instance, the "giant magnetoresistance " of magnetic multilayer structures provides the basis for magnetic sensors used in hard disks and other devices. However, magnetism is not the only route to a large magnetoresistance. Material inhomogeneities or the sample geometry can also generate a substantial magnetoresistance in non-magnetic systems [1]. It has also been demonstrated that a similar effect can be achieved in a semiconductor subjected to a high electric field, where the electrical transport is no longer ohmic [2]. The aim of this project is to determine whether the inhomogeneous electric field present in this non-linear transport regime is sufficient to produce a large magnetoresistance or whether something further is required.

References

[1] M. M. Parish, P. B. Littlewood, Non-saturating magnetoresistance in heavily disordered semiconductors, Nature volume 426, number 6963, 2003

[2] M. P. Delmo et. al., Large positive magnetoresistive effect in silicon induced by the space-charge effect, Nature volume 457, number 7233, 2009

Condensed Matter Physics,
Theoretical & Computational Physics
Associate Professor Meera Parish

Making carbon in the universe: implications for life?

This project will look at the production of carbon by red-giants. The aim will be to take the results from detailed stellar evolution calculations and include these in a new code which simulates the evolution of an entore Galaxy. With some simple approximations, we can produce a "population synthesis" code which can model a large population of stars. Then we can investigate the effect of binary stars, mass-transfer from one to another, the effect of mass-loss etc etc. It is possible, if time permits, to also include other species so we can look at the production of Oxygen also. This has many implications for the appearance of life in the Universe - planets forming in a carbon-rich environment are very different to those forming in a oxygen-rich environment!

This would be a good project for someone who wanted to improve their skills at computer programming: it will start with a very simple code to which you will add more and more and build it into a substantial piece of work.

Astronomy & Astrophysics Professor John Lattanzio,
Associate Professor Amanda Karakas

Merging binary black holes in globular clusters

In February of 2015, the LIGO observatory announced the first detection of gravitational waves from a pair of merging black holes.   Since then, a number of scenarios for forming such binary black holes have been proposed.  One involves dynamical interactions in globular clusters.  3-body encounters could tighten black-hole binaries, bringing them to merger.  They may also be responsible for the ejections of very fast (a few hundred km/s) stars from clusters, which could be probed with the Gaia mission.  In this project, we will analyse the dynamics of globular clusters and investigate gravitational-wave signatures and stellar ejections as tracers of such dynamics.

References

Rodriguez et al., http://adsabs.harvard.edu/abs/2016ApJ...824L...8R ; Mandel & Farmer, http://adsabs.harvard.edu/abs/2018arXiv180605820M

Astronomy & Astrophysics Professor Ilya Mandel,
Dr Eric Thrane

Miniaturisation of Electrical Devices for MBE growth and in-situ Transport Studies

Fuhrer, Hellerstedt and Edmonds have developed unique-in-the-world techniques to measure the electrical properties, such as resistivity and Hall effect, of a thin film of material during growth by molecular bean epitaxy (MBE) as well as post-growth without removing the sample from vacuum. This allows for studies of exotic materials that may be unstable on removal from vacuum. The current techniques use electrical devices on the millimeter length scale. Yet, many interesting and exotic quantum transport phenomena such as ballistic transport and spin field-effect transistors require devices on the micrometer or even nanometer length scale to be observed. This project will have two aims:

(1) Develop fabrication techniques to miniaturise these devices, whilst still maintaining a clean substrate surface capable of growing high-quality MBE films. This part of the project will utilise the fabrication facilities at the Melbourne Centre for Nanofabrication such as lithography, metal deposition and atomic-layer deposition, as well as electron beam lithography in Fuhrer's laboratory.

(2) Carry out MBE film growth and transport measurements on these miniaturised devices using the combined low-temperature scanning tunnelling microscopy and MBE system within the Fuhrer laboratory.

Condensed Matter Physics Professor Michael Fuhrer,
Dr Mark Edmonds,
Dr Bent Weber,
Dr Jack Hellerstedt

Mixing at convective boundaries in 3D and 1D

Stars are not spherically symmetric. One of the most important multi-dimensional phenomena inside stars is convective overturn, which is driven by buoyancy. Stellar evolution models are forced to treat this phenomenon in spherical symmetry by means of effective recipes such as the "mixing-length theory". Such an approach cannot do full justice to the multi-dimensional nature of convective flow. Mixing-length theory often describes the rapid and efficient mixing within convective zones quite well, but it is particularly difficult to adjust or extend it to reproduce what is going on at convective boundaries. Here, the interaction of convective plumes with a stable region can lead to wave excitation and complicated mixing processes. Various recipes have been proposed to treat these phenomena in stellar evolution models [1,2].

Thanks to growing computer power, we can now simulate convection inside stars in three dimensions in certain cases and use these simulations to test and improve one-dimensional recipes for mixing. In this project, we will focus on the process of “turbulent entrainment” in the late stages of shell convection in massive stars. We will compare the results of recent 3D simulations [3,4] to standard recipes for convective boundary mixing in stellar evolution codes [2] and to more sophisticated one-dimensional turbulence models [5] to determine how these one-dimensional models can be calibrated to correctly reproduce the growth of convective shells by entrainment.

References

[1] Viallet, Meakin, Prat & Arnett 2015, Astronomy & Astrophysics 580, 61

[2] Freytag, Ludwig & Steffen 1996, Astronomy & Astrophysics 319, 497

[3] Müller, Viallet, Heger & Janka 2016, The Astrophysical Journal 833, 124

[4] Meakin & Arnett 2007, The Astrophysical Journal 667, 448

[5] Wuchterl & Feuchtinger 1998, Astronomy & Astrophysics 340, 419

Astronomy & Astrophysics Dr Bernhard Mueller,
Professor Alexander Heger,
Professor John Lattanzio

Mixing in red giants – attacked with telescopes and computers

Red-giants are fascinating stars and they continue to provide us with problems that as yet have no solutions. One of these concerns mixing as they stars climb the giant branch. The standard models do not predict as much mixing as is seen in the data. We recently discovered a mechanism that is likely to be involved in the solution of this problem, but there are aspects of the mechanism that are still unknown. We are trying to tackle this problem by combining observations and theoretical models. We will be taking new data in December for stars in the globular cluster NGC1851.

This project involves both running stellar models, so that the student investigates the theory of thermohaline mixing, as well as reducing data taken with Australia's largest telescope. This will likely involve a trip to the Australian Astronomical Observatory (AAO) to work with the co-supervisors in Sydney. The aim is to compare the observed data with the models run at Monash.

References:

[1] Lardo, et al., 2012, Astronomy and Astrophysics, 541, p141

[2] Angelou et al., 2012, Astrophysical Journal, 749, p128

[3] Eggleton et al., 2006, Science, 314, p1580

Astronomy & Astrophysics Professor John Lattanzio,
Dr Simon Campbell

Modeling black hole birth kicks

When massive stars undergo supernovae and collapse into neutron stars or black holes, they may asymmetrically eject mass, leading to a recoil kick.  Such birth kicks for black holes play an important role in the evolution of X-ray binaries and possible future gravitational-wave sources.  However, they are difficult to measure through direct observations.  In this project, we will use a combination of supernova explosion models and indirect observational constraints to constrain black hole natal kicks.

References

Mirabel, http://adsabs.harvard.edu/abs/2016arXiv160908411M ; Mueller et al., http://adsabs.harvard.edu/abs/2016MNRAS.460..742M

Astronomy & Astrophysics Professor Ilya Mandel,
Dr Bernhard Mueller

Modelling the electronic structure of low-dimensional organic nanostructures on surfaces

Supramolecular and metal-organic self-assembly on surfaces holds promise for the synthesis of functional low-dimensional nanostructures with ultimate atomic-scale precision [1]. This approach consists of depositing atoms and functionalised organic molecules onto clean surfaces, in a controlled environment, to achieve well-defined configurations via programmed inter-adsorbate and adsorbate-surface interactions.

Potential functionality of these nano-assemblies arise from their atomic-scale electronic structure, which is dictated by quantum mechanics. Understanding and predicting such electronic structure is non-trivial, given the typically large size of the unit cell of these many-body systems. Density functional theory (DFT) [2] offers a viable numerical method for determining the energy-dependent electronic density in such systems, allowing to understand and predict their morphology and electronic properties at the atomic scale.

The project here consists of using computational DFT tools for determining the atomic-scale morphology and electronic structure of low-dimensional nanostructures on surfaces. Systems of interest will consist of 1D and 2D metal-organic frameworks on metals, semiconductors and atomically thin materials. In particular, these systems are relevant for the design of solid interfaces with functionality in photovoltaics, photocatalysis, molecular nanoelectronics and molecular magnetism. This project will be co-supervised by Dr. N. Medhekar (Department of Materials Science and Engineering).

References

[1] Johannes V. Barth, "Molecular Architectonic on Metal Surfaces", Annual Review of Physical Chemistry, volume 58, 375-407 (2007).

[2] R. O. Jones, "Density functional theory: Its origins, rise to prominence, and future", Reviews of Modern Physics, volume 87, 897 (2015).

Condensed Matter Physics Dr Agustin Schiffrin,
Dr Nikhil Medhekar

Modelling the interaction of atoms with an optically trapped microsphere

The interaction of light with matter invariable involves the exchange of momentum. This exchange of momentum can be exploited to trap and remotely manipulate particles from the size of individual atoms to objects at the micron scale. Typically, the study of optical forces on these vastly different size objects is performed independently. The purpose of this project is to investigate the combined effect of these two, seemingly different, applications of optical forces. In particular, the project will involve modelling the optical field at the surface of a microsphere trapped by a focused laser beam and studying the interaction of laser cooled atoms with the field at the surface of the microsphere.

Imaging Physics Professor Kris Helmerson

Nearby AGNs in the mid-infrared

References

[1] Alonso-Herrero et al.. 2014, MNRAS, 443, 2766 (arXiv:1407.1154.)

[2] Brown et al.., 2014, ApJS, 212, 18 (arXiv: 1312.3029).

[3] Mason et al.., 2012, AJ, 144, 11 (arXiv: 1205.0029).

Astronomy & Astrophysics Associate Professor Michael Brown

Neutron star mergers

Neutron star mergers are thought to be the origin of short gamma-ray bursts. They are also prime candidates for observations with gravitational wave detectors such as LIGO. In this project we will try to adapt our 3D simulation code to model neutron star mergers in 3D with the aim of understanding the physics of these extreme stellar objects.

