Past Events

For the fall 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. September 1, 2021 (Wednesday) 5:00 p.m.
2. September 15, 2021 (Wednesday) 10:00 a.m.
3. September 29, 2021 (Wednesday) 10:00 a.m.
4. October 13, 2021 (Wednesday) 5:00 p.m. *Cancelled*
5. October 27, 2021 (Wednesday) 10:00 a.m.
6. November 10, 2021 (Wednesday) 5:00 p.m. 
7. November 24, 2021 (Wednesday) 10:00 a.m. 
8. December 8, 2021 (Wednesday) 10:00 a.m.

Formation of Coalescing Double Compact Object: What Have We Learnt from Gravitational-wave Observations So Far

Speaker: Prof. Tassos Fragos
Affiliation: University of Geneva
Date: September 1, 2021 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://hku.zoom.us/j/91670913765?pwd=R3czQmkvdzYvRjcyZmRYR0xqbWhvdz09
Meeting ID: 916 7091 3765
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210901_tf
Abstract:
The detection of gravitational waves from coalescing binary black holes by the LIGO/Virgo observatories network allowed for the first time the direct observation of stellar-mass black holes, while the simultaneous gravitational wave and electromagnetic signal from the merger of two neutron stars provided the first direct evidence for origin of short gamma-ray bursts. These gravitational wave events, complemented by a half-a-century-long history of indirect observations of accreting compact objects in X-ray binaries, can give us now a more complete picture of the formation and evolution of binary stellar systems containing compact objects. At the same time, they also revealed weaknesses of the theories of stellar structure, binary evolution and compact object formation. In this talk, I will briefly review the current observed sample of gravitational-wave detections (the GWTC-2 catalogue) and their astrophysical implications, and discuss the different formation pathways that have been proposed in the literature to explain the properties of the observed populations. I will then highlight recent results from recent studies that aim at constraining the formation theories of coalescing double compact objects by statistically comparing them with GWTC-2, as well as other potential electromagnetic counterparts or precursors. Finally, I will close with a discussion of the “next-generation” models of double compact object formation that we are currently working on and what we expect that we will learn from them.

Acoustic Metamaterials with Broadband Tunable Impedance Matching

Speaker: Prof. Nicholas Xuanlai Fang
Affiliation: Department of Mechanical Engineering, Massachusetts Institute of Technology
Date: September 15, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/91929879531?pwd=cEVVS3J5aytqQmFvamFRamdHNlZydz09
Meeting ID: 919 2987 9531
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210915_nxf
Abstract:
Recent development of acoustic metamaterials opens a door to an unprecedented large design space for acoustic properties such as negative bulk modulus, negative density, and refractive index. Such novel concept paves the way for the design of a new class of acoustic materials and devices with great promise for diverse applications, such as broadband noise insulation, sub-wavelength imaging and acoustic cloak from sonar detection.
In this talk, I will present our research progress on advanced design and micro/nanofabrication techniques, to enable exploration and rapid prototyping of architectured metastructures for acoustic waves. These structures show promise on focusing and rerouting ultrasound through broadband metamaterials. As an example, we report a class of impedance transformers to overcome the fundamental limit of narrowband transmission. We experimentally show that the transformer device offers efficient implementation in broadband underwater ultrasound detection with the benefit of being soft and tunable. The broadband impedance matched nonreflecting acoustic metamaterial can also robustly prohibit reflection and reverberation of airborne sound waves over a wide range of incident angles. I will also discuss the acoustic labyrinthine metamaterials which can exhibit extreme constitutive parameters and an exceptional ability to control the phase of sound at deep-subwavelength scale.

Nuclear Spin-isospin Physics: from Phenomena to Fundamentals

Speaker: Dr. Haozhao Liang
Affiliation: Department of Physics, The University of Tokyo
Date: September 29, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/94118563503?pwd=aDcwOTdrTFhLUkVaQ1hXUFJaWElzZz09
Meeting ID: 941 1856 3503
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20210929_hzl
Abstract:
Spin and isospin are essential degrees of freedom in nuclear systems, and the relevant studies on their properties play important roles not only in nuclear physics but also in nuclear astrophysics and particle physics. In this talk, I will first introduce the studies on nuclear spin-isospin properties in the framework of covariant density functional theory (DFT), on the topics of nuclear charge-exchange resonances, beta decays, isospin-symmetry breaking corrections to nuclear superallowed beta transitions, etc. For developing the next-generation nuclear DFT with more fundamental theories, I will also talk about an ongoing work about functional renormalization group and Kohn-Sham scheme in DFT.

Towards a Quantum Internet *Cancelled*

Speaker: Prof. Stephanie Wehner
Affiliation: QuTech, TU Delft
Date: October 13, 2021 (Wednesday)
Time: 5:00 p.m.
Venue: Rm 522, 5/F, Chong Yuet Ming Physics Building, HKU
Zoom Link: https://hku.zoom.us/j/96489097917?pwd=SUlKVWJjeVI1WUdQcis0YjdEL2t6Zz09
Meeting ID: 964 8909 7917
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20211013_sw
Abstract:
The vision of a Quantum Internet is to provide fundamentally new internet technology by ultimately enabling quantum communication between any two points on earth. Such a Quantum Internet would – in synergy with the ‘classical’ internet that we have today - connect quantum processors in order to achieve new capabilities that are provably impossible using classical communication.
In this talk, we provide an introduction to quantum (internet-) working, and present recent results on the path of taking such a network from a physics experiment to a quantum network system. We report on the realization of the first quantum link layer protocol, now implemented on quantum hardware based on Nitrogen-Vacancy Centers in diamond. We conclude by presenting several tools that can be used to examine the experimental requirements of building larger quantum network, and be used by you to learn more about quantum networking beyond this talk.

