# Postgraduate Projects

The following M.Phil./Ph.D. projects are available in 2020/2021 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$16,660 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.

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**Postgraduate Projects**

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__Astronomy and Astrophysics group__

__Astronomy and Astrophysics group__

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Project LXD01: Simulations of Black Hole Accretion Disks and Jets

#### Supervisor: Dr. L.X. Dai

Black hole accretion is the central engine that powers the most luminous sources in the universe, such as quasars and X-ray binaries. Besides radiation, winds and jets are also produced in black hole accretion. The feedback carried by radiation, winds and jets can provide feedback to the hosting galaxy and influence its formation and evolution. Besides powering persistent sources, black hole accretion can also power transient flares such as tidal disruption events and gamma-ray bursts (including the ones produced in neutron star mergers). We study the physics of black hole accretion and emission using general relativistic numerical codes. In particular, we are very interested in studying a class of accretion called super-Eddington accretion, which is believed to happen in high-redshit quasars, tidal disruption events and ultra-luminous X-ray sources. This class of accretion is under-investigated, yet it is important for understanding the formation of massive black holes and galaxies in the early universe. A lot of progress has been obtained lately on the observational front, and theoretical studies are much needed to complement the fast progress in observations. MPhil/PhD projects on this topic include: 1) design and perform new general relativistic simulations of (super-Eddington) accretion flow; 2) analyze previous simulation data and carry out radiative transfer analysis; 3) comparing data to observations (if the student is also interested in working on / is familiar with observations).

### Project KF01: The Origin of the highest-energy particles in the universe

#### Supervisor: Dr. K. Fang

High-energy cosmic particles carry extreme energy that cannot be produced by man-made accelerators. They are unique messengers from distant sources such as active galactic nuclei and starburst galaxies. We will study high-energy sources using gamma-ray data from the Fermi satellite and the High Altitude Water Cherenkov telescope (HAWC), as well as high-energy neutrino data from the IceCube telescope. We will also investigate the radiative transfer of relativistic particles in cosmic environment using numerical simulation. Our work aims at understanding the origin of cosmic rays through both data and theory.

### Project MHL01: 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 JJLL01: Star Formation in Giant Elliptical Galaxies at the Centers of Galax 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. In relaxed clusters where the density of this gas increases rapidly towards the cluster center, the hot gas around the center is predicted to cool rapidly so as to produce an inflow of relatively cool gas (i.e., an X-ray cooling flow). Indeed, relaxed clusters exhibit relatively cool X-ray gas in their cores, and preferentially exhibit relatively large quantities of gas at even lower temperatures. Relativistic jets from the central giant elliptical galaxy, however, can churn and reheat the cool gas, complicating our understanding of the nature of this gas. Our work focuses on determining the origin, excitation and therefore physical properties, and fate of relatively cool gas in the giant elliptical galaxies at the center of galaxy clusters; as well as the recent history of star formation in these galaxies, and the manner in which their AGNs are fueled.

### Project JJLL02: Astrophysical Applications of Gravitational Lensing

#### 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? How did their supermassive black holes grow over time? What is dark matter, which dominates not only matter in galaxies but also matter in the space between galaxies? To address these questions, Dr. Lim, his students, and his collaborators use gravitational lensing by galaxy groups or clusters as cosmic lenses to magnify background galaxies. In this way, we able to detect and study distant, and therefore young, galaxies that would otherwise be too dim to detect and too small to resolve. We can even determine, through geometry, the distances to these galaxies, the redshifts of which are often difficult to measure because these galaxies are so faint. The manner in which the number of these galaxies change over time allows us to test predictions by different forms of dark matter. Through gravitational lensing, we also are able to study the properties of the lensing clusters, allowing us to weigh supermassive black holes in cluster member galaxies, and to search for substructure in dark matter as predicted in some models. We continue to develop different exciting astrophysical applications of gravitational lensing.

### Project CYN01: Mapping the Magnetic Fields of 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 pulsar wind nebulae (PWNe). While the PWN magnetic fields play an important role in the particle acceleration and transport processes, little is known about the field configurations. In this observational project, we will map the PWN magnetic fields using radio interferometric observations. This can offer a powerful probe of the physical conditions and evolutionary history of PWNe. The results will be compared with other systems to understand the critical parameters that determine the field properties.

