Postgraduate Projects

The following M.Phil./Ph.D. projects are available in 2018/2019 academic year.  Students are encouraged to contact their prospective supervisors directly to obtain the further detailed information of the project.  We also welcome students to visit our laboratories and research facilities.

Full-time MPhil and PhD students who hold a first degree with second-class honours first division (or equivalent) or above are normally considered eligible to receive a Postgraduate Scholarship (HK$15,700 per month) during the normative study period. This year we expect to admit a large number of postgraduate students.  Students please visit the homepage of HKU graduate school at www.hku.hk/gradsch/ and get the information as well as application forms there.

For other details, please contact Prof. X.D. Cui (Tel. 2859 8975, email address: xdcui@hku.hk), Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong.

space


Postgraduate Projects

 

Astronomy and Astrophysics

 

Project 1: Gamma-ray Pulsar

Supervisor: Prof. K.S. Cheng

Pulsars are rapidly spinning and strongly magnetized neutron stars. They behave like monopolar generators, for young pulsars, whose potential drop in the pulsar magnetosphere can exceed 1015 volts. Such high potential drop can accelerate charged particles to extremely relativistic speed and emit photons with energies higher than 109 eV when these charged particles are forced to move along strong curved magnetic field lines in the pulsar magnetosphere. These high energy photons play very important role in understanding the electrodynamics of the pulsar magnetosphere. In fact, most pulsars except very young pulsars like the Crab pulsar, whose gamma-rays can directly escape from the emission region with very little attenuation, which can provide direct probe to the properties of acceleration regions including magnetic field structure, local electric field strength, charged density, current etc. Pulsars are only known steady gamma-ray point sources in our galaxy. However, before 2008 only 7 pulsars are confirmed to be gamma-ray pulsars, which can only provide very limited information on studying the properties of gamma-ray pulsars. After the launched of a very powerful gamma-ray satellite called “Fermi” at the end of 2008, the situation has completely changed. In less than one year Fermi has already detected 46 gamma-ray pulsars and in a little bit longer than two years observation 70 gamma-ray pulsars are found. It is interesting to note that many radio pulsars with similar parameters as those detected gamma-ray pulsars have not been detected by Fermi satellite. There are six of these non-Fermi detected young pulsars are detected in hard X-rays and soft gamma-rays and they are called soft gamma-ray pulsars. To explain the nature of soft gamma-rays and discover more pulsars in this category is a meaningful subject as a MPhil/PhD topic. In addition, some pulsars are located in binaries. The interactions between the pulsar and its companion star can produce a time dependent multi-wavelength emission. Electrons can be accelerated to extremely relativistic speed in the shock formed by the interaction between the pulsar wind and the stellar wind. Inverse Compton scattering between the shocked electrons and the stellar photons can produce multi-GeV photons. Searching for new gamma-ray binaries and transient gamma-ray emission from pulsar binaries are also possible topics for MPhil/PhD students.

Project 2: Dynamics and Origins of Planetary Systems

Supervisor: Dr. M.H. Lee (Adjunct with Department of Physics)

Extrasolar planet searches have now yielded thousands of planets around other stars. The discoveries include planetary systems with two or more detected planets and planets in binary star systems. Multiple-planet systems and, in particular, those with planets in or near orbital resonances provide important constraints on the formation and dynamical evolution of planetary systems. We are investigating the current dynamical states and origins of resonant planetary systems and planets in binary star systems. In addition, there are projects related to the formation and dynamical evolution of the planets and their satellites in our Solar System. Prior knowledge of classical mechanics and numerical methods would be an asset.

Project 3: Interstellar Medium and Star Formation in Central Giant Elliptical Galaxies of Cool-Core Clusters