Astronomy & Astrophysics Associate Professor Daniel Price

Nucleosynthesis during the core helium flash

After a star exhausts its central H supply it becomes a red-giant. For low mass stars this He core becomes electron-degenerate and continues to heat. When the temperature reaches about 100 million degrees we find helium burning is ignited. But due to the degenerate equation of state, this is almost explosive. The star generates about one billion times the energy of our Sun, but just for a few days. The structure of the star changes dramatically, of course. Although the details are very complicated, we know that most stars survive this phase, as we see them in the next stage of their lives.

Standard calculations with the standard assumptions (hydrostatic equilibrium, instantaneous convective mixing) seem to get through this phase, although clearly the details require multi-dimensional hydro-dynamical calculations. Nevertheless, since models made with these assumptions match the later phases, these standard assumptions must be OK for most stars. In this project you will evolve stars through this phase using a stellar evolution code, investigating the uncertainties in the calculations. Further, a separate code calculates the detailed nuclear reactions occurring and we will investigate the reactions and possible mixing mechanisms. Are there observational tests we can apply to the models?

Astronomy & Astrophysics Professor John Lattanzio

On-surface design of organic nanostructures with tailored optoelectronic functionality

On-surface supramolecular chemistry - through which molecular and atomic units interact and form well-defined geometries – holds promise for the fabrication of nanostructures with atomic-scale precision and tailored electronic properties [1]. This project consists of using approaches of supramolecular chemistry to synthesise low-dimensional organic and metal-organic nano-assemblies on surfaces. The goal is to achieve solid-state interfaces with atomically precise morphologies, resulting in well-defined electronic structure, and with potential for efficient light absorption and photo-induced charge separation. The motivation stems from designing materials for efficient and cost-effective optoelectronic applications [2].

Systems of interest will consist of organic and metal-organic nano-assemblies on noble metal surfaces and thin insulating films. These systems will be prepared in ultrahigh vacuum (UHV) and characterised in situ by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS), as well as non-contact atomic force microscopy (ncAFM). X-ray-based measurements performed at the Australian Synchrotron will provide complementary chemical characterisation. Optical techniques will provide optoelectronic characterisation. International collaborations will allow for time-resolved studies of ultrafast photo-induced charge dynamics.

References

[1] Johannes V. Barth, "Molecular Architectonic on Metal Surfaces", Annual Review of Physical Chemistry, volume 58, 375-407 (2007).

[2] Gregory D. Scholes, Graham R. Fleming, Alexandra Olaya-Castro, and Rienk van Grondelle, "Lessons from nature about solar light harvesting", Nature Chemistry, volume 3, 763-774 (2011).

Condensed Matter Physics Dr Agustin Schiffrin

Optical photoacoustic tomography sensing

Photoacoustic techniques are seen as the next generation of imaging for biological systems [1]. Photoacoustic tomography allows structures throughout a volume of scattering tissue to be visualized using light, which has many advantages over conventional X-ray or MRI techniques. The main limitation is the lack of high spatial resolution sensors for detecting ultrasound waves. This experimental project will optimize a recently developed optically-based sensor, utilizing a conventional camera, to provide high-resolution, time resolved ultrasound detection of a large region without requiring scanning. A tomographic inversion algorithm will be used to reconstruct the sensing volume.

References

[1] Laufer J., Zhang E., Raivich G., Beard P., Three-dimensional noninvasive imaging of the vasculature in the mouse brain using a high resolution photoacoustic scanner. Applied Optics, 48, 10, D299-D306, 2009

Imaging Physics Dr Alexis Bishop

Optical Lattices for ultracold atoms

Ultracold atoms can be trapped in an optical lattice, the periodic potential formed by optical standing waves. The atoms in the optical lattice exhibit behavior similar to electrons in an ideal crystal. By including the interaction of the atoms, such a system can be used to study many-body phenomena traditionally in the realm of solid-state physics. This project will investigate the emergence of many-body behaviour of particles as various parameters such as interaction strength and disorder as varied in this model system.


Physics,
Condensed Matter Physics
Professor Kris Helmerson

Optical site testing for future robotic telescopes

Demand for future optical observing facilities dedicated to survey and fast-response followup is only likely to increase. One such facility is the Gravitational wave Optical Transient Observatory (GOTO), a project aimed at detecting the electromagnetic counterparts of binary inspiral events detected with the Laser Interferometric Gravitational-wave Observatory (LIGO). GOTO is hoped eventually to consist of a pair of instruments, one in La Palma, Canary Islands, and one in Australia. This project is focussed on measurements of the astronomical "seeing", both the degree of blurring of stellar images by the atmosphere, and the brightness of the sky. Portable test equipment will be developed and tested in the lab, and then deployed at candidate sites to obtain empirical values for the seeing, and verify the remote measurements. The student will visit the observing sites to assist in data collection, and analyse the CCD data to estimate the seeing. Experiments at existing telescope sites will also be necessary to calibrate the test equipment.

Further reading: Hotan et al.. 2013, http://dx.doi.org/10.1017/pasa.2012.002

Astronomy & Astrophysics Associate Professor Duncan Galloway,
Professor Karl Glazebrook (Swinburne)

Optical trapping of microscopic water droplets for single molecule studies

Techniques for optically observing single molecules are extending and even changing our understanding of molecular processes in biology. Often, it is desirable to follow the dynamics of a single molecule for several seconds or longer. Methods have been developed to immobilize and isolate or confine single molecule in order to study their dynamics on such long time scales. One such approach is to confine the molecule of interest in a microscopic water droplet immersed in an immiscible background fluid, and then trap the water droplet using optical tweezers. For a sufficiently small water droplet, the molecule of interest will remain within the detection volume of a confocal microscope allowing continuous measurement of the molecule's behaviour. This approach has advantages over other approaches for immobilizing single molecules, such as surface attachment - the molecule is free to diffuse within the water droplet away from an uncharacterized surface. The goal of this project will be to develop an apparatus capable of trapping and manipulating microscopic water droplets containing single molecules and studying the behaviour of the single molecules. In addition to the development of the necessary optical technologies for such studies, microfluidic-based approaches will be investigated for generating single, microscopic water droplets on demand.

Condensed Matter Physics Professor Kris Helmerson

Periodically driven many-body quantum systems

Periodic driving of a system endows it with a temporal periodicity, much like crystal lattices that have a spatial periodicity. Yet, periodic driving yields a variety of tuning knobs which can in principle allow one to investigate physics that is difficult to access in any equilibrium system. Indeed, even though there is only one time dimension, one can even make the system equivalent to higher-dimensional spatial lattices. This project will explore some of the new physics that can emerge in periodically driven many-body quantum systems.

Quantum Gases,
Condensed Matter Physics,
Theoretical & Computational Physics
Associate Professor Meera Parish,
Dr Jesper Levinsen

Phase-Engineering of Atomically Thin Crystals

Graphene - an atomically thin sheet of carbon atoms - has attracted enormous scientific interest since its discovery in 2003. Awarded with the Nobel Prize only seven years after it was discovered, the material was soon hailed as the next disruptive technology due to superior attributes, being a zero-band gap Dirac semi-metal with large electron mobility.

A related class of atomically thin materials are the layered transition metal dichalcogenides (TMDCs) MX2, composed of hexagonal lattices of transition metal atoms (M) coordinated by chalcogens (X). The more complex composition of TMDCs allows for a number of different crystalline phases with drastically varying electronic properties - ranging from semiconducting, metallic, and superconducting, to more exotic topological phases. The different crystal structures have been shown to be lattice-matched and exhibit atomically sharp boundaries. Considerable interest therefore exists to locally control the phase to define atomic-scale heterostructures to be used in molecular-scale electronic devices.

The project aims at phase-engineering group VI TMDCs (such as MoS2 and WS2). Based on the interest and skill of the student, experimental techniques may include wet-chemical processing of the material, nanofabrication (including electron beam lithography), electron transport, as well as structural and electronic characterization at the atomic-scale, using low-temperature scanning tunnelling microscopy (STM).

Condensed Matter Physics Dr Bent Weber,
Professor Michael Fuhrer

Phase contrast imaging with vortex lattices

We are looking for an enthusiastic student to perform research on practical applications of optical vortex lattices [1, 2] for interferometry [3, 4], to expand the scope of contemporary singular optics.  In prior work [5] we have demonstrated quantitative ‘singularimetry’ [6], where a 3-beam lattice of optical vortices directly measured phase shifts imparted by a specimen in one arm of a 3-beam Mach-Zehnder interferometer.  For certain microscopy techniques, strong radiation-matter interactions and short wavelengths may prohibit such uses of beam splitters.  For example, in electron holography, 3-beam vortex lattices have recently been realized, which could enhance the precision of nanoscale electromagnetic field mapping [7, 8].  However, restricting just one electron beam to pass through a sample poses significant engineering challenges.  We have hence derived a different approach, where all beams pass through the sample. To this end, a new differential form of singularimetry was developed, utilising vortices and gradient singularities as topological fiducial markers in a structured-illumination context.  This approach analytically measures phase gradients imparted by refracting specimens, yielding quantitative information that is local and deterministic.  Recent light-optics experiments of this kind have demonstrated that singularity lattices can detect subtle specimen variations with high precision [9].

We would now like to create new capabilities for these experiments, by exploring the non-trivial movement of oppositely charged vortices in response to specimen attenuation variations.  Similarly, we want to extend these ideas to non-vortical singular points in more general structured optical wave-fields, with applications for other forms of radiation.  The project is open to theoretical, computational and/or experimentally inclined students.

References

[1] K. W. Nicholls and J. F. Nye, "Three-beam model for studying dislocations in wave pulses", Journal of Physics A: Mathematical and General, volume 20, 4673-4696 (1987).

[2] Jan Masajada and Bogusława Dubik, "Optical vortex generation by three plane wave interference", Optics Communications, volume 198, 21-27 (2001).

[3] Jan Masajada, "Small-angle rotations measurement using optical vortex interferometer", Optics Communications, volume 239, 373-381 (2004).

[4] Agnieszka Popiolek-Masajada, Monika Borwinska, and Boguslawa Dubik, "Reconstruction of a plane wave’s tilt and orientation using an optical vortex interferometer", Optical Engineering, volume 46, 073604-073608 (2007).

