News and 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 follows.
For inquiries or suggestions of future speakers, please contact the colloquium working group (Prof. Jane Lixin Dai, Prof. Tran Trung Luu, Prof. Yanjun Tu, Prof. Chenjie Wang, and Prof. Shizhong Zhang).

Electron orbital dynamics in solids

Speaker: Prof. Hyun-Woo LEE
Affiliation: Department of Physics, POSTECH
Date: January 28, 2026 (Wednesday)
Time: 11:15 a.m.
Venue: CYCP1, LG1/F, Chong Yuet Ming Chemistry Building,  Main Campus, HKU

Poster: Download

Abstract:

For a long time, it has been believed that electrons' orbital angular momentum is quenched in solids unless induced by the spin-orbit coupling in magnetic materials. Contrary to common belief, recent studies [1-3] have revealed that electron eigenstates may have finite orbital angular momentum even in the absence of spin-orbit coupling when inversion symmetry is broken. In centrosymmetric systems, on the other hand, electron eigenstates do not have orbital angular momentum unless the spin-orbit coupling is strong. Nevertheless, a flow of electrons with finite orbital angular momentum is generated in a transverse direction when an electric field is applied (orbital Hall effect) [4-9]. The first part of this talk aims to present basic ideas of a few key orbital dynamics [10-12], such as the orbital Rashba-Edelstein effect [3], the orbital Hall effect [4-9], and orbital torque [13-16]. The second part deals with more recent topics such as the differences between spin and orbital dynamics [17], and orbital relaxation dynamics [18,19].

Key Reference:

1. S. R. Park, C. H. Kim, J. Yu, J. H. Han, and C. Kim, Orbital-angular-momentum based origin of Rashba-type surface band splitting, Phys. Rev. Lett. 107, 156803 (2011).
2. V. Sunko et al., Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking, Nature 549, 492 (2017).
3. A. E. Hamdi et al., Observation of the orbital inverse Rashba–Edelstein effect, Nat. Phys. 19, 1855 (2023).
4. B. A. Bernevig, T. L. Hughes, and S.-C. Zhang, Orbitronics: The intrinsic orbital current in 𝑝-Doped silicon, Phys. Rev. Lett. 95, 066601 (2005).
5. H. Kontani, T. Tanaka, D. D. Hirashima, K. Yamada, and J. Inoue, Giant orbital Hall effect in transition metals: Origin of large spin and anomalous Hall effects, Phys. Rev. Lett. 102, 016601 (2009).
6. T. Tanaka et al., Intrinsic spin Hall effect and orbital Hall effect in 4⁢𝑑 and 5⁢𝑑 transition metals, Phys. Rev. B 77. 165117 (2008).
7. D. Go, D. Jo, C. Kim, and H.-W. Lee, Intrinsic spin and orbital Hall effects from orbital texture, Phys. Rev. Lett. 121, 086602 (2018).
8. Y.-G. Choi et al., Observation of the orbital Hall effect in a light metal Ti, Nature 619, 52 (2023).
9. I. Lyalin, S. Alikhah, M. Beritta, P. M. Oppeneer, and R. K. Kawakami, Magneto-optical detection of the orbital Hall effect in chromium, Phys. Rev. Lett. 131, 156702 (2023).
10. D. Go, D. Jo, H.-W. Lee, M. Kläui, and Y. Mokrousov, Orbitronics: Orbital currents in solids, Europhys. Lett. 135, 37001 (2021).
11. D. Jo, D. Go, G.-M. Choi, and H.-W. Lee, Spintronics meets orbitronics: Emergence of orbital angular momentum in solids, npj Spintronics 2, 19 (2024).
12. R. B. Atencia, A. Agarwal, and D. Culcer, Orbital angular momentum of Bloch electrons: equilibrium formulation, magneto-electric phenomena, and the orbital Hall effect, Adv. Phys.: X, 9, 2371972 (2024).
13. D. Go, and H.-W. Lee, Orbital torque: Torque generation by orbital current injection, Phys. Rev. Res. 2, 013177 (2020).
14. J. Kim et al., Nontrivial torque generation by orbital angular momentum injection in ferromagnetic-metal/Cu/Al2O3 trilayers, Phys. Rev. B 103, L020407 (2021).
15. D. Lee, et al., Orbital torque in magnetic bilayers, Nat. Commun. 12, 6710 (2021).
16. R. Gupta et al., Harnessing Orbital Hall Effect in Spin-Orbit Torque MRAM, Nat. Commun. 16, 130 (2024).
17. S. Han, H.-W. Lee, and K.-W. Kim, Orbital dynamics in centrosymmetric, Phys. Rev. Lett. 128, 176601 (2022).
18. J. Sohn, J. M. Lee, and H.-W. Lee, Dyakonov-Perel-like orbital and spin relaxations in centrosymmetric systems, Phys. Rev. Lett. 132, 246301 (2024).
19. S. Peng et al., Unconventional scaling of the orbital Hall effect, Nat. Mater. 24, 1749 (2025).

Speaker: Prof. Zdeněk SOFER
Affiliation: University of Chemistry and Technology Prague
Date: February 11, 2026 (Wed) (Cancelled)

Speaker: Dr. Stanislav KRUCHININ
Affiliation: Microsoft Austria

Date: March 4, 2026 (Wed)
Time: 11:15 a.m.
Venue: CYCP1, LG1/F, Chong Yuet Ming Chemistry Building,  Main Campus, HKU

Poster: Download

Abstract:

Neural networks have progressed beyond their original role as computational tools for pattern recognition and data analysis. Contemporary large language models are reshaping the scientific workflow, transforming it from a linear sequence into an autonomous, data-driven cycle of discovery. This talk will examine this paradigmatic shift in the context of nanostructure physics and ultrafast photonics. Specifically, I will discuss how AI facilitates the research process, ranging from automated retrieval and summarization of scientific literature to accelerated hypothesis generation, design of materials, and real-time analysis of ultrafast spectroscopy data. By integrating AI across the entire research pipeline—from literature review to experimental implementation and theoretical analysis—it becomes possible to accelerate discovery and open new avenues for scientific workflow.