Supervisor: Dr. J.J.L. Lim

Galaxy clusters are immersed in hot X-ray-emitting gas that constitutes the bulk of their baryonic mass (the mass in the X-ray-emitting gas is many times more than that of all the stars in the cluster member galaxies combined). In relaxed clusters where the density of the hot gas increases rapidly towards the cluster center, the hot gas in the central region of the cluster is predicted to cool rapidly so as to produce a flow of relatively cool gas towards the cluster center (i.e., an X-ray cooling flow). Indeed, relaxed clusters exhibit relatively cool X-ray gas in their cores, and are therefore known as cool-core clusters. Radio jets from the active supermassive black hole in the central giant elliptical galaxy of the cluster, however, can reheat the cooling X-ray gas, mitigating if not entirely quenching the X-ray cooling flow. Our (Lim, Ao, & Dinh 2008; Ho, Lim & Dinh 2009) study of the central giant elliptical galaxy in the Perseus Cluster has provided the most direct evidence yet for cool gas - in the form of molecular gas at temperatures ~100 K - deposited by an X-ray cooling flow. We are currently studying gas at other temperature to better understand the composition and excitation of the interstellar medium in this galaxy (e.g., Lim et al. 2012). In addition, we have cataloged and are studying numerous (numbering over ten thousand) star clusters in this galaxy (e.g., Yu et al. 2015) that range in age from ~1 Myr to ~10 Gyr, so as to elucidate the formation of star clusters in this galaxy over cosmic time. Finally, we are extending our study of star formation in central cluster giant elliptical galaxies to other galaxy clusters imaged by the Hubble Space Telescope for the purpose of using these clusters as gravitational lenses.

Project 4: Formation of Binary / Multiple Low-Mass Protostellar Systems

Supervisor: Dr. J.J.L. Lim

Our Sun is unusual in at least one aspect: it is single. Most stars having masses comparable with and higher than that of the Sun are actually members of binary systems; a small fraction are members of multiple system (comprising three or more stars). Although the formation of such systems is therefore the primary mode of star formation at solar and higher masses, we do not have a comprehensive framework for how binary and multiple systems form. Our studies of the binary protostellar systems L1551 IRS5 (Lim & Takakuwa 2006; Lim et al. 2016a) and L1551 NE (Lim et al. 2016b) have provided the first direct evidence for rotationally driven fragmentation of molecular cores, one the leading hypothesis for how binary and multiple systems form. In these systems, we find that the circumstellar disks of the binary protostars are aligned with their orbital plane and also the material enveloping these systems (comprising their parental molecular cores); furthermore, the orbital motion of these protostars is in the same direction as the spin of their surrounding envelope. All these properties imply that the angular momenta of the binary protostars and the material from which they formed share a common axis, the basic premise of models that invoke rotationally driven fragmentation. Our study of L1551 NE has established for the first time that binary protostars have circumbinary disks. The circumbinary disk of L1551 NE is rotationally supported; on opposing segments of the disk, gravitational torques from the binary protostars drive material outwards to produce two spiral arms, and in between these arms drive material inwards. We are currently studying how material in the circumbinary disks accretes onto the circumstellar disks of the binary components.

Project 5: Gravitational Lensing by Galaxy Clusters

Supervisor: Dr. J.J.L. Lim

When did gas in bodies comprising primarily dark matter first turn into stars, making galaxies visible for the first time? How did the different stellar components of galaxies – in the case of galaxies like our own, central bulge, disk (in which our Sun resides), and surrounding halo – assemble over time? What is dark matter, which dominates not only matter in galaxies but also matter in the space between galaxies? Addressing the questions posed above requires overcoming two challenges: (i) finding young and therefore distant galaxies, which are faint not just because of their vast distances but also because they are just beginning to form stars; and (ii) spatially resolving these galaxies, which have small angular sizes not just because of their vast distances but also because they are intrinsically small. To address these challenges, Dr. Lim and his collaborators are using massive galaxy clusters as gravitational lenses to magnify background galaxies, so as to detect and spatially resolve galaxies having lower luminosities and at greater distances than would otherwise be feasible. The lens model we have constructed are able to reproduce the observed positions of the lensed images, as well as the appearances of their individual counter-images. Furthermore, our model shows consistency between the redshifts of the lensed galaxies derived from their colors and the redshifts derived geometrically from the lens model, allowing us to clarify and correct a number of lensed images that were previously ambiguous or unidentified. We also demonstrated, for the first time ever in any cluster, that our lens model is able to accurately reproduce the relative brightnesses of the multiply-lensed images. With this reliable correction for lensing, we are able to obtain the intrinsic properties of all the lensed galaxies, including those more weakly lensed and do not produce multiple images, so as to address the questions posed above.