### Project QAP01: 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 include: Prof. Q.A. Parker, Prof. A. Zijlstra (Hung Hing Ying distinguished visiting professor), Dr. Claire Lykou, Dr. Andreas Ritter, Dr Xuan Fang**

**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 QAP01(a): 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 galaxy 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 QAP01(b): PNe 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 QAP01(c): 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 data-cubes 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 data-cubes. 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 QAP01(d): 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 QAP01(e): 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.

### Project MS01: Studying the Universe using satellites

All the projects described below fall under the main topic of space science using satellites

**Supervisor(s) for all projects: Dr. M. Su**

We have access to some world leading space science facilities, including Chinese space station, Dark Matter Particle Explorer, CMB telescopes. We are also building our own satellites to do focused science, including X-ray telescopes, UV telescopes, gamma-ray detectors, microwave telescopes, and cosmic-ray detectors. A large range of projects are available from numerical simulation, hardware construction, data analysis, science forecast, data mining etc. Broadly speaking, if you are interested in using satellites or building your own ones, please talk to him.

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__Atomic, Optical and Quantum Physics group__

__Atomic, Optical and Quantum Physics group__

### Project HFC01: 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.

### Project HKL01: Quantum Information and Quantum Communication Theory

#### Supervisor: Prof. H.K. Lo

We are in the midst of the Second Quantum Revolution. Quantum mechanics can revolutionize information processing by performing tasks that are difficult or impossible in conventional information theory. For instance, quantum computer can break standard encryption schemes such as RSA. Quantum cryptography can lead to unbreakable codes. Quantum internet enables distributed quantum information processing including blind quantum computing in the cloud. We will study the power and limitation of quantum communication and information. Our work ranges from the foundations of quantum information theory, the foundations of security, and the proposal of quantum repeaters to the design of practical protocols, their simulations and experimental implementations. This project will be on the theoretical side.

The applicant may work either independently or work closely with the experimental group. The experimental group will leverage our generous start-up funding to build the first quantum communication lab in Hong Kong. Our goal is to bring the success of the professor's research from the U. of Toronto to Hong Kong.

See https://www.comm.utoronto.ca/~hklo/

See also https://spectrum.ieee.org/tech-talk/telecom/internet/quantum-repeater-trial-ignites-hopes-for-longdistance-quantum-cryptography-and-computation

A strong academic background in physics (or a closely related subject such as theoretical computer science) will be beneficial.

(Prof. Lo is expected to join the Department in Jan. 2020. To discuss the project with Prof. Lo, please email hklo@comm.utoronto.ca)

### Project HKL02: Experimental Quantum Communication and Quantum Internet

#### Supervisor: Prof. H.K. Lo

We are in the midst of the Second Quantum Revolution. Quantum mechanics can revolutionize information processing by performing tasks that are difficult or impossible in conventional information theory. For instance, quantum computer can break standard encryption schemes such as RSA. Quantum cryptography can lead to unbreakable codes. Quantum internet enables distributed quantum information processing including blind quantum computing in the cloud. We will study the power and limitation of quantum communication and information. Our work ranges from the foundations of quantum information theory, the foundations of security, and the proposal of quantum repeaters to the design of practical protocols, their simulations and experimental implementations. This project will be on the experimental side.

The applicant will play a key role in our experimental efforts with the help of postdocs/research assistant professor. We will leverage our generous start-up funding to build the first quantum communication lab in Hong Kong. Our goal is to bring the success of the professor's research from the U. of Toronto to Hong Kong.

See https://www.comm.utoronto.ca/~hklo/

See also https://spectrum.ieee.org/tech-talk/telecom/internet/quantum-repeater-trial-ignites-hopes-for-longdistance-quantum-cryptography-and-computation

A strong academic background and a strong interest in doing hands-on research in experimental optics will be useful.

(Prof. Lo is expected to join the Department in Jan. 2020. To discuss the project with Prof. Lo, please email hklo@comm.utoronto.ca)

### Project HKL03: Quantum Communication and Quantum Internet (Theory or Simulation or Experiment)

#### Supervisor: Prof. H.K. Lo

We are in the midst of the Second Quantum Revolution. Quantum mechanics can revolutionize information processing by performing tasks that are difficult or impossible in conventional information theory. For instance, quantum computer can break standard encryption schemes such as RSA. Quantum cryptography can lead to unbreakable codes. Quantum internet enables distributed quantum information processing including blind quantum computing in the cloud. We will study the power and limitation of quantum communication and information. Our work ranges from the foundations of quantum information theory, the foundations of security, and the proposal of quantum repeaters to the design of practical protocols, their simulations and experimental implementations. This project will be on the phenomenological side.