[5] Samuel A. Eastwood, Alexis I. Bishop, Timothy C. Petersen, David M. Paganin, and Michael J. Morgan, "Phase measurement using an optical vortex lattice produced with a three-beam interferometer", Optics Express, volume 20, issue 13, 13947-13957 (2012).

[6] M. R. Dennis and J. B. Gotte, "Beam shifts for pairs of plane waves", Journal of Optics, volume 15, 014015 (2013).

[7] T. Niermann, J. Verbeeck, and M. Lehmann, "Creating arrays of electron vortices", Ultramicroscopy, volume 136, 165-170 (2014).

[8] C. Dwyer, C. B. Boothroyd, S. L. Y. Chang, and R. E. Dunin-Borkowski, "Three-wave electron vortex lattices for measuring nanofields", Ultramicroscopy, volume 148, 25-30 (2015).

[9] Timothy C. Petersen, Alexis I. Bishop, Samuel A. Eastwood, David M. Paganin, Kaye S. Morgan, and Michael J. Morgan, "Singularimetry of local phase gradients using vortex lattices and in-line holography", Optics Express, volume 24, issue 3, 2259-2272 (2016).

Imaging Physics,
Condensed Matter Physics,
Theoretical & Computational Physics
Dr Timothy Petersen,
Dr Laura Clark,
Dr Alexis Bishop

Photoacoustic detection of deep targets

There is much interest in being able to detect stroke affected regions in the human brain without using dangerous ionising X-rays or expensive MRI or CT techniques. Recently developed photoacoustic techniques have the ability to provide visualisation of the vascular structure as well as measurements of other properties. For the photoacoustic technique, an intense pulsed laser beam, with a wavelength that is absorbed by the haemoglobin in red blood cells, illuminates the skull, which strongly diffuses the light without appreciable absorption. The blood cells in the vessels of the brain absorb the scattered light, and generate an outward pressure pulse. The pressure pulse can travel largely unimpeded through the brain and skull to the surface where it can be detected by a pressure transducer.  By observing the pressure signal at different delay times, images of structures at different depths in the brain can be made.

This experimentally-based project is to develop a sensor system, based on piezoelectric polymers, that is capable identifying the equivalent of stroke affected brain regions beneath a human skull. The project does not involve human or animal experimentation, and will use simulated brain analogues.

Imaging Physics Dr Alexis Bishop

Planets around evolved stars

The results from stellar evolution calculations are an essential tool for studies of extra-solar planets. While most extra-solar planets are found around solar-type main sequence stars, planets are found around cooler evolved red giant stars. Observational studies suggest these planet-hosting evolved stars may be metal poor, in contrast to the main-sequence stars with planets, which tend to be metal rich (Maldonado et al. 2013). When a star exhausts it's supply of hydrogen the outer layers expand and the star becomes a red giant. Will it expand enough to swallow the planets that orbit it? The answer to that question depends on the initial mass and composition of the star, and masses and orbital characteristics of the planets (e.g., Villaver et al. 2014).

In this project you will use theoretical models of stars from the main sequence to the tip of the asymptotic giant branch to study the impact of stellar evolution on the orbits of planets. You will start with predicting the future of our Sun and Earth (e.g., Schroeder & Connon Smith 2008) and then expand the calculations to include other mass planets, and stars of different mass and composition.

References

[1] Maldonado, J. et al. 2013, Astronomy & Astrophysics, 554, A84

[2] Villaver, E. et al. 2014, The Astrophysical Journal, 794, p3

[3] Schroeder, K.-P. & Connon Smith, R. 2008, Monthly Notices of the Royal Astronomical Society, Volume 386, Pages 155

Astronomy & Astrophysics Associate Professor Amanda Karakas

Probing neutron stars via thermonuclear bursts

Studies of thermonuclear (X-ray) bursts in accreting neutron stars have historically relied on short observations of individual sources, resulting in (usually) a handful of bursts. Another approach is to gather large numbers of bursts from multiple sources and telescopes, and analyse the resulting combined sample to better understand the physics of this phenomena. Such an effort is currently underway at Monash with the Multi-INstrument Burst ARchive (MINBAR), which presently consists of approximately 2200 bursts from NASA's Rossi X-ray Timing Explorer satellite as well as the defunct Dutch/Italian mission BeppoSAX. The principal efforts at present are to add bursts in public data observed by the JEM-X camera onboard ESA's INTEGRAL satellite, likely adding another 2000 events and with new observations continually being added. The catalog, once complete, will prove a vital resource for studies of thermonuclear bursts and will be released to the public.

This project will involve analysis of burst data, cross calibration, and verification for addition to the burst sample. It is expected that the student will also work on burst data from newly-discovered transient neutron stars through the course of the project, and collaborate on papers resulting from this work.

This project will primarily involve analysis of reduced data from the various X-ray satellites with IDL. Opportunities exist for work with project partners at SRON (Netherlands) and DTU Space (Denmark).

Astronomy & Astrophysics Associate Professor Duncan Galloway

Quantitative Electron Tomography at the Nanoscale

Electron tomography is a 3D characterisation technique that can be performed in a scanning transmission electron microscope (STEM) [1,2]. It is capable of sub-nanometre resolution, and in some cases can even resolve individual atoms in 3D [3]. However, its accuracy is primarily geometric - it determines the shape of the object with high resolution but the correlation between the reconstructed intensity in an individual voxel (volume pixel) and the physical properties of the object (density, atomic number, etc.) is poor.

This project seeks to improve the correlation between voxel intensity and atomic number / density of a sample based on recent work that models the physics of electron scattering to put the recorded STEM intensities on an absolute scale [4]. This will primarily be investigated via numerical simulation, though options for combining simulation and experimental data from simple, standard specimens may also be explored. This project involves computation - learning to use programs for generation of crystal structures, multislice image simulation and tomographic reconstruction - and potentially experimental data analysis on data obtained at the Monash Centre for Electron Microscopy. The student may be involved in (but not directly running) sample preparation by focused ion beam milling and electron tomography experiments.

References

[1] M. Weyland, Electron Tomography of Catalysts, Topics in Catalysis volume 21, issue 4, 2002.

[2] M. Weyland et. al., Three-Dimensional Structural and Compositional Characterisation of Nanoscale Materials by Alternative Modes of Electron Tomography, Scripa Materialia 55 (1) 2006.

[3] S. Van Aert et. al., Three-dimensional atomic imaging of crystalline nanoparticles, Nature volume 470, issue 7334, 2011.

[4] James M. LeBeau, Scott D. Findlay, Leslie J. Allen, and Susanne Stemmer, Quantitative Atomic Resolution Scanning Transmission Electron Microscopy, Phys. Rev. Lett. volume 100, issue 20, 2008

Imaging Physics,
Condensed Matter Physics
Dr Matthew Weyland,
Dr Scott Findlay

Quantum engines in many-body systems

In recent years, many concepts belonging to thermodynamics have been translated to quantum systems. While the description of such systems is often highly abstract, this project will explore some of these ideas in the context of physically realisable many-body quantum systems.

Quantum Gases,
Condensed Matter Physics,
Theoretical & Computational Physics
Associate Professor Meera Parish,
Dr Jesper Levinsen

Quantum hearing: Using cold atoms to sense weak signals in noise

Hearing is amazing – your ears and brain can pick our your name dropped in conversation against a noisy background many times louder. Recorded sound needs hundreds of kilobits per second, yet your brain deals with the datastream in real time. What does this have to do with quantum mechanics? In this project, you will create a ‘quantum hearing cell’ made of ultracold atoms which senses audio-frequency magnetic fields with ‘hearing-like’ properties of frequency selectivity, and even with phase selectivity. The ultimate goal is to build a 'quantum cochlea', inspired by the mammalian auditory system - an array of quantum sensors each tuned to a different frequency which will sense complex signals efficiently and at the quantum noise limit.

Quantum Gases Dr Lincoln Turner,
Dr Russell Anderson

Quantum process tomography of an evolving spinor condensate

Quantum process tomography lets you work out what Hamiltonian a quantum system is evolving under, by feeding different states into the process and measuring what comes out. But what if you have only one quantum state, for example a Bose-Einstein condensate you don't want to destroy in a projective measurement? In a previous Honours project, Michael Kewming showed that quantum state tomography could be done on BECs - many times over without destroying the BEC. In this project you'll extend this to quantum process tomography, using many successive measurements on an evolving BEC to measure its mean-field spinor Hamiltonian.

Quantum Gases Dr Lincoln Turner,
Dr Russell Anderson

r-band as a star formation rate indicator

For this project, you will calibrate r-band as a star formation rate indicator, which can then be used to measure the star formation rates of galaxies across the whole sky. Galaxies grow in stellar mass via galaxy mergers and star formation, with the latter process dominating the growth of low mass galaxies. Accurate measurements of star formation rates are thus critical for understanding how galaxies grow over cosmic time. Unfortunately measuring star formation rates in low mass galaxies is challenging, with local dwarf galaxies often being undetected (or lacking data) in ultraviolet, mid-infrared and radio continuum bands. However, the H-alpha emission line is present in the optical r-band, and there will soon be r-band coverage of the whole sky from SDSS, Panstarrs and Skymapper. If one can effectively use i-band and z-band to subtract the stellar continuum, r-band photometry can be used to measure star formation rates for local galaxies across the entire sky.

References:

Brown et al., 2014, ApJS, 212, 18 (arXiv: 1312.3029).

Brown et al., 2017, ApJ, resubmitted (available on request)

Moustakas and Kennicutt., 2006, ApJS, 164, 81

Astronomy & Astrophysics Associate Professor Michael Brown

Rapid x-ray dark field imaging

X-ray imaging has become an essential tool in the medical field, but conventional absorption methods are still limited in their ability to differentiate between different types of soft tissue. Various methods of phase-contrast x-ray imaging enable visualisation of soft tissue by taking advantage of the changes in the direction of x-ray propagation [1].  In addition, a ‘dark field’ signal can be collected that maps scattering by structures that are too small to resolve directly, like the air sacs in the lungs [1].  These image modalities are already being applied in biomedical research [2].  However, many of these methods require multiple images to reconstruct the phase or dark field images, which is an obstacle when imaging breathing, moving subjects.  Therefore, we are developing methods that either work with only a single exposure [3] or that carefully time image capture to match sample/patient movement.