Project 6: Particle Acceleration and Transport in Pulsar Wind Nebulae

Supervisor: Dr. S.C.Y. Ng

Pulsars lose most of their rotational energy through relativistic particle winds. The consequent interactions with the ambient medium result in synchrotron bubbles known as wind nebulae (PWNe). These sources are important cosmic ray accelerators in the Galaxy. We will study the radio and X-ray properties of PWNe using observations taken with the EVLA, ATCA, Chandra, XMM-Newton, and Fermi telescopes, in order to understand the acceleration and transport mechanism of cosmic rays.

Project 7: Late Stage Stellar Evolution

All the projects described below fall under the main topic of Late stage stellar evolution and exploitation of “The new Hong-Kong/AAO/Strasbourg multi-wavelength and spectroscopic Planetary Nebulae database: HASH”
Supervisor(s) for all projects: Prof. Q.A. Parker, Prof. A. Zijlstra, Dr. D.J. Frew, Dr. I Bojcic

Some scientific background to the projects listed below
Stars, the key building blocks of all galaxies, are born in collapsing gas clouds, live their lives as nuclear fusion reactors, and eventually die. Massive stars live fast and die young, exploding as supernovae after only a few million years. However, the vast majority of stars have lower mass and may live for billions of years. PNe derive from stars in the range ~1-8 times the mass of the Sun, representing 90% of all stars more massive than the sun. PNe form when only a tiny fraction of unburnt hydrogen remains in the core. Radiation pressure expels much of this and the hot stellar core can shine through. In a few thousand years the effective temperature rises from ~5000 degrees to as high as 250,000 degrees before falling as the core fades and contracts to a so-called White-Dwarf (WD). The radiation field ionizes the final ejected shell which is called a PN as well as the faint halo of material ejected at earlier times, providing a visible fossil record of the entire mass loss process. PNe have nothing to do with planets but acquired this name because the glowing spheres of ionized gas around their hot central stars resembled planets to early observers.

The study of PNe is crucial to understand both late stage stellar evolution, and the chemical evolution of our entire Galaxy. The ionised shell exhibits strong and numerous emission lines that are excellent laboratories for plasma physics. PNe are also visible to great distances where their strong lines permit determination of the size, expansion velocity and age of the PN, so probing the physics and timescales of stellar mass loss. We can also use them to derive luminosity, temperature and mass of their central stars, and the chemical composition of the ejected gas. Their radial velocities can trace a galaxy’s kinematic properties and test whether the galaxy contains a substantial amount of dark matter. The kinematic properties of PNe in galaxy halos also give strong constraints both on the mass distributions and formation processes of giant elliptical galaxies. The PN formation rate also gives the death rate of lower mass stars born billions of years ago and they directly probe Galactic stellar and chemical. Their complex shapes provide clues to their formation, evolution, mass-loss processes, and the shaping role that may be played by magnetic fields, binary central stars or even massive planets. As the central star fades to a WD and the nebula expands, the integrated flux, surface brightness and radius change in ways that can be predicted by current hydrodynamic theory. PNe are thus powerful astrophysical tools, providing a unique window into the soul of late stage stellar evolution.

We are also in a golden age of PN discovery and Prof Parker and his team have lead programs that have more than doubled the totals accumulated by all telescopes over the previous 250 years. The scope of any future large-scale PNe studies, particularly those of a statistical nature or undertaken to understand true PNe diversity and evolution should now reflect this fresh PN population landscape of the combined sample of ~3500 Galactic PNe now available. Such studies should take into account these recent major discoveries and the massive, high sensitivity, high resolution, multi-wavelength imaging surveys now available across much of the electromagnetic spectrum.

Following this motivation we provide, for the first time, an accessible, reliable, on-line "one-stop" SQL database for essential, up-to date information for all known Galactic PN.

All the projects below will make use of and build on this world-leading new resource.

Project 7a: The PNe luminosity function (LMC, SMC, Bulge and local volume)

This PNLF provides the co-eval brightness distribution of a population of PNe in a given system (such as an entire Galaxy). An exponential fit to the bright end cut off of the PNLF is a potent cosmological standard candle but how and why it works so well across all galay hubble types is a mystery while the detailed form and features seen in various PNLFs (so called “Jaboby dips”) are hard to interpret. Access to our highly complete PNLFs across 10dex in [OIII] magnitudes for the Bulge and LMC in particular offers strong opportunities to tackle these problems.