Depending on the applicant's expertise, he/she/they can work on either theory or simulation or experiment. We will leverage our generous start-up funding to build the first quantum communication lab in Hong Kong. Our goal is to bring the success of the professor's research from the U. of Toronto to Hong Kong.

See https://www.comm.utoronto.ca/~hklo/

See also https://spectrum.ieee.org/tech-talk/telecom/internet/quantum-repeater-trial-ignites-hopes-for-longdistance-quantum-cryptography-and-computation

A strong academic background in physics or a closely related subject will be beneficial.

(Prof. Lo is expected to join the Department in Jan. 2020. To discuss the project with Prof. Lo, please email hklo@comm.utoronto.ca)

### Project TTL01: Ultrafast spectroscopy of condensed matters

#### Supervisor: Dr. T.T. Luu

We have been actively working on and contributing to the field of high-order harmonic generation in solids and its spectroscopic applications. Once we drive a condensed matter system using a strong electric field that is beyond perturbation theory, ultrafast electronic currents, generated inside the materials, give rise to the generation of coherent, intense extreme ultraviolet radiation in the form of high-order harmonics. Careful studies of these harmonics and the related time-resolved measurements would allow to study very interesting electronic properties and dynamics of the involved system. In this project, we will first construct a state-of-the-art experimental apparatus (involving high power laser pulses and its applications in nonlinear optics) that would not only allow us to do attosecond streaking measurements (direct measurement of light waves) but also generate high-order harmonics from novel condensed materials. Direct spectroscopic applications will follow immediately.

### Project ZDW01: 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 SZZ01: 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.

### Project SZ01: Scattering processes in topological Weyl metamaterials

#### Supervisor: Prof. S. Zhang

Topological physics concerns with exploration and implementation of non-trivial band structures in periodic systems and the related topological protection. The most well known examples are topological insulators that are insulating in the bulk but conductive on the surface. The study of topological states has recently expanded into other fields, such as cold atoms, photonics, acoustics, and mechanical systems. In particular, the development of topological systems in photonics has attracted widespread attention. The design and implementation of photonic edge/surface state in photonic structures that support topological protection immune from scattering has become a frontier research direction. While topologically protected surface states have been intensively studied in photonics, less attention has been paid to the bulk wave propagation inside the topological systems such as photonic Weyl media. This project aims to investigate the scattering of wave inside Weyl metamaterials by introducing resonant scatterers/defects into the periodic Weyl metacrystal. It is expected that due to the diminishing density of states close to the Weyl frequency, the resonant scattering cross section will diverge, leading to strong interaction between the wave and the introduced defects. This feature will further be exploited to design various photonic devices.

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__Experimental Condensed Matter and Material Science group__

__Experimental Condensed Matter and Material Science group__

### Project XDC01: Optical Properties in Emerging 2 Dimensional Materials

#### Supervisor: Prof. X.D. Cui

The emerging atomic 2D crystals offer an unprecedented platform for exploring physics in 2 dimensional systems. As the material dimension shrinks to atomically thin, quantum confinements and enhanced Coulomb interactions dramatically modify the electronic structure of the materials from the bulk form and incur sophisticated consequences featuring strong electron-electron interactions and robust quasiparticle of excitons. We are to investigate physics properties in emerging 2D materials with emphasis in optical properties with semiconductor optics technique.

### Project AD01: Perovskite Optoelectronic Devices

#### Supervisor: Prof. A.B. Djurišić

Recent advances in organometallic halide perovskite solar cells have resulted in increasing interest in next generation solar cell based on these materials. In spite of great interest for practical applications, 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 charge transport layer 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. Some knowledge of chemistry would be beneficial.

