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Spring 2021 Virtual Physics Colloquium
Next Talk: 21 Apr 2021 (Wed) 10:00 a.m.


Date:January 27 - May 5, 2021 (Every other Wednesday)
1. January 27, 2021 (Wed) 5:00 p.m.
2. February 10, 2021 (Wed) 5:00 p.m.
3. February 24, 2021 (Wed) 10:00 a.m.
4. March 10, 2021 (Wed) 5:00 p.m.
5. March 24, 2021 (Wed) 10:00 a.m.
6. April 7, 2021 (Wed) 10:00 a.m.
7. April 21, 2021 (Wed) 10:00 a.m. *Next Talk*
8. May 5, 2021 (Wed) 5:00 p.m.

PhysicsColloquium

For the spring semester of 2021, the virtual colloquium will happen on every other Wednesday either at 10-11am or 5-6pm. The detailed schedule and talk information are as below.
For inquiries or suggestions of future speakers, please contact the colloquium working group (Dr. Jane Lixin Dai, Dr. Tran Trung Luu, Dr. Yanjun Tu, Dr. Chenjie Wang, and Dr. Shizhong Zhang).

1. January 27, 2021 (Wednesday) 5:00 p.m.
2. February 10, 2021 (Wednesday) 5:00 p.m. 
3. February 24, 2021 (Wednesday) 10:00 a.m. 
4. March 10, 2021 (Wednesday) 5:00 p.m.
5. March 24, 2021 (Wednesday) 10:00 a.m.
6. April 7, 2021 (Wednesday) 10:00 a.m.
7. April 21, 2021 (Wednesday) 10:00 a.m. *Next Talk*
8. May 5, 2021 (Wednesday) 5:00 p.m.

Next Generation Materials for Solar Energy Conversion

Speaker: Prof. James Durrant
Affiliation: Imperial College London
Date: January 27, 2021 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://hku.zoom.us/j/91321297721?pwd=U1k1b0ZhR3Y5R2JUWmEvVWFvUm5mUT09
Meeting ID: 913 2129 7721
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210127_jd
Abstract:
The development of renewable, low cost energy technologies is now widely considered to be a key scientific challenge for the 21st century. My group’s primary research interest is the development of new chemical approaches to solar energy conversion – harnessing solar energy either to produce electricity (photovoltaics) or molecular fuels (e.g. hydrogen). We undertake fundamental scientific studies of new materials and device concepts, aiming to elucidate design principles which enable technological development. Our research is based around using transient laser spectroscopies to undertake transient optical and optoelectronic studies of light driven electron and energy transfer reactions. Such studies are undertaken in parallel with device development and functional characterisation, employing a wide range of molecular, polymeric and inorganic materials. In my talk, I will start by introducing some of the new materials currently under development for solar energy conversion, identifying the key opportunities and challenges. I will then go on to address the lessons for material and device design which can be learnt from transient spectroscopy studies – focusing in particular on energy and electron transfer kinetics, drawing upon examples of my group’s research on organic and perovskite solar cells, as well as metal oxides, conducting polymers and carbon nitrides for photoelectrochemical and photocatalytic fuel synthesis.

Heat Flow in Quantum Hall States

Speaker: Prof. Moty Heiblum
Affiliation: Weizmann Institute of Science
Date: February 10, 2021 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://hku.zoom.us/j/93129507589?pwd=NXJ5M3NaUVV5STVSY0xNYkJVd05UQT09
Meeting ID: 931 2950 7589
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210210_mh
Abstract:
Quantum mechanics sets an upper bound on the amount of charge flow as well as on the amount of heat flow in ballistic one-dimensional channels. The two relevant upper bounds, that combine only fundamental constants, are the quantum of the electrical conductance and the quantum of the thermal conductance. Remarkably, the latter does not depend on particles charge; particles statistics; and even the interaction strength among the particles.
Unlike the relative ease in determining accurately the quantization of the electrical conductance, measuring accurately the thermal conductance is more challenging - as heat flow is not conserved, and accurate and noninvasive temperature measurements are not trivial.
The universality of the thermal conductance was already demonstrated for weakly interacting particles: phonons, photons, and electronic Fermi-liquids. I will describe our work on thermal conductance measurements in the fractional QHE regime. I will concentrate on the method used in the measurements, which were focused on Laughlin’s states and hole-conjugate states – in order to prove the universality of the thermal conductance (which violates the Wiedemann-Franz conjecture). The studies were extended to fractional states in the first-excited Landau level, and in particular on the five half state, which was predicted to be non-abelian. We found a fractional thermal conductance coefficient, indeed proving the non-abelian character of the state. Note our observed topological order of the state (PH-Pfaffian) was not expected in current numerical works (which expect the anti-Pfaffian order). However, our current measurements (using a different method) strengthen the observed topological order.