Project 7b: PPNe AGB haloes and the ejected mass budget

The main shells of PNe typically contain only ~0.1 Msun in ejected material while the residual core – on the way to becoming a white dwarf are only ~0.6Msun. However, the progenitor star may have had a mass of between one and up to 8 solar massess. The “missing mass” has been lost on the AGB and particularly post AGB and pre PNe phases of evolution. At least part of this is detectable in terms of so called AGB haloes. These can be extensive but of a surface brightness that could be 1/1000 times weaker than the main PN shell. Detailed study of such haloes especially in terms of abundances is currently lacking as is a proper understanding of where are the previously ejects mass is to be found.

Project 7c: Morpho-kinematic modelling of PNe and insights in bipolarity

The advent of powerful integral field units (IFU) on major telescopes to perform areal point-to-point spectroscopy of resolved objects has enabled detailed 3-D datacubes to be obtained. This has enabled both kinematic and line intensity maps to be produced for significant numbers of PNe for the first time. These data can be combined with morpho-kinematic modelling with sophisticated visualisation software such as SHAPE to permit the de-projection of 2-D PNe images into more accurate 3-D representations as matched and informed by the kinematic data available from the IFU datacubes. More accurate determinations of true PNe morphologies can be obtained particularly for bi-polar PNe where the major axis might otherwise be poorly constrained and provide insights into connections between CSPN properties, nebular characteristics binarity and morphology.

Project 7d: Central stars of PNe – discovery, description and diversity

Currently less than 25% of known PNe have unequivocally identified central stars (CSPN). The availability of significant new PNe samples, new wide field surveys and particularly access to new u-band imaging from VPHAS+ and UVEX promises to dramatically improve this number. It is the characteristics of these CSPN and possible binarity that directly affects the observed properties of the ionised nebulae. This project seeks to both discover new CSPN candidates and study their properties and diversity to inform our understanding of PN shaping, expansion and evolution.

Project 7e: Abundances of planetary nebulae, Galactic gradients and the local group

Obtaining accurate abundances for PNe is a difficult enterprise. Very high S/N spectra are required for large numbers of faint emission lines in order to provide sufficient species to allow proper abundance estimates. So far only ~150 PNe have well determined abundances from a total population of over 3200 Galactic PNe. Most of these are also for the highest surface brightness PNe as these are the easiest to observe but they may not be representative of the underlying abundance patterns of most PNe. This project will attempt to improve this situation in terms of both available abundances and breadth of PNe sample selection. Results will be used to improve our understanding of nebula abundance variations as a function of PNe CSPN properties (mass and likely progenitor mass), environment and other variables.


 

Experimental Condensed Matter Physics group


Project 8: In-depth Investigation of Fundamental Optoelectronic Processes in Advanced Organic and Inorganic Layered Solar Cells

Supervisor: Prof. S.J. Xu

High-efficiency devices for converting sunlight energy to electrical energy are highly desirable for the great purpose of energy saving and sustainable development. Such energy converting devices are usually called solar cells. Currently, some organic materials such as polymers and various inorganic semiconductor materials are employed to fabricate solar cells. Getting a better understanding of some fundamental optoelectronic processes such as light absorption, photo-generated carriers’ diffusion and transport, radiative and non-radiative recombination of photo-generated and electrically-injected carriers in solar energy materials and solar cell structures is vital for enhancing and improving the performance of solar cells. In this project, we employ a variety of optical spectroscopic techniques to investigate some fundamental optoelectronic processes occurring in energy materials and solar cell structures in detail. We aim at getting an in-depth insight into the main energy losing mechanisms via studying the transport behavior and various mid-way annihilation channels of charge carriers in the materials and practical devices.

Project 9: Optical Characterization of GaN-based Nanopillars and Quantum Dots

Supervisor: Prof. S.J. Xu

GaN based light emitting diodes (LEDs) are emerging as the core technology for solid state lighting. How to further increase light emission efficiency, brightness and reduce cost of GaN based LEDs are the practical requirements for developing GaN based solid state lighting. GaN based nonplanar nanostructures such as nanowires and nanodots are recognized as promising materials to meet these requirements. In the project, we will employ various state-of-the-art precise optical spectroscopic techniques to characterize GaN-based nanowires (nanopillars) and nanodots. Various key factors affecting light emission efficiency, such as defect and stress states, effect of built-in piezoelectric-field, carriers’ localization, electron (exciton)-phonon interactions, and their internal relationships will be systematically investigated.