Key Reference:

1. Wang, H., Fu, T., Du, Y. et al. Scientific discovery in the age of artificial intelligence. Nature 620, 47–60 (2023). https://doi.org/10.1038/s41586-023-06221-2 
    

Speaker: Prof. Kohei INAYOSHI
Affiliation: Peking University
Date: March 18, 2026 (Wed)
Time: 10:30 a.m.
Venue: CPD-G.02, Central Podium Levels – Ground, Centennial Campus, HKU

Poster: Download

Abstract:

Little Red Dots (LRDs) are a newly identified population of active galactic nuclei (AGNs) uncovered by JWST deep surveys. Their enigmatic properties challenge the standard AGN paradigm and offer new clues to the early formation and rapid growth of massive black holes. LRDs show distinct UV-optical spectral energy distributions, including a clear V-shaped continuum with a universal turnover near 4000 Å, broad emission lines, and the absence of classical AGN signatures such as hot dust and X-ray corona emission. The coexistence of broad-line emission with prominent Balmer absorption and break features on LRD spectra, none of which can be explained by evolved stellar populations, suggests that nuclear black holes are deeply enshrouded by dense gas with a nearly 100% covering fraction. Such conditions can naturally reproduce the red optical and flat near-infrared continuum spectra without invoking dust reddening, when the absorbing gas clouds are optically thick and their reprocessed emission shapes a thermal spectrum. In this talk, I will review the physical processes regarding the gas-enshrouded AGN interpretation for the LRD’s mysterious nature and discuss a crucial role of LRDs bridging the first-generation of early black holes and ultra-massive black holes powering luminous quasars at lower redshifts. 

Key References:

1. Inayoshi K., 2025, ApJL, 988, L22.
2. Inayoshi K., Maiolino R., 2025, ApJL, 980, L27.
3. Inayoshi K., Visbal E., Haiman Z., 2020, ARA&A, 58, 27.

Speaker: Prof. Farhad YUSEF-ZADEH
Affiliation: Northwestern University
Date: April 1, 2026 (Wed)
Time: 11:15 a.m.
Venue: CYCP1, LG1/F, Chong Yuet Ming Chemistry Building,  Main Campus, HKU

Poster: Download

Abstract: 

Over the past two decades, precise measurements have established that a 4.2-million-solar-mass black hole, Sgr A*, resides at the center of our Galaxy, in agreement with predictions from Einstein’s general theory of relativity. Current flaring activity from Sgr A* offers a unique probe of accretion processes near the event horizon, within a few Schwarzschild radii, and is considered a fundamental property of emission from the accretion disk. While the origin of these flares, spanning radio to X-ray wavelengths, remains debated, recent simulations suggest that magnetic reconnection events may drive them.

I will present highlights from recent multi-wavelength observations of Sgr A* with JWST, NuSTAR, and VLA (2023–2024) and propose a unified physical framework for variable emission across infrared, X-ray and radio bands.

Speaker: Prof. Jian GE
Affiliation: Shanghai Astronomical Observatory, Chinese Academy of Sciences
Date: April 15, 2026 (Wed)

Time: 11:15 a.m.
Venue: CYCP1, LG1/F, Chong Yuet Ming Chemistry Building,  Main Campus, HKU

Poster: Download

Abstract:

The Earth 2.0 (ET) space mission is one of four missions under China’s Space Origins Exploration Program (2024–2031). Currently in Phase C and scheduled for launch in fall 2028, ET aims to discover habitable Earth-like exoplanets using a dedicated observatory at the Earth–Sun L2 point. The ET payload includes six wide-field transit telescopes and one microlensing telescope, all equipped with large-format CMOS detectors designed for high-precision photometry. During its four-year prime mission, ET will monitor more than 3 million nearby dwarf stars for transits and about 30 million stars in the Galactic bulge for microlensing events. The mission’s daily data downlink rate will reach 2.5 Gb. Simulations predict the detection of ~20 Earth 2.0s, ~25 free-floating Earth-mass planets, more than 4,000 terrestrial-like planets, and tens of thousands of additional exoplanets. This talk will present the mission design, technical innovations, and its scientific potential in exoplanet, stellar, and time-domain astrophysics.

Key Reference:

1. Ge, J. Chen, W., Chen, Y.H., et al. 2024, “Search for a Second Earth - the Earth 2.0 (ET) Space Mission”, Chinese Journal of Space Science, Volume 44, Issue 3, pp. 400-424, 25 pp.
2. Ge, J., Zhang, H., & Deng, H.P., et al. 2022, “The ET mission to search for earth 2.0s”, The Innovation 3(4), 100271
3. Ge, J. Zhang, H., Zhang, Y.S.,et al. 2024, “Progress in the Earth 2.0 (ET) Space Mission”, Proceedings of the SPIE, Volume 13092, id. 1309218 16 pp.
4. Ge, J., Zhang, H., & Deng, H.P., et al. et al. 2022, “The Earth 2.0 Space Mission for Detecting Earth-like Planets around Solar Type Stars”, Proceedings of the SPIE, Volume 12180, id. 1218015 13 (2022)
5. Ge, J., Zhang, H., Zang, W.C., et al. (2022). ET white paper: to find the first earth 2.0. Preprint at arXiv. https://doi.org/10.48550/arXiv.2206.06693.

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Date: April 29, 2026 (Wed)
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