Physics of the Large Hadron Collider 

Speaker: Prof. Claudio Campagnari
Affiliation: Chair of Physics Department, University of California, Santa Barbara
Date: February 24, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/98767573700?pwd=bzJ3SXh5aEJsQnozMHIwVi8zZ3VVZz09
Meeting ID: 987 6757 3700
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210224_cc
Abstract:
The Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) has now been operating for almost a decade. The highlights of the program have been the discovery of the Higgs Boson in 2012 and the first searches for new physics at the Tera-electronVolt energy scale. In the next decade the LHC is expected to deliver a factor of 30 more data than what have been collected to date. In this talk we will review the most important results and lessons learned from the data taken in the 2010s, and discuss the prospects for the science that will become accessible with the larger dataset.

BCS-BEC Crossover in a 2D Superconductor

Speaker: Prof. Yoshihiro Iwasa
Affiliation: QPEC & Department of Applied Physics, University of Tokyo & RIKEN Center of Emergent Matter Science
Date: March 10, 2021 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://hku.zoom.us/j/91059281191?pwd=WUVuZ2ZtaTRDS0Fla2VlcldYZ2NjZz09
Meeting ID: 910 5928 1191
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210310_yi
Abstract:
The Bardeen-Cooper-Schrieffer (BCS) condensation and the Bose-Einstein condensation (BEC) are the two extreme limits of the ground state of the paired fermion systems, which are theoretically predicted to continuously connected through an intermediate regime [1]. We report the two-dimensional (2D) BCS-BEC realized in a gate-controlled superconductor, electron doped layered material ZrNCl. To observe this phenomenon, we utilized an ionic gating method, which is well known as a powerful tool to control the carrier density in a large scale and induced 2D superconductivity [2].
We have succeeded in controlling the carrier density by nearly two-orders of magnitude, and established an electronic phase diagram through the simultaneous experiments of resistivity and tunneling spectra on the ionic gating devices. We found Tc exhibits dome-like behavior, and more importantly, a wide pseudogap phase was discovered in the low doping regime. In the low carrier density limit, Tc scales as Tc/TF = 0.12, where TF is the Fermi temperature [3], which shows fair agreement with the theoretical prediction in the 2D limit of BEC [4].
References:
[1] M. Randeria and E. Taylor, Annu. Rev. Condens. Matter Phys. 5, 209 (2014).
[2] Y. Saito, T. Nojima and Y. Iwasa, Nat. Rev. Mater. 2, 16094 (2017).
[3] Y. Nakagawa et al., arXiv:2012.05707
[4] S. S. Botelho and C. A. R. Sá de Melo, Phys. Rev. Lett. 96, 040404 (2006).

Supermassive Black Holes and Low-Frequency Gravitational Waves

Speaker: Prof. Chung-Pei Ma
Affiliation: University of California, Berkeley
Date: March 24, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/99973046894?pwd=bFpHTW51Yy9GbkpaUmQwVFJKSjNvUT09
Meeting ID: 999 7304 6894
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210324_cpm
Abstract:
Supermassive black holes are a fundamental component of galaxies. Residing at the centers of galaxies, these black holes have masses up to tens of billion suns and directly impact the evolution of their host galaxies. Professor Ma will describe recent progress in discovering new populations of massive black holes, and the implications for the theoretical understanding of the symbiotic relationships between black holes and galaxies. She will discuss the prospects for detecting low-frequency gravitational waves from merging binaries of supermassive black holes in the next decade.

Gravitational Wave Sources at the Hearts of Galaxies

Speaker: Prof. Smadar Naoz
Affiliation: University of California at Los Angeles
Date: April 7, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/98351009114?pwd=bFpHTW51Yy9GbkpaUmQwVFJKSjNvUT09
Meeting ID: 983 5100 9114
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210407_sn
Abstract:
The recent gravitational-wave detections by LIGO/Virgo revolutionized the way we sense our Universe. However, it remains challenging to explain the formation channels of these sources. Motivated by these challenges, recent studies have emphasized the significant contribution of dynamical formation channels in dense stellar environments to the overall gravitational-wave signals. Focusing on the dense stellar clusters surrounding supermassive black holes at the center of galaxies, I will outline stellar binaries' evolution from birth up to possible gravitational-wave mergers. The supermassive black hole can induce collisions between binary members, while the frequent interactions with the neighbors in this dense environment can sometimes tend to unbind the binary. I will highlight some exotic outcomes, including gravitational-wave emission, for this dynamical evolution channel. I will show how this channel can leave a clear signature on the gravitational-wave signals, allowing differentiation between different merger mechanisms. The Laser Interferometer Space Antenna (LISA) can potentially be used to distinguish between channels.