 

Experimental High Energy Particle Physics Group

 

Project 10: Searching for Supersymmetry at the Large Hadron Collider

Supervisor: Dr. Y.J. Tu

The Standard Model (SM) has worked beautifully to predict and explain various experimental results. However, the SM has many open questions thus it is believed not a complete theory. Among many models, supersymmetry is the most promising candidate for new physics. SUSY predicts a partner particle for each particle in the SM. These new particles would solve a major problem in the SM, hierarchy problem - The masses of the W, Z particles are 1016 smaller than that of the Planck mass. SUSY also provides good dark matter candidate and a solution to the baryon asymmetry of the universe. We will search for super particles decaying into SM leptons plus missing transverse energy. Such experimental signatures have rich interpretations in various new physics scenarios, e.g. in SUSY, when the charginos and neutralinos (mixtures of superpartners of the gauge bosons and the Higgs bosons) produced via electroweak interactions and decay into the W, Z or H plus the lightest neutralino or gravitino (Dark Matter candidate), where W, Z further decay into leptons and Higgs decays invisibly, the final state will contain leptons plus missing transverse momentum. The same final states also appear in the slepton decays, which are superpartners of the SM leptons. Therefore, the projects are not only key searches for SUSY, but also good probes for Dark Matter and beyond the SM Higgs physics.

Project 11: Searching for Higgs Beyond the Standard Model at the Large Hadron Collider

Supervisor: Dr. Y.J. Tu

The Standard Model (SM) has worked beautifully to predict and explain various experimental results. However, the SM has many open questions thus it is believed not a complete theory. Among various new theories, models with an extended Higgs sector are extensively existing and well motivated, such as SUSY, Two Higgs Doublet Model (2HDM) and Composite Model. The group will work on searching for Higgs predicted in physics beyond the Standard Model. The focus will be in the scenario where such Higgs decays into top quarks.


 

Experiemental Nuclear Physics Group


Project 12: Spectroscopy of neutron-rich Ca isotopes

Supervisor: Dr. J.H.C. Lee

We will perform in-beam gamma spectroscopy measurements of 56Ca and 53, 55Ca at RIBF facility (RIKEN) via one nucleon knockout reactions, with the use of MINOS device coupled with DALI2 gamma spectrometer and ZeroDegree Spectrometer. The measurement of 56Ca extends the systematic studies of the energies of 21+ and other low-lying states beyond 54Ca (N=34). The location of 21+ energy of 56Ca gives a direct measure of the difference between 0+ and 2+ two-body matrix elements in the f5/22 which has not yet been determined. This new experimental data is also valuable in accessing the accuracy of the calculated Ex(21+) of 56Ca using different effective interactions in shell-model theories and ab-initio calculations. The spectroscopy of 53, 55Ca could reflect the nature of the N=34 shell closure and the contribution of the g9/2 state. The single-particle properties (angular momentum and spectroscopic factor) of the low-lying states will be extracted from the cross sections and parallel momentum distributions of the residues.


 

Materials Science group

 

Project 13: Organic and Perovskite Optoelectronic Devices

Supervisor: Prof. A.B. Djurišić

Even though the present state-of-the-art realizations are still less efficient compared to the inorganic ones, the low cost production of large-area organic solar cells represents a great attraction of these devices and the feature of a greatly reduced fabrication cost adds significant impetus to research in this area. In particular, recent advances in organometallic halide perovskite solar cells have resulted in increasing interest in next generation solar cells based on organic materials. In spite of great interest for practical applications of organic materials and devices, there are still a number of unanswered questions concerning their fundamental properties and principles of operation. The objective of this project is to investigate the influence of doping, interface modifications and device architecture changes on the performance of solar cells. The objectives are to improve the device efficiency and stability, as well as develop devices on flexible substrates. Particular emphasis is placed on the development of novel perovskite materials for both LED and solar cells applications, and studies of the device degradation and improvement of the device stability. The student should have basic knowledge of optics and solid state physics.