### Project AD02: Wide Band Gap Nanostructures

#### Supervisor: Prof. A.B. Djurišić

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, TiO_{2}, SnO_{2}, CeO_{2} 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 DKK01: Topological quantum states at artificial 2D interfaces

#### Supervisor: Dr. D.K. Ki

Topological states of matter represent the new class of materials that are characterized by their low-energy quasiparticles at the boundaries, such as Majorana Fermions in topological superconductors and non-Abelian anyons in even-denominator fractional quantum-Hall insulators. These states are under intense focus as they have exotic topological properties that are not only fundamentally interesting but also offer a great promise for realizing new types of device applications (e.g., topological quantum computing). In this project, we will focus on ‘artificially designing’ the new topological states by creating atomically sharp interfaces between different 2D crystals where van der Waals interactions can induce new properties in the system, on-demand. Examples include graphene-on-transition metal dichalcogenides (where the spin-orbit coupling—the critical element for realizing topological states—can be controlled) and multi-domain Moiré superlattices (where two topologically different states can be joined to reveal new effects). Having known that nearly hundreds of 2D crystals exist with diverse properties, we expect this project will expand the ‘zoo’ of topological materials available and bring us a step closer to the realization of topological electronics

### Project DKK02: New many-body physics in engineered 2D materials

#### Supervisor: Dr. D.K. Ki

Electrons in solid interact with each other and studying the resulting many-body effects is one of the recurring main themes of condensed matter physics. Recently, atomically thin 2D crystals, such as graphene, have emerged as an interesting material system with novel electronic properties that can be tuned widely in the experiments. The goal of this project is therefore to take a full advantage of such large experimental flexibilities to explore new many-body phenomena in these materials. To this end, we will realize the devices with extremely high quality in various geometries and approach as close as possible to zero Fermi energy where interactions are known to be most dominant.

### Project CCL01: Room temperature ferromagnetism in ZnO related materials

#### 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 and experimental results showing that room temperature ferromagnetism (RTFM) in ZnO:Tm (Tm=transition metal) could be stabilized by electron and hole mediations. It is interesting to notice that RTFM can also be achieved in non-magnetic element doped (like Cu) ZnO and undoped ZnO, which is proposed to be associated with the intrinsic defects (like VO and VZn). However, the origins of the observed room temperature ferromagnetism are controversial and the physics is far from completely understood. The present project aims to fabricate non-magnetic element doped ZnO with RTFM and to understand the physics and the origin of the RTFM. It is also to achieve electric bias modulated magnetization in the corresponding device structure.

### Project CCL02: High dielectric constant oxides via defect engineering

#### Supervisor: Dr. C.C. Ling

Materials with high dielectric constant and low dielectric loss are essential for the miniaturization of capacitive microelectronic devices, and also have the potential in compact high-density energy storage applications. Colossal dielectric constant >104 with low dielectric loss <0.1 have been acheived in acceptor-donor co-doped oxides, and was speculated to be associated with the electron-pinning defect complex. The physics and the action of the electron-pinning defect is totally unclear. The current project aims to fabricate oxide materials with high dielectric constant and low dielectric loss, to study the physics of the colossal dielectric constant, as well as to explore the identity and the action of the electron-pinned defect.

### Project CCL03: Zn vacancy cluster and complex in Zinc Oxide Materials

#### Supervisor: Dr. C.C. Ling

Zinc oxide is a wide band gap II-VI semiconductor with a band gap of 3.4 eV. Because of its excellent electrical and optical properties, it is a potential material for a variety of applications in devices including ultra-violet optoelectronic, spintronic, photovoltaic, sensor and transparent conductive electrode. Although defects play crucial role in determining the material electrical and optical properties and there are studies involving simple point defects, not much is known in defect complex and vacancy cluster. Despite that majority of device applications involves ZnO thin film, relatively few defect studies were performed in particular in thin film focusing in optoelectronic applications. This project aims to study Zn-vacancy cluster and complex in ZnO films grown by pulsed laser deposition (PLD), focusing on how they influence the materials’ electrical and optical properties as well as on manipulating the film electrical and optical properties via the defect engineering approach.

### Project MHX01: MBE Growth and Surface Studies of Two-Dimensional Materials

#### Supervisor: Prof. M.H. Xie

Two-dimensional (2D) materials exhibit many interesting properties, which are attracting extensive research attentions in recent years. Examples include monolayers of transition metal dichalcogenides (TMDCs) and phosphorene, which hold potentials for nano electronic, optoelectronic, spin- and valley-tronic applications. In this project, ultrathin films of 2D materials and their heterostructures will be fabricated by molecular beam epitaxy and characterized by the surface tools, such as electron diffraction (LEED/RHEED), scanning tunneling microscopy and spectroscopy (STM/S), ultraviolet photoemission spectroscopy (UPS), etc.

