Past Events

The Physics Colloquium has been arranged regularly since the fall semester of 2021. The detailed schedule and talk information of this semester 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).

Neutrino Astronomy: From Dream to Reality

Speaker: Prof. Naoko KURAHASHI NEILSON
Affiliation: Drexel University
Date: September 13, 2023 (Wednesday)
Time: 10:00 a.m.
Zoom Link: https://shorturl.at/wxy57
Meeting ID: 962 1808 1400
Password: 2859
Poster: Download
Abstract:
The Universe has been studied using light since the dawn of astronomy, when starlight captured the human eye. The IceCube Neutrino Observatory, located at the geographic South Pole, observes the Universe in a different and unique way: in high-energy neutrinos. IceCube's discovery in 2013 of a diffuse celestial neutrino radiation, in other words, high-energy neutrinos from beyond the solar system, started an era of neutrino astronomy. Searches for astronomical sources responsible for creating these neutrinos have covered broad source types while combating background event rates that are 6 orders of magnitude higher.

1.  https://www.science.org/doi/10.1126/science.adc9818

Unconventional superconductivity beyond the BCS paradigm 

Speaker: Prof. Hai-Hu WEN
Affiliation: Nanjing University
Date: September 27, 2023 (Wednesday)
Time: 4:00 p.m.
Venue: MB237, 2/F, Main Building, The University of Hong Kong
Poster: Download
Abstract:
High temperature superconductors and related pairing mechanism are the very basis for large scale applications of superconductors. There are two family members of unconventional high temperature superconductors: cuprates and iron based superconductors (IBS). Their parent phase seems different, but anti-ferromagnetism is a common feature. In IBS, there are multibands, thus an S± pairing manner was proposed basing on the picture by exchanging anti-ferromagnetic spin fluctuations. However, due to the distinct Fermi surface contours in different systems, it remains to know whether there is a universal pairing mechanism in all IBS. We first show the solid evidence of S± pairing by using the non-magnetic quantum impurities [1], then we show bosonic modes on the tunneling spectra in two types of IBS, which can also be traced back to the spin fluctuation induced pairing[2]. In some IBS with only electron pockets, we used the phase referenced quasiparticle interference technique to reveal that the repulsive interaction, namely exchanging spin fluctuations, is still the driven pairing mechanism[3,4]. We also used this technique to confirm that the d-wave pairing gap in a typical cuprate superconductor Bi-2212 [5], indicating a strong magnetic coupling induced pairing. In IBS superconductors, the Fermi energies are generally quite small, which shows the strong deviation from the weak coupling BCS theory and possible crossover to the BEC scenario. We show the consequence of the small Fermi energy in IBS [6,7]. Recently, superconductivity was discovered in thin films of the infinite-layer nickelate Nd_1-xSr_xNiO₂. Here we report single particle tunneling measurements on the superconducting nickelate thin films. We find predominantly two types of tunneling spectra, one shows a d-wave gap function, another one exhibits a full gap [8]. We also reveal the strong local pairing in very underdoped cuprates [9,10]. Finally we give a perspective about unconventional superconductivity and possible route to explore more high temperature superconductors.
Key References:
1. Huan Yang, et al., Nature Communications 4, 2947 (2013).
2. Zhengyu Wang, et al., Nature Physics 9, 42(2013).
3. Zengyi Du, et al., Nature Communications 7, 10565(2016).
4. Zengyi Du, et al., Nature Physics 14, 134 (2018).
5. Qiangqiang Gu, et al. Nature Communications 10, 1603 (2019).
6. Mingyang Chen, et al., Nature Communications 9, 970(2018). 
7. Xiaoyu Chen et al., Phys. Rev. Lett. 126, 257002(2021).
8. Qiangqiang Gu, et al. Nature Communications 11, 6027(2020).
9. Siyuan Wan, et al. PNAS 118, No. 51, e21153171182021 (2021).
10. H. Z. Li, et al. npj Quantum Materials 8, 18 (2023).

Probing dark matter self-interactions with collapsed dark matter halos

Speaker: Prof. Yiming ZHONG
Affiliation: City University of Hong Kong
Date: October 25, 2023 (Wednesday)
Time: 4:00 p.m.
Venue: MB237, 2/F, Main Building, The University of Hong Kong
Poster: Download
Abstract:
Over the past decades, we have accumulated compelling evidence for the existence of dark matter, yet its nature remains elusive. Dark matter could be part of the "dark sectors," comprising hidden particles with new interactions. These new particles and interactions can alter the formation and evolution of various small-scale structures in the Universe. This talk will focus on the impacts of dark matter self-interactions, a common feature within the dark sector paradigm, on the properties of dark matter halos. Self-interacting dark matter halos experience gravothermal evolution, where the central halo initially forms a core that ultimately collapses. The core-collapsed phase diversifies the density distributions of dark matter halos. It also showcases the universality in the evolution of self-interacting dark matter halos as thermal systems. Current and future observations, such as the James Webb Space Telescope and the Large Synoptic Survey Telescope, could probe the properties of the collapsed dark matter halos through strong lensing and weak lensing and shed light on dark matter self-interactions. Furthermore, the collapsed halos may give birth to the supermassive black holes observed at high redshifts, a longstanding puzzle in astrophysics.
Key References:
1. R. Essig, S. D. Mcdermott, H.-B. Yu, and Y.-M. Zhong, Phys. Rev. Lett. 123, 121102 (2019), 1809.01144.
2. W.-X. Feng, H.-B. Yu, and Y.-M. Zhong, Astrophys. J. Lett. 914, L26 (2021), 2010.15132.
3. D. Gilman, Y.-M. Zhong, and J. Bovy, Phys. Rev. D 107, 103008 (2023), 2207.13111.
4. Y.-M. Zhong, D. Yang, and H.-B. Yu, Mon. Not. Roy. Astron. Soc. 526, 758 (2023), 2306.08028.