The honours project will involve looking at the information provided by this ‘dark field’ signal from these two methods, specifically when resolved over time. There are opportunities for computational modelling to elucidate the imaging process, experimental implementation on various x-ray sources and numerical analysis of the resulting images.  Depending on the preferences of the student, the project could also look at incorporating new algorithms into the image analysis code.

References

[1] Marco Endrizzi, “Phase contrast x-ray imaging”, Nuclear Inst. And Methods in Physics Research A (2017).

[2] PhysicsWorld –“Making the invisible visible” -

[3] Kaye Morgan, Timothy Petersen, Martin Donnelley, Nigel Farrow, David Parsons and David Paganin, “Capturing and visualizing transient x-ray wavefront topological features by single-grid phase imaging”, Opt. Express 24 (2016).

Imaging Physics Dr Kaye Morgan,
Dr Marcus Kitchen

Rolling the dice on X-ray scattering

Conventional X-ray tomography involves imaging an object at a series of angles. The 2d images are combined using a reconstruction algorithm to obtain 3d structural information about the object. The X-ray imaging interaction also generates inelastically scattered Compton photons which scatter at all angles, not just directly in line with the source and object. We are developing a tomographic imaging system that uses these usually ignored photons, which also carry information about the object’s structure, but intrinsically at all angles, enabling us to obtain 3d information potentially without moving the object. This would allow instantaneous 3d measurement of, say, lung air volume of a breathing animal, with much lower X-ray dose than required from approaches requiring multiple images from multiple angles.

To optimise the positioning of our detectors and optical elements, and to ensure that our 3d reconstruction software is capturing the important physics, this project involves the development of a flexible Monte Carlo simulation of the imaging system combining the geometry and interaction physics. The student undertaking this project will then perform computational experiments to compare various experimental parameters, or perhaps use a computational optimisation framework to obtain the optimal arrangement subject to the constraints.

This project will likely involve experiments at the Australian Synchrotron and in our X-ray lab.

Imaging Physics Dr Marcus Kitchen,
Dr Gary Ruben

Rotation in Massive stars with Varying Spin Direction

With LIGO and its cousins and successors, it has now become possible to detect the gravitational waves from merging close binary star systems. Indeed, the the detections and analyses have become so sophisticated that they place constrains on the rotation axes of the merging stars. What is a lot less well known, however, is how we may get there, in particular, how the rotation evolves inside these massive stars that make the black holes or neutron stars that merge.  Simple one-dimensional models do exist that follow the rotation from star formation (pre-main sequence) to stellar death (core collapse), even including magnetic fields [Ref 1], however, they all assume that the rotation axis of the star remains fixed and is the same for all layers.

This, however, may well be a crude oversimplification.

As stars form inside a turbulent molecular cloud, they may accrete material with varying angular momentum direction.  For low-mass star such as the sun, they will be fully convective in their infancy, likely aligning the entire spin.  For the sun we find that the orbital plane in which the planets move is misaligned with the spin of the sun by some 7 degrees, suggesting notable misaligned of the last accretion phase that made the planets with the average of the earlier accretion that made the sun.

For massive stars that die as neutron stars or black holes, accretion is still continuing for a major fraction of their lifetime, i.e.,  while the star is already evolving and is no longer fully convective.  With variation of angular momentum direction of accreted material continuing, the star may build up layers rotating about different axes from the core to the surface of the star.

The goal of this project is to understand the relative motion and shear between these layers.  This can include potential instabilities that lead to transport of angular momentum and composition.  The project can also involve understanding hydrodynamic fluid instabilities and implementing some of the ideas into a code, in collaboration with the supervisors, and performing simulations of the evolution of such stars until core collapse.  The results will be an original new contribution helping to shed new light on interpreting gravitational wave signals.

References

[1] Heger, A.; Woosley, S. E.; Spruit, H. C.: "Presupernova Evolution of Differentially Rotating Massive Stars Including Magnetic Fields", The Astrophysical Journal, Volume 626, Issue 1, pp. 350-363, 2005.

Astronomy & Astrophysics Professor Alexander Heger,
Dr Bernhard Mueller,
Dr Rosemary Mardling

Searching for Beyond Standard Model physics with the COMET detector

The Standard Model of Particle Physics has been incredibly successful at describing every fundamental particle interaction which we have measured over the last decades. Interactions which are either forbidden by the Standard Model, or extremely rare offer an opportunity to look for effects which result from new physics contributions not described by the Standard Model.

The COMET experiment currently being constructed in Japan will search for the Charged Lepton Flavour Violating (CLFV) decay of muons to electrons.  The Standard Model calculation for this decay predicts an extremely small rate while many Beyond Standard Model (BSM) physics contributions would potentially give much larger rates.  If evidence of this reaction is seen at COMET it would point to the existence of new physics previously undetected.

The project will involve working with Simulations of the COMET detector, and working to help prepare for initial running of the detector due to take place in the next few years. Any students who would like to take part should have some experience of programming, with experience of C++ being particularly useful.

If you would like to discuss this project please contact Jordan.Nash@monash.edu

Particle Physics Professor Jordan Nash

Searching for lepton number violating decays with data from the Large Hadron Collider

One of the fundamental assumptions in the Standard Model (SM) of particle physics is that lepton numbers for the individual families are conserved. So according to the SM, a decay that produce a pair of leptons will always produce e+e-, µ+µ- or τ+τ- but never e+τ- or other such combinations. However, the recent observation of hints for physics beyond the SM in decays of B mesons have given rise to speculation about new particles that would not conserve lepton number and there is thus a strong motivation to search for decays violating it.

In the project, you will analyse data from the LHCb experiment at the Large Hadron Collider. You will search for the decay Λb → pKµ+e- and Λb → pKe+µ- as an observation would be clear evidence for these new particles. The project will involve analysing data from the LHCb experiment located at the Large Hadron Collider. You will during the project analyse the data from the detector, learn about how measurements are made in a very complex system and get to use the latest software developments in Big Data analysis and artificial intelligence. The programming environment will be Python.

Particle Physics Professor Ulrik Egede

Shattering insights towards the atomic structure of glasses

The formation and atomic structure of glasses remains a long-standing important and unsolved problem in condensed matter physics. Despite continued international efforts, there are many intriguing scientific questions which remain unanswered. For example, how can a brittle solid maintain short range order yet still possess the structure of a liquid, supposedly devoid of long range order? What is the atomic structure of an archetypal vitrified monatomic solid, such as pure amorphous silicon or diamond like carbon? Do the atoms form continuous random networks or, as recently and hotly debated in Science [1, 2, 3], are there nano-scopic 'para-crystals' inter-dispersed within a structurally-frustrated meta-stable ensemble? For low density carbonaceous solids, do Fullerenes, such as interwoven, buckled, graphene sheets and nanotubes, provide an adequate description of the medium range order [4, 5]?

Using high-resolution transmission electron microscopes (TEMs), this project will acquire experimental data to address several of these fundamental questions. There are also opportunities to implement modelling techniques to interrogate the structure of such glasses, ranging from Monte Carlo integration techniques to the advanced processing of scanning electron nano-diffraction patterns [6]. Theoretically inclined students could also develop associated Hybrid Reverse Monte Carlo source code [7, 8], to increase the number of many-body inter-atomic potentials for describing bonding configurations in multi-component glasses.

Modern wonder materials like metallic glasses have interesting properties, such as substantial hardness, and can undergo super-plastic deformation [9]. The glass forming abilities of bulk metallic glasses are sensitively tied to composition and generally a multitude of elements are required to cause structural arrest in rapid thermal quenching from the liquid state. However, several binary and ternary alloys have been identified as good glass formers. Such metallic glasses are being synthesized by collaborators at CSIRO and are available for this project. Several pertinent questions concerning these alloys need to be addressed. For example, on the atomic scale, are these glasses partially crystallized? Can the pair correlation functions be reliably measured and how do these correlate with the alloy properties? Experimental students would be trained to prepare ultra-thin metallic glass foils and will analyze high quality experimental data from TEMs at the Monash Centre for Electron Microscopy.

References:

[1] "The local structure of amorphous silicon", M. M. J. Treacy and K. B. Borisenko, Science 335, 950 (2012).

[2] Comment on "The local structure of amorphous silicon", S. Roorda and L. J. Lewis, Science 338, 1539 (2012).

[3] Response to Comment on "The local structure of amorphous silicon", M. M. J. Treacy and K. B. Borisenko, Science 338, 1539(2012).

[4] "High-resolution transmission electron microscopy study of a cross-linked fullerene-related multilayer graphitic material", L. N. Bourgeois and L. A. Bursil, Phil. Mag. A, 79, 1155 (1999).

[5] "Curved-surface atomic modelling of nanoporous carbon", T. C. Petersen, I. K. Snook, I. Yarovsky, D. G. McCulloch, and B. O'Malley, J. Phys. Chem. C; 111, 802 (2007).

[6] "Systematic mapping of icosahedral short-range order in a melt-spun Zr36Cu64 metallic glass", A. C. Y. Liu, M. J. Neish, G. Stokol, G. A. Buckley, L. A. Smillie, M. D. de Jonge , R. T. Ott, M. J. Kramer and L. Bourgeois, Phys. Rev. Lett. 110, 1539 (2012).

[7] "Structural analysis of carbonaceous solids, using an adapted Reverse Monte Carlo algorithm", T. Petersen, I. Yarovsky, I. Snook, D. G. McCulloch, G. Opletal, Carbon, 41, 2403-2411 (2003).

[8] "HRMC_2.0: Hybrid Reverse Monte Carlo method with silicon, carbon and germanium potentials", G. Opletal, T. C. Petersen, I. K. Snook and S.P. Russo, Computer Physics Communications 184, 1946 (2013).

[9] "Superplastic deformation of Zr65Al10Ni10Cu15 metallic glass", Y. Kawamura, T. Shibata, A. Inoue and T. Masumoto, Script. Mat. 37, 431 (1997).

Condensed Matter Physics Dr Timothy Petersen

Spectroscopy and the composition of stars in globular clusters

Globular clusters are the oldest and most populous stellar aggregates in existence. Recent studies have shown that the stars in globular clusters show abundance patterns that are unique to the clusters. We do not know why they are not seen in the Galaxy, but only within the globular clusters. They may even be the remnants of collisions between dwarf Galaxies and our Milky Way. A fuller understanding requires us to determine the abundances of many stars in many clusters and to compare with theoretical models so we can see what stars produced the existing patterns.

