Joint Colloquia

ASIAA/CCMS/IAMS/LeCosPA/NTU-Phys/NTNU-Phys

Coordinators:

Chia-Hung Chang (NTNU Phys), Kai-Feng Chen (NTU Phys), Shao-Yu Chen (CCMS), Cheng-Tien Chiang (IAMS), Jiwoo Nam (LeCosPA), Tomomi Sunayama (ASIAA), Shang-Min Tsai (ASIAA)

Sep. 02, 2025 (week 01)

Christopher Ford

Semiconductor Physics Group, Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge, UK.

Nanoscale electron channels – probing fundamental physics in 1D wires

Host: Chi-Te Liang

Time: 4:20 pm ~ 6:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

Observing separate spin and charge Fermi seas in a strongly correlated one-dimensional conductor: An electron is usually considered to have only one form of kinetic energy, but could it have more, for its spin and charge, by exciting other electrons? In one dimension (1D), the physics of interacting electrons is captured well at low energies by the Tomonaga-Luttinger model, yet little has been observed experimentally beyond this linear regime. Here, we report on measurements of many-body modes in 1D gated wires using tunnelling spectroscopy. We observe two parabolic dispersions, indicative of separate Fermi seas at high energies, associated with spin and charge excitations. By comparing a series of devices consisting of 1D wires between 1 and 10 μm long, made using large numbers of air bridges, we observe the emergence of two additional 1D “replica” modes that strengthen with decreasing wire length, as predicted by the most recent theory. The interaction strength is varied by changing the amount of 1D intersubband screening by more than 45%. Our findings not only demonstrate the existence of spin-charge separation in the whole energy band outside the low-energy limit of the Tomonaga-Luttinger model but also set a constraint on the validity of the newer nonlinear Tomonaga-Luttinger theory. We have also measured the power-law suppression of tunnelling at electron temperatures down to 10mK and explain why it changes when the second and third 1D subbands become occupied.

Brief Bio

Chris Ford is Professor of Quantum Electronics at the Cavendish Laboratory, University of Cambridge, UK. After studying at Cambridge, he worked as a postdoc at the IBM TJ Watson Research Center, Yorktown Heights, USA in 1988-9 before returning to Cambridge as a Junior Research Fellow at Girton College and then becoming a lecturer and later a full professor. He is an experimentalist who has pioneered measurements of the Aharonov-Bohm effect in semiconductor rings and antidots, and quenching of the Hall effect in ballistic junctions. He introduced the plunger gate for tuning gate-defined quantum dots, and has demonstrated single-photon emission from single electrons carried by surface acoustic waves (SAWs), and his group is currently working on

Moving single-electron quantum dots carried by Surface Acoustic Waves (SAWs).
Nonlinear interaction effects in 1D wires beyond the Luttinger-liquid regime.
Transport through self-assembled monolayers of nanocrystals and/or molecules.

VIDEO

20250902.MP4

Sep. 09, 2025 (week 02)

Graham R. Fleming

Department of Chemistry, University of California, Berkeley, USA

Photosynthetic Energy Transfer: Design Principles and Questions Illustrated by the Photosystem II Supercomplex

Host:

Time: 2:20 pm ~ 4:20 pm

Place: Dr. Poe Lecture Hall, IAMS

Abstract

Brief Bio

For the inaugural lecture, we are honored to welcome Professor Graham R. Fleming, a leading figure in ultrafast spectroscopy, former Vice Chancellor for Research at UC Berkeley, and a member of the U.S. National Academy of Sciences. Prof. Fleming shared a formative period of his early academic life with Prof. Lin. During Lin’s sabbatical in England in the early 1970s, they collaborated on pioneering studies in molecular spectroscopy and energy relaxation. Prof. Fleming has often acknowledged Prof. Lin as a key mentor who profoundly shaped his scientific thinking and approach. His participation as the inaugural speaker honors both their long-standing academic relationship and the profound impact Prof. Lin had on generations of scientists.

VIDEO

20250909.MP4

Sep. 16, 2025 (week 03)

Ran Cheng

University of California, Riverside

Artificial spacetime, topological instanton, and emerging electrodynamics in pseudo-Hermitian quantum mechanics