The Enigma of the Deconfined Quantum Critical Point 

The deconfined critical point has been proposed to describe "beyond Landau" quantum phase transitions between different ordered ground states of 2D quantum antiferromagnets. Despite years of efforts by many groups using computational model studies to characterize the ground-state transition between an antiferromagnet and a non-magnetic state with a pattern of singlets, the nature of this phase transition is still puzzling.
Moreover, a host of related phenomena that were not part of the original proposal have been discovered, e.g., emergent symmetries, unusual weak first-order transitions, helical dimer order, and possibly spin liquid phases terminating at deconfined critical points. I will summarize the current status of the deconfined critical point emerging from large-scale computer simulations.
I will also discuss recent experiments that finally show real promise to realize the deconfined critical point and many of its associated unusual quantum effects.

Ionic Gating of 2D Semiconductors 

Speaker: Prof. Alberto Morpurgo
Affiliation: University of Geneva
Date: November 10, 2021 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://hku.zoom.us/j/98010138189?pwd=cGxUaFZOM21xRVJHeGp6QjlqNktwdz09
Meeting ID: 980 1013 8189
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20211110_am
Abstract:
Ionic gating is a technique to implement electrostatic gating in electronic devices by using electrolytes to transfer very efficiently the electrostatic potential from a metallic electrode –the gate– to the surface of a semiconductor. The first mention of ionic gating can be found already in Bardeen’s Nobel lecture discussing the development of the transistor, but the technique actually started to be systematically developed approximately 15 years ago. After an initial period plagued by difficulties associated to problems of irreproducibility and incompatibility with different classes of materials, ionic gating has developed into a powerful technique, enabling experiments beyond what had been initially envisioned.

In my talk I will discuss different classes of experiments performed in my group, in which we apply ionic gating to 2D semiconductors, and most notably to many different 2D semiconducting transition metal dichalcogenides. After a general introduction, I will briefly touch upon gate induced superconductivity, showing that the use of van der Waals structures and the combination with different experimental probes allows sophisticated experiments to measure the density of states and to gain important information about electron-phonon coupling. I will then discuss in detail how ionic gating can be used as a precise quantitative spectroscopic technique, to measure band gaps of 2D semiconductors, as well as band offsets between different atomically thin materials. As a last topic, I will discuss very recent experiments in which we succeeded to realize double gated ionic transistors, providing independent control of the accumulated charge density and of the applied perpendicular electric field. I will show that these devices allow an electric field to be applied perpendicularly to atomically thin 2D semiconductors that is so large (in excess of 2 V/nm) to completely quench the 1.6 eV band gap of trilayer WSe₂. Our measurements show that, in the presence of such a large field, the conductance and valence band overlap, transforming the semiconductors in a semimetal (and possibly –according to theory– in a quantum spin Hall system).

Topological Metamaterials: New Physics and Applications 

Speaker: Dr. Yidong Chong
Affiliation: Nanyang Technological University
Date: November 24, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/98816374807?pwd=eWFmQW1IYXIvOTQ4bVJIRkx2VTlhUT09
Meeting ID: 988 1637 4807
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20211124_yc
Abstract:
Topological phases of matter, such as topological insulators and Weyl semimetals, originated in the study of quantum materials. Over the past decade, the special features of topological phases have been reproduced using various specially designed metamaterials, in which electromagnetic, acoustic, or electrical waves take over the role of the electronic wavefunction. In this talk, I will describe how these synthetic structures provide exciting new playgrounds for testing our theoretical understanding of topological phases. I will survey the possibilities for using topological metamaterials to realize phenomena that are challenging or impossible to access in real quantum materials, including engineered disorder, high spatial dimensions, nonlinearity, and non-Hermiticity. The talk will conclude with a discussion of the prospects for new technological applications, such as disorder-resistant waveguides, topological lasers, and efficient photon sources.

Fast Radio Bursts

Speaker: Prof. Victoria Kaspi
Affiliation: McGill University
Date: December 8, 2021 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://hku.zoom.us/j/95472595515?pwd=QjVBWHF6QzlnTktsd2psQ2trOGFyZz09
Meeting ID: 954 7259 5515
Password: 2859
Poster: https://www.physics.hku.hk/Seminar/Col_20211208_vk
Abstract:
Fast Radio Bursts (FRBs) are a newly recognized and puzzling phenomenon consisting of few-millisecond bursts of  radio waves coming from cosmological distances.  They are ubiquitous, with all-sky rates of roughly 1000 per day, signaling a not uncommon yet powerful explosion of presently unknown origin.  Recently there has been significant observational progress on understanding FRBs and recognizing their potential as novel cosmological probes, in large  part thanks to the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB).  In this talk I review what is known about FRBs, and describe some of the latest CHIME/FRB results related to FRB origin and distribution in the Universe.

Anyone interested is welcome to attend.