We will source original data form the world's largest telescopes and then analyse this to determine the abundances of key species in globular cluster stars: perhaps Li, C, N, O, Mg, Al, Fe as well as the heavy elements made by neutron capture, such as Sr, Y, Zr, Ba, La. Stellar models that can produce these species will be compared with the abundances we measure.

This project will involve travel to the Australian Astronomical Observatory (in Sydney) to visit and work with Dr de Silva. There is also the opportunity to visit the 4m Anglo-Australian Telescope in Coonabarabran, NSW.

Astronomy & Astrophysics Professor John Lattanzio,
Dr Simon Campbell

Split-Field Spectral Computed Tomography

X-ray computed tomography (CT) is a method to non-destructively image the internal structures of a three dimensional object. However, conventional CT suffers from the inability to distinguish different materials. It has been shown that this can be overcome by combining data sets taken at two or more different X-ray energies, which is often called dual-energy, or spectral CT, respectively [1]. Common implementations of dual-energy CT make use of energy resolving detectors, or a change of the X-ray illumination between two acquisitions [2, 3].

The goal of this project is to design and implement a method to perform dual-energy CT that can be readily implemented into existing imaging systems. We plan to combine X-ray filters and exploit intrinsic symmetries present in X-ray imaging to simultaneously acquire CT data at different X-ray energies. This would circumvent the need for an energy resolving detector or multiple sequential acquisitions and their respective drawbacks.

Experiments will be conducted both at the Australian Synchrotron, and Monash University. Data analysis includes image processing, tomographic reconstruction, and implementation of algorithms for elemental separation. Depending on the progress, the possibility to work towards combining these methods with phase-contrast X-ray imaging exists.

References

[1] Robert E. Alvarez and Albert Macovski , “Energy-selective reconstructions in X-ray computerised tomography”, Phys. Med. Biol., volume 21, 733 (1976)

[2] M. Paziresh, A. M. Kingston, S. J. Latham, W. K. Fullagar, and G. M. Myers, “Tomography of atomic number and density of materials using dual-energy imaging and the Alvarez and Macovski attenuation model”, Journal of Applied Physics, volume 119, 214901 (2016)

[3] S. Ehn, T. Sellerer, K. Mechlem, A. Fehringer, M. Epple, J. Herzen, F. Pfeiffer and P. B. Noël, “Basis material decomposition in spectral CT using a semi-empirical, polychromatic adaption of the Beer-Lambert model”, Phys. Med. Biol., volume 62, N1–N17 (2017)

Imaging Physics Dr Florian Schaff,
Dr Kaye Morgan,
Dr Marcus Kitchen

Statistical mechanics of random Graphs and complex networks

Many systems in nature can be described in terms of complex networks. Non-trivial examples include genetic networks, ecological networks and spin networks. This project will apply the principles of statistical mechanics to explore the organising principles and dynamics of random graphs and complex networks. Field theoretic techniques will be used to investigate an ensemble of random graphs, including networks that exhibit a Bose-Einstein phase transition.

References:

[1] R. Albert, A-L Barabasi, "Statistical mechanics of complex networks", Reviews of Modern Physics, Volume 74, Issue 47 (2002).

Theoretical & Computational Physics Professor Michael Morgan

Stellar population synthesis

Stars are born in clusters. These are very valuable tools for learning about stellar processes, because they provide us with a large number of stars, all born at the same time, but with different masses. Hence we can do statistical studies to understand what is happening to the stars. However, detailed stellar models take a lot of computer time. Yet it is possible to make some approximations that are very accurate, informed by the results of detailed models, which enable us to produce a statistical model of a cluster of stars. This is called "Population Synthesis". One application of this is to investigate the evolution of red-giants that become carbon stars. Does the predicted distribution match what we see in real clusters?

Skills required: Some interest in programming is needed. That could be fortran or another high level language, or even MATLAB or something similar. Some astronomy would be an advantage, especially second year units. But is not essential.

Astronomy & Astrophysics Professor John Lattanzio,
Dr Simon Campbell

Study of Flavour Anomalies

An important component of particle physics research is the study of rare processes in 'flavour physics'. In current experiments this typically refers to decays of bottom or charm mesons that have very small branching fractions. Recent experimental studies of these decays have produced a number of 'anomalies', or disagreements with the standard model expectations.

In this project we will study these anomalies from the theoretical perspective, looking for common threads that can lead to global explanations in terms of new physics. When good explanations are found, they also produce predictions for upcoming experiments that can rule the out (or, with much luck, confirm them).

Particle Physics Professor German Valencia

Strings in Quantum Chromodynamics

In this project, we will consider the physics of “hadronisation” - the process by which high-energy quarks and gluons turn into hadrons. This is intimately related to a fundamentally unsolved question in theoretical physics - the problem of confinement. (There is even a $1M prize for finding a solution to it.) Today, the most successful and widely applied physical model of the process of hadronisation is based on strings. Essentially, a high-mass string, with a single quark and antiquark as its endpoints, breaks into a series of smaller string pieces - hadrons - via a series of tunnelling events. The model makes several intrinsic assumptions however, which are now beginning to be challenged by new data from the Large Hadron Collider. Motivated by this data, we will consider simple alternative formulations of the string model, and compare these with data. This project is well suited for students with an interest in theoretical physics (note, however, that the strings we consider are not superstrings; there is no connection between this project and quantum gravity), computer physics (the string model is implemented as a C++ code), and physics at the Large Hadron Collider.

Particle Physics Associate Professor Peter Skands

Supernova Explosions and General Relativity

Core-collapse supernovae mark the death of massive stars: In an evolved star that has gone through the various burning stages, its iron core eventually collapses, leaving behind a neutron star, while the envelope is expelled in a violent and very bright explosion [1]. These explosions are fascinating laboratories for matter under extreme conditions and cosmic furnaces that make many of the chemical elements that are the building block of planets like ours. Supernovae are also promising sources of neutrinos and gravitational waves.

As the engine powering the explosion is hidden deep inside the stellar core, we need numerical simulations to understand the supernova mechanism. Simulation codes need to include include a variety of physical ingredients such as multi-dimensional fluid flow [2], general relativistic effects, and radiation transport.

In this project, we will explore the role of general relativistic effects in a more controlled manner than hitherto possible. Previous comparisons of relativistic and Newtonian models [3,4] have been plagued by differences in grid resolution and numerical methods, whereas this project will use a single code to eliminate these confounding factors. The project will entail one- and two-dimensional simulations, some of whom will be performed on national supercomputers.

References

[1] Janka 2012, Annual Review of Nuclear and Particle Science 62, 407

[2] Mueller 2016, Publications of the Astronomical Society of Australia 33, e48

[3] Mueller, Janka & Marek 2012, The Astrophysical Journal 756, 84

[4] Liebendoerfer et al. 2005, The Astrophysical Journal 620, 840

Astronomy & Astrophysics Dr Bernhard Mueller,
Professor Alexander Heger

Supernovae Making Neutron Stars or Black Holes?

When a massive star reaches the end if its life, the core collapses into a neutron star or, possibly, a black hole. In many cases, at first a shock is launched moving outward, ejecting the outer layers of the star. But there may not be enough energy to eject the entire core, or there can be hydrodynamic interactions in the envelope that push some of the matter onto the central object. How much of the material falls back will determine the final mass of the compact remnant that is left behind. If the mass exceeds the maximum mass for a neutron star, it will collapse to a black hole.

For this project you will use an analytic model for supernova explosions and their energies to simulate the explosion of these stars. You will then use a one-dimensional hydrodynamic code modified for proper inner boundary conditions, to simulate the dynamics of the explosion and how much mass is ejected or fall back. This will allow you to estimate the remnant mass (some of the rest mass is carried away by neutrinos). Using a range of supernova progenitor models, you can make perditions about the distribution of neutron star and black hole masses.

References

[1] W. Zhang, S. E. Woosley, and A. Heger, "Fallback and Black Hole Production in Massive Stars ", The Astrophysical Journal, Volume 679, Number 1 (2008)

[2] H-T. Janka, "Explosion Mechanisms of Core-Collapse Supernovae ", Annual Review of Nuclear and Particle Science, Volume 62, pages 407-451 (2012).

[3] O. Pejcha and T. A. Thompson, "The landscape of the neutrino mechanism of core-collapse supernovae: neutron star and black hole mass functions, explosion energies, and nickel yields ", The Astrophysical Journal, Volume 801, Number 2 (2015).

[4] S. E. Woosley, A. Heger, and T. A. Weaver, "The evolution and explosion of massive stars ", Reviews of Modern Physics, Volume 74, 1015 (2002).

[5] A. Heger, C. L. Fryer, S. E. Woosley, N. Langer, and D. H. Hartmann, "How Massive Single Stars End Their Life ", The Astrophysical Journal, Volume 591, Number 1 (2003).

Astronomy & Astrophysics Professor Alexander Heger

Supersymmetric dark matter

When interpreted within the standard (ΛCDM) cosmological framework astrophysical observations indicate that some 85 percent of the matter in the Universe is non-luminous (dark). Supersymmetry offers a natural explanation for this dark matter in the form of the lightest supersymmetric particle.

While the minimal supersymmetric extension of the standard particle model is constrained into a fine-tuned theoretical region by experiments, extending the Higgs (electroweak symmetry breaking) sector of these models can remove this problem.

This project examines whether we can obtain an amount of dark matter consistent with observations within a particular realization of the next-to-minimal supersymmetric standard model while staying in the natural part of the parameter space and remaining consistent with various collider, astrophysical and low energy measurements.

References

[1] H. Baer, C. Balazs, "χ2 analysis of the minimal supergravity model including WMAP, g(mu)-2 and b → gamma constraints", arXiv:hep-ph/0303114

[2] C. Balazs, M. S. Carena, A. Menon, D.E. Morrissey, C. E. M. Wagner, "The Supersymmetric origin of matter", Physical Review D, Volume 71, 075002 (2005).

[3] C. Balazs, D. Carter, "Discovery potential of the next-to-minimal supergravity motivated model", Physical Review D, Volume 78, 055001 (2008).

[4] C. Balazs, D. Carter, "Likelihood analysis of the next-to-minimal supergravity motivated model", AIP Conference Proceedings (2009).

Particle Physics Professor Csaba Balazs

Supersymmetric origin of matter

Supersymmetry has the potential to explain the origin of all, baryonic (visible) and non-luminous (dark), matter in the Universe. While dark matter may be the lightest supersymmetric particle, a baryon-antibaryon asymmetry can be generated by electroweak baryogenesis in the (next-to-)minimal supersymmetric extension of the standard particle model.