Host: Danru Qu

Time: 2:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

Geometric phase (or Berry phase) has profound implications for the physical behavior of quantum systems. A prototypical scenario is the two-level quantum system, where the Berry phase can be formulated as the magnetic flux emanating from a fictitious magnetic monopole located at the energy-degenerate point in parameter space. If the Hamiltonian becomes explicitly time-dependent, the Berry phase also evolves over time, thereby generating an accompanying electric field by virtue of the Faraday effect. This insight has led to important applications such as the spin-motive force in a moving magnetic texture. Here we explore a hitherto overlooked case: two-level pseudo-Hermitian (PH) quantum systems with real eigenvalues. Our generic model system supports a unique Berry curvature resembling a 2+1 dimensional electromagnetic field, yet it cannot be mapped onto a static magnetic field as in its Hermitian counterpart, even when the PH Hamiltonian does not explicitly depend on time. The key ingredient here is an emergent spacetime metric characterizing the parameter space such that one component of the adiabatic parameter naturally plays the role of time, while the others act as spatial coordinates. Consequently, the electric and magnetic components of the Berry curvature are inherently connected by the 2+1 dimensional Faraday equation in the presence of spacetime singularities—dubbed instantons—which carry quantized topological charges. These topological instantons in (real-spectral) PH quantum systems parallel the role of magnetic monopoles in Hermitian quantum mechanics. Our discovery unifies non-Hermitian Berry phase physics with field-theoretic concepts, offering a novel handle on topology in open and dissipative quantum systems.

Brief Bio

Dr. Ran Cheng obtained his Ph.D. in Physics from the University of Texas at Austin in 2014, followed by a postdoc appointment at Carnegie Mellon University. In 2018, he joined the University of California, Riverside, as an Assistant Professor and was promoted to Associate Professor in 2025. He holds joint appointments in the Departments of Electrical and Computer Engineering, Physics and Astronomy, and Materials Science and Engineering. Dr. Cheng leads a research group in theoretical Condensed Matter Physics with a focus on spintronics, magnetism, and topological materials. He explores a broad range of fundamental physical phenomena and their implications in advanced materials, especially in magnetic topological insulators and antiferromagnetic nanostructures. His research is driven by both experimental insights and mathematical intuitions. His research is supported by the DoD MURI Award, the NSF CAREER Award, funding from the W.M. Keck Foundation, and several intramural grants.

VIDEO

20250916.MP4

Sep. 23, 2025 (week 04)

Andreas Schäfer

Universität Regensburg

Quark-gluon interactions and Quantum Information Science

Host: Jiunn-Wei Chen

Time: 2:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

Quantum Chromo Dynamics (QCD) is the fundamental theory of strong interactions, i.e. of the interactions of gluons and quarks. QCD is time reversal invariant, and, therefore, QCD processes cannot produce thermal entropy. However, a thermal, so called "fireball", description of high-energy heavy ion collisions with large thermal entropy is applicable. This apparent contradiction is explained by Quantum Information Science, namely by the Eigenstate Thermalization Hypothesis (ETH). For small systems, certain aspects of quantum computing can be evaluated on classical, digital computers by explicit Hilbert space construction. This we did and we confirmed the validity of ETH for SU(2) gauge theory. Encouraged by this success we have continued to analyze other aspects. Roughly speaking, all of these agree with theoretical expectations (up to finite volume and truncation artefacts). This additional success has ecouraged us to start work on an even far more ambitious project, namely the first principle QCD (or more precisely AdS/CFT) treatment of hadronization.

VIDEO

20250923.MP4

Sep. 30, 2025 (week 05)

Tadashi Takayanagi

Yukawa Institute for Theoretical Physics, Kyoto University

Quantum Entanglement and Gravitational Spacetime

Host: Pei-Ming Ho

Time: 2:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

Recently, a new interpretation of gravitational spacetime in terms of quantum entanglement has been developed. The idea of holography in string theory provides a simple geometric computation of entanglement entropy. This generalizes the well-known Bekenstein-Hawking formula of black hole entropy and strongly suggests that a gravitational spacetime consists of many qubits with quantum entanglement. Also a new progress on black hole information problem has been made recently by applying this idea. A new insight on holography for de Sitter spaces have also been obtained from quantum information viewpoints. I will explain these developments in this lecture.

VIDEO

20250930.MP4

Oct. 07, 2025 (week 06)

Feng-Li Lin

Dept. of Physics, National Taiwan Normal Univ.

Thermodynamics for the NISQ era

Host: Kai-Feng Chen

Time: 2:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

In this talk, I will discuss two topics in quantum thermodynamics for the noisy intermediate-scale quantum (NISQ) systems. The key features of these systems are their mesoscopic size, so that the quantum or thermal fluctuations cannot be neglected by taking the thermodynamic limit. This then prompts the extension of classical thermodynamics to encompass the platforms for quantum computing and communications. The first topic is thermalization of closed quantum systems, which extends the zeroth law of thermodynamics to the quantum regime. The second is the fluctuation theorem, which extends the second law of thermodynamics by treating energy and information exchanges as random quantities. The results of these studies demonstrate how quantum information enhances our understanding of thermodynamics.