Project 14: Wide Band Gap Nanostructures

Supervisor: Prof. A.B. Djurišić and Prof. M.H. Xie

Due to exceptional properties different from bulk materials, nanostructures of different semiconductors have been attracting increasing attention. The obtained morphology of the nanostructures and their optical properties are strongly dependent on the fabrication conditions. The objective of this work is to investigate the dependence of structural and optical properties of wide band gap (ZnO, TiO2, SnO2, CeO2 and GaN) on the fabrication conditions. The fabricated nanostructures will be characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X ray diffraction (XRD), photoluminescence and photoluminescence excitation (PL and PLE). The project will involve extensive experimental work. The application of prepared nanomaterials in LEDs, solar cells, photocatalysis, catalysis, sensors, or Li-ion batteries (depending on the material chosen) will also be studied.

Project 15: Fabrication and Characterization of Transition-metal Doped ZnO: Tm Materials and Structures

Supervisor: Dr. C.C. Ling

Dilute magnetic semiconductor (DMS) is a class of material receiving extensive attention because of its potential application spintronic, which is a new class of device based on the degree of freedom of the electron spin. For practical device applications, the Curie temperature of the DMS material has to be above the room temperature. There was theoretical calculation showing that room temperature ferromagnetism in ZnO:Tm could be stabilized by electron and hole mediations. The present project aims to fabricate ZnO:Tm materials, characterize their magnetic, electrical and structural properties, and to find the origin of the room temperature ferromagnetism.

Project 16: Pulsed Laser Deposition Growth of ZnO Related Structures

Supervisor: Dr. C.C. Ling

ZnO is a wide band gap semiconductor recently received extensive attention because of its potential applications in UV optoelectronic devices. Technologies for dopings of ZnO are of essential for ZnO-based device development. However, p-type doping is very difficult, not reliable and non-reproducible. The present project aims at the fabrication of high quality n-type and p-type ZnO related structures using the method of pulsed laser deposition with the well control of the film electrical and optical properties. Characterizations of the film structures and devices using a comprehensive spectroscopy approach will also be carried out.

Project 17: Vacancy Related Defects in Zinc Oxide

Supervisor: Dr. C.C. Ling

Zinc oxide is a wide band gap II-VI semiconductor with a band gap of 3.4 eV. Similar to the extensively studied wide band gap materials like GaN or SiC, ZnO is now being considered as the potential material for fabricating short-wavelength optoelectronic devices, a well as electronic devices operating at high temperature, high frequency and high radiation environment. As defect plays an important role in determining the electrical and optical properties of the material, knowledge concerning the defects has to been known in order to have successful device fabrication. However, information of defects in ZnO is far from complete. In this project, we suggest to investigate the native electrical active defects in ZnO by temperature dependent Hall (TDH) measurement, deep level transient spectroscopy (DLTS), photoluminescence (PL) and positron annihilation spectroscopy (PAS). TDH and DLTS measurements are used to identify and characterize the electrical active defects, while PL is a technique to reveal optical active defects. On the other hand, PAS is selectively sensitive to vacancy type defects and can offer defect information such as the ionization energy, the concentration, the charge state and the microstructure. Studying the correlations between the signals from these characterizing techniques can possibly explore the microstructures of the identified defects, and also the involvements of the vacancy type defects in determining the material’s electrical and optical properties.

Project 18: MBE of Thin Films and Layered Structures

Supervisor: Prof. M.H. Xie

Recent discovery of topological insulator (TI) state of matter causes a lot of attention for new physics and properties. Transition metal dichalcogenides (TMDCs) are two dimensional semiconductors that attract lots of attention for future miniature electronics, spintronics, and valeytronics. In this project, ultrathin films of TIs and TMDs, their heterostructures will be fabricated by molecular-beam epitaxy and characterized by surface tools.


 

Quantum Computing and Information Theory

 

Project 19: Quantum Information Theory

Supervisor: Prof. H.F. Chau

A lot of activities are going on in the field of quantum information theory recently. This field is about the study of quantum mechanical system from an information theoretical point of view. We ask questions like what information can be stored, transmitted and extracted using quantum mechanical systems. In this theoretical Ph.D. project, one is expected to focus on the tradeoff between different resources in quantum information processing such as energy, time, space and communication. Knowledge in the following fields is required: quantum mechanics in Sakauri level, quantum optics, statistical mechanics, coding theory, classical information theory, computational complexity, functional analysis and algebra. Although it is not necessary for you to have all the above subjects, but the more you know them the better prepared you are. I am looking for a hardworking, self-motivated individual who is both physically and mathematically sound to take up the challenge.