Electroweak baryogenesis is typically driven by charged superpartners of the standard gauge bosons (gauginos). While in the minimal models the matter content of the Universe is successfully reproduced, these scenarios face stringent constraints from experiments measuring the electric dipole moments of electrons.

In this project, we explore the possibility that electroweak baryogenesis can be driven by neutral gauginos, gauge singlet scalars or by non-standard gauge bosons. In such models the electric dipole moment constraints would not apply, but the question whether the dark matter content is consistent with measurements remains to be examined.

References:

[1] C. Balazs, M. S. Carena, A. Menon, D.E. Morrissey, C.E.M. Wagner, The Supersymmetric origin of matter, arXiv:hep-ph/0412264.

[2] C. Balazs, M. S. Carena, A. Freitas, C.E.M. Wagner, Phenomenology of the nMSSM from colliders to cosmology, arXiv:0705.0431.

[3] Y. Li, S. Profumo, M. Ramsey-Musolf, Bino-driven Electroweak Baryogenesis with highly suppressed Electric Dipole Moments, arXiv:0811.


Particle Physics,
Theoretical & Computational Physics
Professor Csaba Balazs

The aftermath of merging neutron stars

When two neutron stars collide they emit large quantities of energetic electromagnetic radiation and gravitational waves. To date, we have observed the gamma-ray and X-ray emission of these catastrophic collision, but have not yet observed their gravitational wave signal. Regardless, what happens following the merger is heavily debated, and depends on complicated details of the way matter behaves in these hot, dense environments. Whether the post-merger remnant is a black hole, an unstable neutron star that eventually collapses into a black hole, or an eternally stable neutron star, is uncertain. In this project, we will use gamma- and X-ray observations, as well as state-of-the-art neutron star models to understand electromagnetic and gravitational-wave observations that are the smoking gun for determining whether a black hole or a neutron star is born during these mergers.

Astronomy & Astrophysics Dr Paul Lasky

The ages of the star clusters

Stars are largely born in clusters. They are mostly born at the same time - the spread is usually very small, certainly small compared to the lifetime of a star. Hence a star cluster represents a collection of stars of the same age but different masses. When we observe such a cluster in the HR diagram we are seeing a superposition of many evolutionary points for lots of different masses but at the same age. When we calculate stellar evolution we select a mass and calculate how the star ages. To compare with a cluster we need to interpolate within the individual tracks to find how they would all look at the same age. Such a line in the HR diagram is called an "isochrone". In this project you will devise a good way to interpolate within existing evolutionary tracks to determine how cluster of different ages will look. If time permits we can compare with some real clusters and get estimates for the age of the cluster.

Astronomy & Astrophysics Professor John Lattanzio,
Associate Professor Amanda Karakas

The biomagnetic microscope: Non-invasive imaging of neurocurrents with ultracold atoms

This project will bring together physicists, physiologists and engineers to pioneer a new method of imaging microscale biological functions from their emitted magnetic fields. The ultimate goal is real-time mapping of neurocurrent networks without invasive electrodes or sectioning.

Quantum sensors based on cold atom sensors are capable of fast, sensitive and high resolution measurements of magnetic fields, even in unshielded environments. These sensors may displace the 1970s technology of SQUIDs used in magnetoencephalography but being much smaller and free of cryogens, opening the prospect of micron scale magnetic imaging in vivo. We will utilise the ultimate source of ultracold atoms, a Bose-Einstein condensate, as an ultrasensitive magnetic field sensor to make the first measurements of functional magnetic fields emanating from life on the cellular scale.

Livings things are warm, wet, require oxygen and emit miniscule magnetic fields. Bose-Einstein condensates sense tiny fields but are extremely cold, and require ultrahigh vacuum. A major challenge of this Honours project is the development of the key technology required to bring condensates within microns of living cells: a bio-quantum interface.

Imaging Physics,
Quantum Gases
Professor Kris Helmerson,
Dr Lincoln Turner,
Dr Russell Anderson,
Dr Tuncay Alan (Engineering),
Professor David Spanswick (Physiology)

The end of nucleosynthesis: The case of very metal-rich stars

Low and intermediate-mass stars evolve through core hydrogen and helium burning before ascending the asymptotic giant branch phase. It is during this phase that the richest nucleosynthesis occurs deep in the stellar interior, the products of which may be mixed to the stellar envelope. Insights from theoretical models tell us that the mixing in very metal-rich stars is less than in models with the same metallicity as the sun. Does this mean that less material exposed to nuclear reactions is mixed to the surface? The goal of this project is to model the nucleosynthesis of very metal-rich stars in order to: 1) quantify how the surface composition varies as a function of mass and metallicity, 2) calculate the stellar yields, which are the integrated amount of an element (e.g., carbon) expelled into the interstellar medium by the star. Using the stellar yields, you will be able to determine if stellar nucleosynthesis comes to an end in very metal-rich stellar populations!

Astronomy & Astrophysics Associate Professor Amanda Karakas

The Fate of the Biggest Stars

One of the biggest puzzles in understanding the formation and structure of Galaxies are the huge black holes in their centres. Some of them have a billion time the mass of the sun, even when they are only a tenth of their percent age. One, highly speculative, theory is that they may start as the collapse of supermassive stars of maybe a million times the mass of the sun, from the first, or very early, generation of stars that precede the first galaxies ("pre-galactic stars"). Whereas supermassive stars of primordial composition either undergo hydrostatic burning or collapse to black hole, stars that have some enrichment in material from a previous generation of stars may instead explode, probably the most powerful explosions in the universe other than the big bang itself. But where exactly are the boundaries between explosion, collapse, and hydrostatic burning?

The goal of this project is to find the boundaries between hydrostatic burning, thermonuclear explosion, and collapse to a black hole for supermassive stars, i.e., stars of some 100,000 times the mass of the sun. You will use a hydrodynamic stellar evolution code that includes thermonuclear burning and post-Newtonian corrections for general relativity for non-rotating stars. The simulations will start with stars of different initial mass and different initial composition and will follow the early evolution of supermassive stars until they either collapse, explode, or reach hydrostatic burning. A possible extension of the project is to modify the stellar evolution code to include post-Newtonian corrections for rotating stars.

References

[1] P. J. Montero, H-T. Janka, E. Muller and B. Muller, "Influence of thermonuclear effects on the collapse of supermassive stars", Journal of Physics: Conference Series, Volume 314, conference 1.

[2] P. J. Montero, H-T. Janka and E. Muller, "Relativistic collapse and explosion of rotating supermassive stars with thermonuclear effects", The Astrophysical Journal, Volume 749, Number 1 (2012).

[3] Z. Haiman and A. Loeb, "What Is the Highest Plausible Redshift of Luminous Quasars?", The Astrophysical Journal, Volume 552, Number 2 (2001).

[4] G. M. Fuller, S. E. Woosley, and T. A. Weaver, "The evolution of radiation-dominated stars. I - Nonrotating supermassive stars", The Astrophysical Journal, Volume 307, p. 675-686 (1986).

Astronomy & Astrophysics Professor Alexander Heger,
Anthony Lun

The Formation of Supermassive Black Holes

One of the biggest puzzles in understanding the formation and structure of Galaxies are the huge black holes in their centres. Some of them have a billion time the mass of the sun, even when they are only a tenth of their percent age. One, highly speculative, theory is that they may start as the collapse of supermassive stars of maybe a million times the mass of the sun, from the first, or very early, generation of stars that precede the first galaxies ("pre-galactic stars"). Whereas supermassive stars of primordial composition either undergo hydrostatic burning or collapse to black hole, stars that have some enrichment in material from a previous generation of stars may instead explode, probably the most powerful explosions in the universe other than the big bang itself. But where exactly are the boundaries between explosion, collapse, and hydrostatic burning?

The goal of this project is to find how such stars with primordial composition and high accretion rates evolve and approach the point of collapse to a supermassive black hole, as a function of this accretion rate. And, in particular, what the mass of the star is by the time it collapses, i.e., what is the mass of the black holes formed. For example, is there an upper mass limit, and is this different from the one obtained for stars with a given fixed initial mass (see other project).

References

[1] P. J. Montero, H-T. Janka, E. Muller and B. Muller, "Influence of thermonuclear effects on the collapse of supermassive stars", Journal of Physics: Conference Series, Volume 314, conference 1.

[2] Z. Haiman and A. Loeb, "What Is the Highest Plausible Redshift of Luminous Quasars?", The Astrophysical Journal, Volume 552, Number 2 (2001).

[3] G. M. Fuller, S. E. Woosley, and T. A. Weaver, "The evolution of radiation-dominated stars. I - Nonrotating supermassive stars", The Astrophysical Journal, Volume 307, p. 675-686 (1986).

[4] D. J. Whalen, J. L. Johnson, J. Smidt, A. Heger, W. Even, and C. L. Fryer, "The biggest explosions in the universe. II.", The Astrophysical Journal, Volume 777, Number 2 (2013).

Astronomy & Astrophysics Professor Alexander Heger,
Anthony Lun

The holographic Universe

Astrophysical observations indicate that about 70 percent of the Universe is made up by a substance with negative pressure, popularly referred to as "dark energy". The only known substance with negative pressure consists of quantum fluctuations. It is also known that the Universe contains a substantial amount of quantum fluctuations. However, quantum field theory cannot predict the energy density of quantum fluctuations. Thus the measured energy density of the Universe is a complete mystery.

The holographic principle restricts the energy density of any gravitating systems. In this project, we study how the holographic constraint can be imposed on a quantum system. Since the root of the problem is the infinite number of degrees of freedom in a quantum field, we investigate how to limit the number of degrees of freedom in a quantum field in a holographic manner.