### Project SJX01: In-depth Investigation of Fundamental Optical and Optoelectronic Processes in Semiconductors and Luminescent Materials

#### Supervisor: Prof. S.J. Xu

In this project, we employ a variety of optical spectroscopic techniques to get know more about some fundamental optical and optoelectronic processes occurring in semiconductors and luminescent materials, focusing on luminescence and energy transfer mechanisms inside the materials. We try to understand these complicated processes and phenomena in terms of a few simple principles.

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__Theoretical and Computational Condensed Matter group__

### Project GC01: Spin-orbit-coupled correlated materials

#### Supervisor: Dr. G. Chen

The discovery of topological insulator and semimetal has pushed the spin-orbit coupling to the forefront of modern condensed matter physics. As we know, topological insulator and semimetal with protected surface states are non-interacting electron band structure physics. It is naturally to understand the effect of the correlation on top of the non-trivial band structure topology. Besides this theoretical motivation, the 4d/5d transition metal compounds with iridium, osmium, even 4f rare-earth compounds are natural material systems to explore such phenomena. This is a field where both theoretical ideas and experimental efforts converge. In spin-orbit-coupled correlated material, we have discovered and/or proposed novel quantum phases of matter and the unconventional multipolar orders. We will continue to explore the rich and fascinating behaviors of quantum materials with both strong spin-orbit coupling and strong correlation.

### Project GC02: Frustrated and quantum magnetism

#### Supervisor: Dr. G. Chen

Frustration in condensed matter physics usually means competing interactions that cannot be optimized simultaneously. When frustration meets with quantum mechanics in quantum many-body systems, it not only enhances the effect of the quantum fluctuations but also enriches the quantum phenomena. Various exotic and quantum phases such as the quantum spin liquid with emergent excitations and gauge structures are proposed.

In last 1-2 decades, the field of frustrated quantum magnets has grown rapidly. Many quantum magnets have been discovered, studied and characterized. Frustration often but not always comes in the form of geometrical frustration. That is the reason that many existing frustrated quantum magnets come in the form of geometrical lattice such as triangular, kagome, FCC, pyrochlore, hyperkagome lattices. Our work is to provide physical and realistic models to describe the interaction between the microscopic degrees of freedom, and give explanation and prediction of interesting experimental phenomena.

### Project GC03: Ultracold atoms on optical lattices

#### Supervisor: Dr. G. Chen

Ultracold atomic and molecular systems provide another but very different fertile ground for looking for novel quantum phenomena. The most distinct and exciting part in this field is the tunability of experimental parameters and the new probing methods (that are special to the atomic systems). Both (especially the former) are often very difficult in a regular solid state system. For example, magnetic or optical Feshbach resonance can vary the effective interaction from weak to strong in a continuous fashion. A well-known application is the unitary boson or fermion gas with infinite scattering length that will be discussed in the next section. The SU(N) Heisenberg model and Hubbard model are a very good example of quantum many- body problems that can be realized in ultracold atomic and molecular systems but are almost impossible in solid state systems. We propose an novel chiral spin liquid phase for a SU(N) Hubbard model on an optical lattice. In cold atom systems, new experimental probes (like noise correlation, quenched measurement, etc) are available. We want to understand the experimental consequence of the various quantum phases in these new measurements. In general, we are interested in the many-body problems that can be realized in cold atom systems and also support interesting experimental consequences.

### Project ZYM01: Numerical investigations in the zoo of correlated topological state of matter

#### Supervisor: Dr. Z.Y. Meng

In this project, we will make use of large-scale quantum Monte Carlo simulations and theoretical analysis to study interacting electron systems and pursue the understanding of interaction effects on topological state of matter, such as the validity of topological index in the interaction-driven topological phase transitions, the identification and classification of emergent bosonic and fermionic symmetry protected topological phases in interacting models [such as in Phys. Rev. B 93, 115150 (2016)]; to reveal the duality relations between the interaction-driven topological phase transition and the deconfined quantum critical point via numerical investigations [such as in Phys. Rev. X 7, 031052 (2017)]; to discover quantum spin liquids, representatives of topological ordered states, with our large-scale quantum Monte Carlo simulations on frustrated spin systems [such as in Phys. Rev. Lett. 121, 057202 (2018)]; and to discover the manifestation of the symmetry fractionalization and emergent gauge structures in these topological ordered phases [such as in Phys. Rev. Lett. 121, 077201 (2018), Phys. Rev. Lett. 120.167202 (2018)].