 

Theoretical Atomic Physics and Degenerate Quantum Gases

 

Project 20: Spin dynamics in ultracold atomic gases

Supervisor: Dr. S.Z. Zhang

Recent experimental advances in the manipulation of ultra-cold alkali atomic gases have made it possible to engineer synthetic gauge fields and spin-orbit interaction for neutral atoms. Together with the possibility of modifying the inter-atomic interactions using Feshbach resonance, this has led to multitude of possibilities in the investigations of interacting quantum many-body systems. It has been suggested that the new system might support exotic excitations like Majorana fermions or exhibit high transition temperature into the superfluid state. In this project, we will investigate a few aspects of the system, including its novel spin resonance and spin diffusion behavior, which is also likely to shed light on the analogous problems in solid state physics.


 

Theoretical Condensed Matter Physics Group

 

Project 21: Novel Topological States of Quantum Matter

Supervisor: Prof. S.Q. Shen

A topological insulator is a novel topological state of quantum matter which possesses metallic edge or surface states in the bulk energy gap. The edge or surface states consist of an odd number of massless Dirac cones, and result in quantum spin Hall (QSH) effect, which is analogous integer quantum Hall effect. The physical properties of this kind of insulator are unchanged by smooth modifications to their geometry and are robust against non-magnetic impurities and interactions. The edge states and surface states are robust against the nonmagnetic impurities. The primary objective of this proposal is to explore novel topological quantum materials, and to investigate quantum transport in topological insulators, metals and superconductors. Quantum transport and quantum phenomena will be investigated in various forms for the purpose of application.

Project 22: First Principles Calculation of Quantum Transport through Nanostructures

Supervisor: Prof. J. Wang

Currently we are interested in the field of nano-scale physics and technology. It has been demonstrated in several laboratories that many important quantum interference features such as the conductance quantization are observable for atomic wires at the room temperature. As a result, atomic device has important potential device applications and can be operated in room temperature. As theoreticians, we investigate quantum transport through atomic and molecular scale structures where a group of atoms are electrically contacted by metallic leads. Using Density functional analysis and the non-equilibrium Green's function method, we study conductance, capacitance, current-voltage characteristics, and other molecular device characteristics.

Project 23: Quantum Computation

Supervisor: Prof. Z.D. Wang

Quantum computers, based on principles of quantum mechanics, could efficiently solve certain significant problems which are intractable for classical computers. For the past several years, they have become a hot topic across a number of disciplines and attracted significant interests both theoretically and experimentally. In physical implementation of quantum computation, a key issue is to suppress a so-called decoherence effect, which can lead to major computing errors. A promising approach to achieve built-in fault tolerant quantum computation is based on geometric phases, which have global geometric features of evolution paths and thus are robust to random local errors. In this project, it is planned to first study geometric phases in relevant physical systems and then to design geometric quantum gates. Physical implementation of these gates in solid state systems will be paid particular attention.

Project 24: Topological Metals/Semimetals and Quantum Simulations

Supervisor: Prof. Z.D. Wang

Topological quantum materials have significantly intrigued research interest. Investigations of the gapless and gapped systems pave the way for discovering new topological matter. Recently, our group at HKU established a unified theory for topological gapless systems, including novel metals and semimetals consisting of topological Fermi surfaces. Based on our basic theory, we plan to explore various exotic quantum properties of topological metals/semimetals for different dimensions and their quantum simulations with artificial systems.

Project 25: Valley-spintronics in 2D materials and their van der Waals heterostructures

Supervisor: Prof. W. Yao

A trend in future electronics is to utilize internal degrees of freedom of electron, in addition to its charge, for nonvolatile information processing. Suitable candidates include the electron spin, and the valley pseudospin. The latter labels the degenerate valleys of energy bands well separated in momentum space. 2D materials, in particular the group-VI transition metal dichalcogenides, offer an exciting platform to explore valleytronics and spintronics. Van der Waals stacking of the 2D materials further provide a powerful approach towards designing quantum materials that can combine and extend the appealing properties of the building blocks. In this project, we will investigate the valley dependent physics in a group of 2D materials, namely, group VI transition metal dichalcogenides. We will look into the possibility of controlling valley dynamics in these materials and their van der Waals heterostructures by external magnetic, electric and optical fields.

Last updated on 01 September 2017