References

[1] P. Horava, Quantum Gravity at a Lifshitz Point, Phys. Rev. D volume 79, 084008, 2009

[2] C. Charmousis, G. Niz, A. Padilla, P. M. Saffin, Strong coupling in Horava gravity, Journal of High Energy Physics, Volume 2009

[3] M. Li, Y. Pang, A Trouble with Horava-Lifshitz Gravity, Journal of High Energy Physics, Volume 2009

[4] E. Verlinde, On the Origin of Gravity and the Laws of Newton, Journal of High Energy Physics, Volume 2011

[5] T. Banks, TASI Lectures on Holographic Space-Time, SUSY and Gravitational Effective Field Theory, arXiv:1007.4001

[6] R. Bousso, TASI Lectures on the Cosmological Constant, arXiv:0708.4231

[7] J. Martin, Everything You Always Wanted To Know About The Cosmological Constant Problem (But Were Afraid To Ask), arXiv:1205.3365

Particle Physics,
Astronomy & Astrophysics
Professor Csaba Balazs

The influence of the accretion disk on thermonuclear burning in neutron stars

Accreting neutron stars appear in two main “spectral states” in terms of their persistent X-ray emission, that have a profound effect on their behaviour. The “low” state generally has a lower intensity, and a “hard” spectrum (with a greater fraction of higher-energy X-rays). The “high” state has instead a “soft” spectrum with a smaller fraction of high-energy X-ray photons. These spectral states are associated with different geometry of the accretion disk; the “hard” state, with a disk truncated outside the neutron star surface, and the “soft” state with a disk that comes in to meet the stellar surface (e.g. Done et al. 2007). Markedly different behaviour of thermonuclear bursts are observed in the different states, with regular, consistent bursts seen in the hard state, and irregular bursts in the soft. This project would use long-duration X-ray measurements of intensity of many neutron stars to identify transitions between these states, to seek to better understand how frequently they occur, and what conditions may trigger them. Analysis of the thermonuclear bursts observed in different states will seek to understand fundamentally how the accretion geometry affects the burning processes on the neutron star surface.

References

Done et al. 2007, “Modelling the behaviour of accretion flows in X-ray binaries. Everything you always wanted to know about accretion but were afraid to ask”, http://adsabs.harvard.edu/abs/2007A%26ARv..15....1D

Kajava et al. 2014, “The influence of accretion geometry on the spectral evolution during thermonuclear (type I) X-ray bursts”, http://adsabs.harvard.edu/abs/2014MNRAS.445.4218K
Astronomy & Astrophysics Associate Professor Duncan Galloway

The magnetorotational instability in warped accretion discs

Rotating magnetised flows are unstable to an instability known as the Magnetorotational instability (MRI). This is believed to give rise to turbulence and hence angular momentum transport in most astrophysical accretion discs, solving the problem of how material accretes onto protostars, black holes, neutron stars and white dwarfs. The aim of this project will be to carry out 3D modelling of the MRI in action, with the ultimate aim of understanding warped and misaligned flows near black holes and binary stars.

Astronomy & Astrophysics Associate Professor Daniel Price

The origin of inertia

While Isaac Newton and Albert Einstein created remarkable theories explaining the dynamics of gravity, they both omitted to clarify the meaning of mass in their equations. Today, in the context of the standard particle model, we understand inertial mass as a consequence of an interaction with a hypothetical Higgs field filling space and "dragging" on matter similarly to viscose fluid.

In 2012 the LHC discovered a new bosonic state and over 2013 the experimentalists established that its properties are consistent with those of a Higgs boson. (It is remained to be seen whether the particle is composite or an elementary Higgs without substructure.)

The next step is to find out if it is the standard Higgs boson. This step will require the detailed measurements of the properties of the particle which in turn will need lot more data. Fortunately the number of LHC collisions still increases exponentially.

The long-term question: is there anything else to discover at the LHC? The Higgs might hold part of the answer because if it's a non-standard Higgs than it's almost certain that there's more to come. Supersymmetry, extra dimensions, strings? In this project we investigate some of these possibilities.

References:

[1] C. Balazs, E. L. Berger, P. M. Nadolsky, C.-P. Yuan, Calculation of prompt diphoton production cross-sections at Tevatron and LHC energies, arXiv:0704.0001.

[2] P. M. Nadolsky, C. Balazs, E. L. Berger, C.-P. Yuan, arXiv:hep-ph/0702003.

[3] C. Balazs, E. L. Berger, P. M. Nadolsky, C.-P. Yuan, All-orders resummation for diphoton production at hadron colliders, arXiv:hep-ph/0603037.

[4] C. Balazs, C.P. Yuan, Higgs boson production at the LHC with soft gluon effects, arXiv:hep-ph/0001103.

[5] C. Balazs, P. M. Nadolsky, C. Schmidt, C.P. Yuan, Diphoton background to Higgs boson production at the LHC with soft gluon effects, arXiv:hep-ph/9905551.

[6] C. Balazs, C.P. Yuan, Higgs boson production at hadron colliders with soft gluon effects: Backgrounds, arXiv:hep-ph/9810319.


Particle Physics,
Theoretical & Computational Physics
Professor Csaba Balazs

The size-luminosity relation of galaxies

The size-luminosity relation of galaxies is both a constraint on galaxy formation models and a tool useful for the analysis of observations of galaxies. For example, while in-situ star formation can increase a galaxy’s mass without increasing it’s diameter, mergers of galaxies tend to increase galaxy diameters. Modelling the size-luminosity relation is also important for modelling measurements of galaxy photometry, which often don’t directly measure all the light from galaxies. While Shen et al. (2003) is the benchmark study of the size-luminosity relation, that study also has several flaws that hinder its utility. For example, the quality of the imaging data used and the simplified models used to fit that data can produce systematic errors. You will use the GAMA survey to produce a revised benchmark for the galaxy size-luminosity relation. You will also undertake some basic tests of this relation, including whether galaxies with enhanced star formation fall below the size-luminosity relation for all galaxies.

References:

[1] A. M. Brooks, A. R. Solomon, F. Governato, J. McCleary, L. A. MacArthur, C. B. A. Brook, P. Jonsson, T. R. Quinn, and J. Wadsley, "Interpreting the evolution of the size-luminosity relation for disk galaxies from redshift 1 to the present", The Astrophysical Journal, volume 728, 51 (2011).

[2] Aaron A. Dutton, Frank C. van den Bosch, Sandra M. Faber, Luc Simard, Susan A. Kassin, David C. Koo, Kevin Bundy, Jiasheng Huang, Benjamin J. Weiner, Michael C. Cooper, Jeffrey A. Newman, Mark Mozena and Anton M. Koekemoer, "On the evolution of the velocity–mass–size relations of disc-dominated galaxies over the past 10 billion years", Monthly Notices of the Royal Atronomical Society, volume 410, 1660 (2011).

[3] Lee S. Kelvin, Simon P. Driver, Aaron S. G. Robotham, David T. Hill, Mehmet Alpaslan, Ivan K. Baldry, Steven P. Bamford, Joss Bland-Hawthorn, Sarah Brough, Alister W. Graham, Boris Häussler, Andrew M. Hopkins, Jochen Liske, Jon Loveday, Peder Norberg, Steven Phillipps, Cristina C. Popescu, Matthew Prescott, Edward N. Taylor and Richard J. Tuffs, "Galaxy And Mass Assembly (GAMA): Structural Investigation of Galaxies via Model Analysis", Monthly Notices of the Royal Atronomical Society, volume 421, 1007 (2012).

[4] Shiyin Shen, H. J. Mo, Simon D. M. White, Michael R. Blanton, Guinevere Kauffmann, Wolfgang Voges, J. Brinkmann and Istvan Csabai, "The size distribution of galaxies in the Sloan Digital Sky Survey", Monthly Notices of the Royal Atronomical Society, volume 343, 978 (2003).

Astronomy & Astrophysics Associate Professor Michael Brown

The slow neutron capture process from proton-ingesting episodes and the chemical composition of post-asymptotic giant branch (AGB) stars

The abundances of the elements heavier than iron in post-AGB stars cannot be explained by current models of slow neutron captures in AGB stars (see references below). This project investigates the hypothesis that these abundances are the signature of proton-ingestion episodes driven by overshoot in He-burning convective regions. It involves computing stellar structure models of stars of 1.3 solar masses and metal content ~ solar/10 (to match the properties of the observed post-AGB stars) including a parametrized proton-ingestion episode to derive detailed predictions for the abundances of the elements from carbon to lead. Comparison of the results with the post-AGB star observations will allow us to understand if this is a viable hypothesis, generating new knowledge on the slow neutron capture process and on mixing in stars.

References:

[1] de Smedt et al.. 2012, Astronomy & Astrophysics, Volume 541, id.A67.

[2] van Aarle et al.. 2013, Astronomy & Astrophysics, Volume 554, id.A106.

Astronomy & Astrophysics Dr Simon Campbell

The stars that shouldn’t exist

In the hope of improving our understanding of nucleosynthesis (and other fields of astrophysics), astronomers have acquired spectra for millions of stars in our galaxy. The spectrum of a single star can tell us what it is made of: the abundances of dozens of different chemical abundances can be precisely measured. Having spectra for millions of stars is great, but in reality, we always learn the most physics from the rarest stars, the ones that have the most peculiar chemical abundances which cannot be explained by (our current understanding of) physics.

In this project you will find rare stars that are either enhanced, or depleted, in particular chemical abundances. Some of these kinds of stars cannot be explained by standard models of stellar evolution and nucleosynthesis, and they're so rare (~0.01%) that very little progress has been made in understanding them. Put simply, more need to be discovered before strong conclusions can be drawn.

During this project you will chose a particular kind of unusual chemical pattern to look for (and a corresponding longstanding problem in astrophysics to solve), construct perhaps the largest collection of those stars in the galaxy, combine your sample with existing literature sources, and provide interpretation to explain the origin of those stars.

More details about the project, including literature references, are available at http://astrowizici.st/honours18/

Astronomy & Astrophysics Dr Andrew Casey

The ultra-relativistic particle in a box: electrons in atomically thin Na3Bi

Three-dimensional Dirac semi-metals such as Na3Bi are a new class of material where electrons behave as relativistic Dirac-like fermions, moving at constant velocity independent of energy, much like massless neutrinos. In the Fuhrer laboratory we utilize a low-temperature (4K) scanning tunnelling microscope (STM) equipped with a molecular beam epitaxy chamber to study Na3Bi grown under ultra-high vacuum conditions. The primary goal of this project will be to investigate the role of quantum confinement on the electronic structure of Na3Bi by growing films of just a few atomic layers (see image below). The confinement of electrons in such thin films gives them a mass, and opens an energy gap between electrons and holes, creating an insulator which may be conventional or topological. We will use scanning tunnelling microscopy and spectroscopy to search for signs of topological insulator behaviour in ultra-thin Na3Bi.