### Project ZYM02: Dynamical signatures in frustrated systems and quantum magnetism

#### Supervisor: Dr. Z.Y. Meng

With the fast development of modern computational technology, we are now able to compute the excitation spectrum in quantum magnetic systems and provide explanation beyond simple mean-field analysis on the nature of exotic magnetic excitations [such as in Phys. Rev. X 7, 041072 (2017)]. A particular interesting point is that we could calculate the magnetic spectra of frustrated magnetic systems to reveal the existing of the topological order and fractionalized excitations, including Z2 quantum spin liquid in kagome lattice [such as in Phys. Rev. Lett. 121, 077201 (2018)] and U(1) quantum spin liquid in pyrochlore lattice [such as in Phys. Rev. Lett. 120.167202 (2018)]. Moreover, the new types of quantum phase transitions, that are beyond the Landau-Ginzburg-Wilson paradigm of phase and matter, can be also investigated in large-scale quantum Monte Carlo simulations. Example including deconfined quantum critical point, in which the emergent spinon and gauge field are strongly coupled with each other [such as in Phys. Rev. B 98, 174421 (2018) Editors' Suggestion]. We will continue our pursuit along this line to build the new paradigm of quantum phase transitions.

### Project ZYM03: Fundamental properties of metallic quantum critical point

#### Supervisor: Dr. Z.Y. Meng

Landau’s Fermi-liquid theory is the cornerstone in the condensed matter physics. However, in many modern correlated electron systems, ranging from Cu- and Fe-based superconductors, heavy-fermion compounds and the recently discovered twist angle graphene layer systems, metallic behaviors that deviated from the Fermi-liquid paradigm are universally presented, such as pseudogap, anomalous transport and vanishing of quasiparticle fractions. These novel phenomena, associated with quantum critical fluctuations coupled to low-energy fermionic degrees of freedom, are dubbed non-Fermi-liquid in the metallic quantum critical regions.

In this project, we will develop relevant models and numerical methodologies to study various metallic quantum critical points, such as ferromagnetic, antiferromagnetic and nematic fluctuations coupled to different Fermi surface geometries. With the help of numerical method developments, such as the self-learning Monte Carlo invented by us, and the guidance of advanced field-theoretical approaches, we will be able to address the problem of fermions coupled to critical bosons, although highly non-perturbative in nature, with better affirmative than previously known.

Furthermore, many aspects of frustrated magnetism and deconfined quantum critical points also belong to similar setting of fermion and boson coupled systems at their quantum criticality, for example, emergent fractionalized anyons (spinons and visons) coupling with emergent gauge fields in frustrated magnets and deconfined quantum criticality, can also be addressed with aforementioned combined numerical and theoretical approaches [for example, see Refs. Phys. Rev. X 9, 021022 (2019) and Phys. Rev. B 98, 174421 (2018) Editors' Suggestion] . Therefore the outcome of this project will give rise to building a bulk of the new paradigms in quantum matter that are beyond Fermi liquid theory for metals and the Landau-Ginzburg-Wilson framework of phases and phase transitions.

### Project ZYM04: Thermodynamics and dynamics in quantum magnets

#### Supervisor: Dr. Z.Y. Meng

To understand the experimental results in quantum magnetic systems and in particular the frustrated ones, in which the putative quantum spin liquid state might emerge, it is of vital importance that thermodynamic and dynamic results can be captured and explained in unbiased quantum many-body calculations. This is a new research direction in which both the understanding of experiment results including the material properties and measurement details, and more importantly, the quantum many-body methodologies that could capture the thermodynamic and dynamic responses, are required to their best level.

In this project, we will employ and develop Density Matrix Renormalization Group (DMRG) and Tensor-network Renormalization Group (TRG) methods, combined with quantum Monte Carlo (QMC) calculations, to find way to calculate phase transition and thermodynamic properties of quantum many-body models, and then compare the obtained results with experimental results of promising quantum magnetic compounds which might realize quantum spin liquid states or other novel quantum many-body phases and phase transitions. These comparisons would help us to find the correct model description of the quantum magnetic systems and could eventually lead to discovery of quantum states of matter that are beyond the Landau-Ginzburg-Wilson paradigm of phases and phase transitions.