Emergent Pseudo-Gauge Field in Dirac Materials *Next Talk*

Speaker: Prof. Chaoxing Liu
Affiliation: Department of Physics, Pennsylvania State University
Date: April 21, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/98656531617?pwd=VlBUMU5kcDJXd0pOTFhCMENQSmF2Zz09
Meeting ID: 986 5653 1617
Password: 2859
Abstract:
Electrons in solids are usually described by non-relativistic Schrodinger equation since electron velocity is much slower than the speed of light. However, the relativistic Dirac/Weyl equation can emerge as a low energy effective theory for electrons in certain solid materials. These systems are dubbed “Dirac materials” and provide a tunable platform to test quantum relativistic phenomena in table-top experiments. More interestingly, different types of perturbations in these Dirac materials, such as magnetic fluctuations, lattice vibration, strain, and material inhomogeneity, can couple to relativistic electrons in a similar form as the minimal gauge coupling. We thus dubbed these types of perturbations as the emergent “pseudo-gauge field”. It can be shown that under certain condition, the pseudo gauge field mimics the axial gauge field in quantum electrodynamics and thus can lead to rich and intriguing physical phenomena in these Dirac materials. In this talk, I will show that the concept of pseudo-gauge field in these Dirac materials can help us to predict a variety of physical phenomena, including chiral modes in a magnetic vortex core of Weyl semimetals [1] or in inhomogeneous optical Weyl metamaterials [2], topological piezo-magnetoelectric response [3,4], and Berry-curvature induced phonon dynamics. Moreover, exploring physical phenomena induced by pseudo-gauge field in Dirac materials may also deepen our understanding of some fundamental physics, such as chiral anomaly and axion electrodynamics.
References:
[1] Physical Review B, 2013, 87(23): 235306;
[2] Science, 363, 148-151 (2019);
[3] Nature Communications 11, 2290 (2020);
[4] ArXiv:2008.10620, 2020.

Spin-orbit Interactions and Topology of Electromagnetic Fields

Speaker: Prof. Anatoly V. Zayats
Affiliation: Department of Physics and London Centre for Nanotechnology, King’s College London
Date: May 5, 2021 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://hku.zoom.us/j/92484383141?pwd=TERVbVgwNFVrcVZUMWVNRzBVcE9adz09
Meeting ID: 924 8438 3141
Password: 2859
Abstract:
Photonic spin-orbit interactions are responsible for coupling of spin angular momentum of light, associated with circular polarisation of an electromagnetic wave, to orbital angular momentum, associated with the energy flow and propagation direction. Being strongly enhanced in a nanostructured environment, spin-orbit coupling provides interesting and important applications in polarisation-enabled control of optical signals, or in reverse, controlling light polarisation, optical forces, sensing applications and quantum optical processes. Near nanostructures capable of supporting waveguided modes, the spin-orbit coupling is mediated by the transverse spin carried by evanescent waves: electric field spins around an axis perpendicular to the wavevector, with the spinning sense determined solely by the propagation direction. The spin-orbit coupling in such type of modes, results in the so-called photonic spin-Hall effect, in analogy to spin-Hall effect for electrons.
In this talk we will overview the effects associated with the photon spin and angular orbital momenta when light beams interact with plasmonic nanostructures and metamaterials. Various unusual dipolar sources which either are forbidden to excite waveguiding modes or allow complete freedom in tailoring excitation of multiple waveguided modes with a single dipolar source will be reviewed. Spin coupling to orbital angular momentum in complex beams will also be discussed, resulting in deep-subwavelength features of the resulting photonic textures, termed photonic skyrmions. Photonic spin-orbit interactions provide an important tool for harvesting new functionalities and applications of polarised light in numerous photonic and quantum technologies and metrology.

Anyone interested is welcome to attend.


Emergent Pseudo-Gauge Field in Dirac Materials