Photo-induced hidden states in quantum materials: from high-Tc superconductor to colossal magnetoresistant materials

Speaker: Prof. Jingdi ZHANG
Affiliation: The Hong Kong University of Science and Technology 
Date: November 8, 2023 (Wednesday)
Time: 4:00 p.m.
Venue: MB237, 2/F, Main Building, The University of Hong Kong
Poster: Download
Abstract:
In quantum materials, exotic quantum states can emerge as a result of strong many-body interaction that are of charge, magnetic, orbital and structural origins. The delicate balance among these interacting degrees of freedom engenders not only a ground state, but also many other competing metastable states with distinct macroscopic properties. Despite static tuning methods, the rapidly developing ultrafast science has now made it possible to dynamically control quantum materials at an unprecedented level, that is, the direct manipulation of elementary excitations at their fundamental time and energy scales. Here, we show examples on how ultrafast laser excitation can lead to enhanced superconductivity in copper oxide and unconventional superconductors. The other example is shot-by-shot laser control of hidden metastable states in manganite thin films, evidenced by combined methods of ultrafast terahertz spectroscopy, scanning near-field microscopy and X-ray scattering.
Key References:
1. J. Zhang and R. D. Averitt, Annu. Rev. Mater. Res. 44, 19 (2014)
2. K. Cremin, et. al, Proc. Natl. Acad. Sci. 116, 19875 (2019)
3. J. Zhang, et. al, Nat. Mater. 15, 956 (2016); A. S. McLeod, J. Zhang, M. Gu et al. Nat. Mater. 19, 297 (2020)

Comets, unseen planets, and the outer fringes of the solar system

Speaker: Prof. Scott D. TREMAINE
Affiliation: Institute for Advanced Study, Princeton
Date: November 14, 2023 (Tuesday)
Time: 2:00 p.m.
Venue: CPD-LG.10, Centennial Campus,  The University of Hong Kong
Poster: Download
Abstract: Comets have inspired awe since prehistoric times, but even today there are only a few thousand comets with well-determined orbits. Nevertheless, the analysis of this limited sample yields a compelling model for the formation, evolution and present distribution of comets. This model implies that the primary source of comets is the Oort cloud, containing over 100 billion comets at 5,000 to 100,000 times the Earth-Sun distance. I will review our current understanding of the history and structure of the Oort cloud, and what comets can tell us about possible undiscovered planets beyond Neptune.
Key Reference:
1. Nathan Kaib and Kathryn Volk, Dynamical Population of Comet Reservoirs, arXiv:2206.00010

Quantum Geometry Effects in Topological and Flat Band Superconductors

Speaker: Prof. Kam Tuen LAW
Affiliation: The Hong Kong University of Science and Technology 
Date: Novmerber 22, 2023 (Wednesday)
Time: 4:00 p.m.
Venue: MB237, 2/F, Main Building, The University of Hong Kong
Poster: Download
Abstract: The study of quantum geometric properties of Bloch wavefunctions in crystals has deepened our understanding of quantum states of matter. The quantum geometry of a material is characterized by the quantum geometric tensor, the imaginary part of which defines the Berry curvature. The study of Berry curvature effects in condensed matter systems has been a central topic in physics for several decades. However, much less attention has been given to the real part of the quantum geometric tensor, known as the quantum metric.

Key References:
1. The Ginzburg-Landau theory of flat band superconductors with quantum metric, Shuai A. Chen, K. T. Law, arXiv:2303.15504.
2. Anomalous Coherence Length in Superconductors with Quantum Metric, Jin-Xin Hu, Shuai A Chen, KT Law, arXiv:2308.05686.

The Golden Age of Neutron Stars

Speaker: Prof. Gordon BAYM
Affiliation: University of Illinois
Date: December 1, 2023 (Friday)
Time: 4:00 p.m.
Venue: CYPP3, LG1, Chong Yuet Ming Physics Building, The University of Hong Kong
Poster: Download
Abstract: Neutron stars were first posited in the early thirties, and discovered as pulsars in the late sixties; however we are only recently beginning to understand the matter they contain.  I will describe the ongoing development of a consistent picture of the liquid interiors of neutron stars, now driven by ever increasing observations as well as theoretical advances.  These include in particular observations of at least three heavy neutron stars of about 2.0 solar masses and higher; ongoing simultaneous inferences of masses and radii of neutron stars by the NICER telescope; and past and future observations of binary neutron star mergers, through gravitational waves as well as across the electromagnetic spectrum.   Theoretically an understanding is emerging in QCD of how nuclear matter can turn into deconfined quark matter in the interior, and be capable of supporting heavy neutron stars, which I will illustrate with a discussion of modern quark-hadron crossover equations of state.

Quantum Networks: From a physics experiment to a quantum network system

Speaker: Prof. Stephanie WEHNER
Affiliation: QuTech, Delft University of Technology
Date: December 6, 2023 (Wednesday)
Time: 5:00 p.m.
Zoom Link: https://rb.gy/z34fta
Meeting ID: 957 8712 9577 
Password: 2859
Poster: Download
Abstract: The internet has had a revolutionary impact on our world. The vision of a quantum internet is to provide fundamentally new internet technology by enabling quantum communication between any two points on Earth. Such a quantum internet can —in synergy with the “classical” internet that we have today—connect quantum information processors in order to achieve unparalleled capabilities that are provably impossible by using only classical information.