### Project ZYM05: Towards next-generation scientific computing via neuromorphic-AI accelerators

#### Supervisor: Dr. Z.Y. Meng

The futuristic advancement in technology will involve, to a large extent, the engineering of artificial intelligence into almost all aspects of our industry. The widespread adoption of AI is becoming increasingly challenging to 1) remain sustainable at the current power consumption rate, and 2) become comparable with human intelligence.

As a first step, we need to establish a datacenter that is capable of neuromorphic-AI acceleration within the design of modern-age Infrastructure-as-a-Service (IaaS) / Platform-as-a-Service (PaaS) architecture. Whilst the core research will be done in the Jupyter-Python layer -- dockerized within Kubernetes, the target architecture is one that is resilient, which is capable of handling bigdata and service redundancy. With Elastic schema-free NoSQL database and Kafka/Solace bigdata messaging bus (with Golang/gRPC proxy), our core research is immediately deployable as business logic implemented within Java-Spring connected via Kafka. The server-client architecture ensures that our research is architectural compatible and integrable with current modern-age technologies, especially Google APIs. This connects the possibilities of industrial-standard AI technologies such as Dialogflow and Tensorflow. For the purpose of core research, the performance of Python code can further be enhanced with C++. Last but not least, the data I/O will be streamed to/from the neuromorphic accelerators via the underlying Kafka/Solace architecture.

### Project SQS01: 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 CJW01: Topological Phases of Matter with Strong Correlation

#### Supervisor: Dr. C.J. Wang

Topological phases of matter have gained lots of attention due to their richness and wide connections to other fields of physics. In particular, in certain systems, there exist so-called non-Abelian anyon excitations that can be used for fault-torrent quantum computation. While topological phases with weak correlation can be well understood through conventional mean field theories, it requires many new concepts and tools to understand strongly correlated topological phases. We work on two general aspects of topological phases: (i) fundamental theories of topological phases, in particular in higher dimensions and (ii) search of anyons in experimental systems such as fractional quantum Hall liquids and quantum spin liquids. More specifically, we will investigate the deep interplay between symmetry and topology --- two key fundamental concepts in modern physics --- in various quantum systems. Also, we study quantum transport properties for detecting experimental topological systems. When it comes to realistic models, we also plan to perform numerical studies, e.g., using algorithms based on tensor network states.

### Project JW01: 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 ZDW02: 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 WY01: 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 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 physics of valley and spin and their control in 2D materials and their van der Waals heterostructures by external magnetic, electric and optical fields. We will also explore the exciting opportunities to manipulate valley and spin from their emergent properties in the moiré patterns formed by the inevitable lattice mismatch and twisting between the 2D building blocks in heterostructures.

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__Experimental Nuclear and Particle Physics group__

__Experimental Nuclear and Particle Physics group__

### Project JHCL01: Spectroscopy of neutron-rich Ca isotopes

#### Supervisor: Dr. J.H.C. Lee

We will perform in-beam gamma spectroscopy measurements of ^{56}Ca and ^{53, 55}Ca 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 ^{56}Ca extends the systematic studies of the energies of 2_{1}^{+} and other low-lying states beyond ^{54}Ca (N=34). The location of 2_{1}^{+} energy of ^{56}Ca gives a direct measure of the difference between 0^{+} and 2^{+} two-body matrix elements in the f_{5/2}^{2 }which has not yet been determined. This new experimental data is also valuable in accessing the accuracy of the calculated E_{x}(2_{1}^{+}) of ^{56}Ca using different effective interactions in shell-model theories and ab-initio calculations. The spectroscopy of ^{53, 55}Ca could reflect the nature of the N=34 shell closure and the contribution of the g_{9/2 }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.

### Project YJT01: Searching for Supersymmetry at the Large Hadron Collider

#### Supervisor: Dr. Y.J. Tu

The Standard Model (SM) works 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 (SUSY) 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 10^16 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, H plus the lightest neutralino or gravitino (Dark Matter candidate), where W, H further decay, the final state will contain leptons plus missing transverse momentum. The same final states also appear in the Heavy Higgs association production. Therefore, the projects are not only key searches for SUSY, but also good probes for Dark Matter and beyond the SM Higgs physics.

### Project YJT02: Searching for Higgs Beyond the Standard Model at the Large Hadron Collider

#### Supervisor: Dr. Y.J. Tu

The Standard Model (SM) works 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 the 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.