Condensed Matter Physics Professor Michael Fuhrer,
Dr Mark Edmonds

Thermonuclear X-ray bursts with ESA’s INTEGRAL’s satellite

Thermonuclear bursts arise from nuclear burning of accumulated fuel on the surface of accreting neutron stars. The JEM-X camera onboard ESA’s INTEGRAL satellite (http://sci.esa.int/integral), launched in 2002, has already accumulated a substantial share of the total sample of thermonuclear bursts ever observed. This sample continues to grow, with new examples observed regularly. This project will involve working with Monash researchers and the instrument team to identify new examples of bursts in newly-available data, and perform standard analysis, including time-resolved spectroscopy. Analyses of newly-discovered sources will provide the first opportunity to compare observations to numerical models and hence constrain the system parameters. The resulting analysis products will be included in the Multi-INstrument Burst ARchive (MINBAR), the largest such sample to date.

References

Winkler et al., 2003, “The INTEGRAL mission”, http://adsabs.harvard.edu/abs/2003A%26A...411L...1W

Chenevez et al. 2011, “Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721”, http://adsabs.harvard.edu/abs/2011MNRAS.410..179C
Astronomy & Astrophysics Associate Professor Duncan Galloway

Tidal disruption of stars near supermassive black holes

We will attempt to simulate the tidal disruption of stars near supermassive black holes, considering the role of various orbital effects in circularising the debris stream and forming an accretion disc.  We will also consider the subsequent evolution of stars that have been tidally disrupted, and the role of relativistic effects.

References

Bonnerot, Rossi, Lodato, Price, http://adsabs.harvard.edu/abs/2016MNRAS.455.2253B ; Mandel & Levin, http://adsabs.harvard.edu/abs/2015ApJ...805L...4M

Astronomy & Astrophysics Professor Ilya Mandel,
Associate Professor Daniel Price

Towards the spacetime limit of magnetometry

Everything has a limit, but what is the precision limit of a magnetic field measurement? Quantum limits of things that are quantized – photons, atoms – are well understood, but what is the quantum limit for measuring a static magnetic field? You know how to calculate the energy in a magnetic field in a volume of space. You've also seen that there is an energy-time uncertainty principle that restricts how well we can measure this energy in a given duration. We think we can do better than this measurement in the lab – but can we? Do we measure magnetostatic vacuum noise? Does our magnetic sensor start measuring itself? In this project you'll explore the fundamental limits of magnetometry experimentally through approaching this limit in the lab. A stretch goal for this project is using two small BECs to detect the magnetic field dipole created by a nearby larger BEC. This would be the first magnetic detection of a quantum gas.

Quantum Gases Dr Russell Anderson,
Dr Lincoln Turner

Towards topological electronics via assembly of 2D atomic legos

Topological insulators are a class of materials which are electronically insulating in the bulk and highly conducting at the surface. These special surface states provide unique access to highly mobile spin-polarised electrons, which makes these materials a desirable component for future electronics.

This project aims to create an ultraclean, protected environment for the surface states of the layered topological insulating Bi2(Se, Te)3 by encapsulating them with layers of clean insulating hexagonal boron nitride. These heterostructures will be created by mechanical pick and place method which exploits the strong van der Waals interaction between atomically smooth surfaces, allowing the creation of new layered materials from few-atom thick building blocks. The robustness and performance of the topological surface states will be electronically tested by electrical transport measurements down to ultracold temperatures and high magnetic field.

Condensed Matter Physics Professor Michael Fuhrer,
Dr Semonti Bhattacharyya

Ultrafast dynamics of quantum matter

Understanding the response of quantum matter to changes in the system parameters is crucial in developing new technologies. Recent advances in ultracold atomic gases have enabled the systematic study of the fastest collective response possible in any quantum system (relative to system density). Specifically, the low particle density and large atom mass compared with electrons in solids means that the relevant time scale for quantum dynamics is slowed down from attoseconds to microseconds, by more than 10 orders of magnitude. At the same time, ultracold gases feature unprecedented control of interactions, atom spins, and geometry, allowing the study of a wealth of possible out-of-equilibrium scenarios.

This theoretical project aims to predict the collective response of quantum matter to interferometric techniques originally envisioned for two-level systems using newly developed tools. It will explore which aspects of dynamics stem from the behaviour of few particles, and which are unique to many particle systems.

Quantum Gases,
Theoretical & Computational Physics
Associate Professor Meera Parish,
Dr Jesper Levinsen

Understanding Mixing Processes in Massive Stars with the Help of Supernova Explosions

The interiors of massive stars are largely hidden from our view. This is precarious as there are still many uncertainties concerning the burning and mixing processes in stellar interiors, and we cannot be certain that current stellar evolution codes correctly predict the core structure of massive stars before their explosion as a supernova. What is particularly uncertain is how convective flow and its interaction with shell boundaries affects the onion shell structure of massive stars [1,2].

The supernova explosion itself offers a prime opportunity to probe the shell structure of massive stars. As the explosion proceeds, the photosphere moves deeper and deeper into the star, so that the composition of the inner shells can eventually be determined by nebular spectroscopy [3]. Interpreting the nebular emission lines is not trivial, however. The emission lines of abundant elements (O, Si, Ca, Fe, Ni) depend not only on the thermal structure of the exploding star, they can also be strongly affected by mixing processes because certain atoms and molecules are much more efficient coolants than others. For example, tiny amounts of Ca can strongly suppress O line emission.

In this project, we will investigate whether the mixing across shell boundaries in massive stars can be constrained by supernova spectroscopy. We will estimate the mixing between some of the interior shells by turbulent entrainment for a number of stellar evolution models based on empirical scaling laws found in multi-D simulations of convective burning. We will revisit observed nebular spectra [3] to determine whether they can constrain convective boundary mixing during the late stages of massive stars.

References

[1] Meakin & Arnett 2007, The Astrophysical Journal 667, 448

[2] Muller, Viallet, Heger & Janka 2016, The Astrophysical Journal 833, 124

[3] Maguire et al. 2012, Monthly Notices of the Royal Astronomical Society, 420, 3451

Astronomy & Astrophysics Dr Bernhard Mueller,
Professor Alexander Heger

Weak Helium Flashes in Accreting Neutron Stars

Many stars are not single stars like the sun, but are born as binary stars, two stars in a close orbit about each other. If one of the stars is "massive," more than about ten times the mass of the sun, it may end is life in a supernova and leave bind a neutron star. In some cases where the other star in the system is of lower mass, and hence loves longer, the orbit could be tight enough that as this star evolves it swells up enough to transfer mass to the neutron star. The accreted mass accumulates in a layer at the surface, and usually starts some burning immediately (hot CNO cycle). When the layer gets thick enough, it may burn in a brief powerful flash burning material all the way to quite heavy material. This is observed as a Type I X-ray burst. If the accretion is very slow, however, the layer may be so cool, the burning does not start immediately, and when it starts, it may just start hydrogen burning, then subside. Only after several of these weak flashes, a more powerful burst might result.

We will use a hydrodynamic stellar evolution code including an extended nuclear reaction network to follow the accretion and burning flashes. The goal is to explore the regime of weak flashes and where they occur and what is their behaviour as a function of neutron star properties and accretion rate and composition (originating from the companion star). A possible extension of the project is to implement the physics of gravitational settling in the present code.

References

[1] S. E. Woosley et.al., "Models for Type I X-Ray Bursts with Improved Nuclear Physics", The Astrophysical Journal Supplement Series, Volume 151, Number 1 (2004).

[2] F. Peng and C. D. Ott, "Helium ignition on accreting neutron stars with a new triple-α reaction rate", The Astrophysical Journal, Volume 725, Number 1 (2010).

[3] R. Narayan and J. S. Heyl, "Thermonuclear Stability of Material Accreting onto a Neutron Star", The Astrophysical Journal, Volume 599, Number 1 (2003).

[4] F. Peng, E. F. Brown, and J. W. Truran, "Sedimentation and Type I X-Ray Bursts at Low Accretion Rates", The Astrophysical Journal, Volume 654, Number 2 (2007).

Astronomy & Astrophysics Professor Alexander Heger,
Associate Professor Duncan Galloway

What is in the purest stars?

After the Big Bang it took only a few minutes to synthesise the primordial composition of the universe, essentially only hydrogen and helium, with traces of lithium and negligible amounts of everything else. All heavier elements were synthesised in stars. From the Big Bang it would take a few hundred thousand years before atomic nuclei and electrons combine to neutral atoms and molecules. And few hundred million years before the first stars formed. This first generation of stars forged the first heavy elements in the universe and released them back into outer space when these stars exploded as supernovae. The material was diluted with the vast amounts of gas left by the big bang, and then incorporated into the next generation of stars. This way the universe became increasingly enriched in heavy elements as we find them in the crust of the earth, to make up planets, and being necessary to life. The very first generation of stars is thought to be quite short-lived, and all of them are gone by now. The second generation would only have very small trace of the ashes of the first generation of stars, and being much longer-lived, we can find them in our galaxy today. The ratio of elements in these ashes provides important clues as to the nature of the elusive first generation of stars. But for many of the elements the abundances are so small that only upper limits can be determined. Yet even these upper limits provide important clues that we want to use.

Recently the most iron-poor star known was discovered by Australian Astronomers. Only for a hand full of elements abundances could actually be measured, while for many other just upper limits could be estimated. Currently there is, however, no good statistical model in Astronomy how to best estimate these upper limits and how to use these upper limits to constrain the nature of the first stars.

The goal of this project is to derive upper limits for abundances of chemical elements and confidence level for these upper limits provided observational and model data. Abundances of chemical elements are determined by matching spectral lines from atomic and hydrodynamical stellar atmosphere calculations to data taken by the Hubble Space Telescope and by some of the world's largest telescopes in Chile and on Hawaii. A possible approach would be to simulate observations given the model data and the same level of noise, to then determine the detectability and confidence levels. A second goal is to develop a model to constrain theoretical data for production of heavy elements by the first stars.

References

[1] S. C. Keller et. al., Nature, Volume 506, Issue 7489 (2014).

[2] M. S. Bessell et.al., Nucleosynthesis in a primordial supernova: Carbon and oxygen abundances in SMSS J031300.36–670839.3, The Astrophysical Journal Letters, Volume 806, Number 1 (2015).

Astronomy & Astrophysics Professor Alexander Heger,
Kais Hamza (Maths),
Mike Bessell (RSAA/ANU)