VIDEO

20251007.MP4

Oct. 14, 2025 (week 07)

Changyoung Kim

Department of Physics and Astronomy, Seoul National University

Altermagnetism: electronic structures and potential applications

Host: Dr. Ryo Noguchi

Time: 2:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

The third class of magnetism, dubbed as altermagnetism, has been a recent hot topic in the field of magnetism. Some of the collinear antiferromagnets have spin-split bands contrary to the conventional wisdom. These altermagnets therefore have characteristics of both ferro- (FM) and antiferro-magnetism (AFM): spin split bands (thus broken time reversal symmetry) and zero net magnetization. These traits are believed to be important in the fundamental scientific point of view as well as for spintronic applications.In this colloquium talk, I first wish to introduce the concept of altermagnetism. I will first talk about ABCs of altermagnetism – collinear AFM (zero net magnetization) and spin split bands (time reversal symmetry breaking) in terms of symmetries. Then, I will discuss the microscopic origin of the spin splitting based on the AFM order and structural distortion. This shows how spin and orbital are entangled, eventually leading to the multipole order, that is, the order parameter of altermagnets.Experimental verification of altermagetism is inherently difficult due to the zero net magnetization as well as domain formation. Yet, evidences for spin split bands can be obtained in some cases. I will introduce our recent ARPES work on an altermagnet MnTe. ARPES data show split bands which merges to a single band above the Neel temperature, strongly indicating the magnetic origin of the splitting. Finally, I will also briefly introduce recent study results on magnetic responses of RuO2 and domain switching behavior in MnTe/Bi2Te3 hetero structures.

VIDEO

20251014.MP4

Oct. 21, 2025 (week 08)

Midterm week

Oct. 28, 2025 (week 09)

Martin Kalbáč

Heyrovsky Institute of Physical Chemistry of the Czech Academy of Sciences

Exploring and Engineering 2D Materials in Ultra-High Vacuum: From Pristine Surfaces to Controlled Functionalization

Host: 謝雅萍

Time: 2:20 pm ~ 3:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

2D materials are extremally sensitive to their environment. This is because they do not have a bulk component and thus literally all atoms can interact with species adhering to the surface of the 2D material. In ambient conditions the surface gets immediately covered by impurities. In order to preserve a clean surface, we propose a method for exfoliation of the 2D materials in ultra-high vacuum conditions. This allowed us to explore the properties of these materials in a pristine state. To further tailor the properties of 2D materials one can exploit chemical functionalization. However, this process is generally not compatible with ultraclean environment like ultra-high vacuum. I will present our strategies which provides pathway to reach this challenging goal.

VIDEO

20251028-1.mp4

Oct. 28, 2025 (week 09)

Guy Cohen

Department of Chemical Physics, Tel Aviv University, Israel

From Diagrammatic Monte Carlo to Dynamical Localization

Host: 許良彥

Time: 3:20 pm ~ 4:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

Quantum matter driven away from equilibrium is a rich and fascinating physical playground. However, except in a few very special limits, many-body systems are challenging to understand. I’ll briefly discuss how we use field-theoretical techniques to develop diagrammatic Monte Carlo methods that can reliably access numerically exact dynamics in a large and important class of many-body systems: quantum impurity models. These are small interacting quantum systems coupled to a noninteracting continuum, and they can be used to model electronic dynamics and transport in mesoscopic systems coupled to an environment. They can also be used to approximately describe extended interacting quantum systems through embedding frameworks like the dynamical mean field theory (DMFT).
These tools give us unique insight into many fundamentally and technologically interesting problems. I’ll focus on two recent examples where we explored the competition between classical localization effects and quantum delocalization in the presence of many-body interactions. In one project, we applied our methods to the phase diagram of the spin–boson model—a very simple impurity model—as extracted from its transient relaxation dynamics. We showed that this reveals a transient dynamical phase diagram with different universal behavior from those of its equilibrium counterpart. In another, we proposed and solved a numerically tractable model for many-body Stark localization in the limit of large dimensions, based on an exact application of the DMFT mapping. We tracked the decay of spin-density waves, showing that as the field strengthens, transport evolves nonmonotonically from a subdiffusive regime and through a superdiffusive window, eventually becoming suppressed. This final high-field regime embodies a clear demonstration of many-body localization in the thermodynamic limit. 
Our results illustrate the impact of numerically exact algorithms for simulating quantum dynamics: nonequilibrium phases are no longer speculative. Rather, they are computable, testable, and ready to be confronted with experiment.

VIDEO

20251028-2.mp4

Nov. 04, 2025 (week 10)

Joel W. Ager

University of California Berkeley and Lawrence Berkeley National Laboratory

Bio-inspired Approaches for Carbon Dioxide Conversion

Host: Li-Chyong Chen

Time: 2:20 pm ~ 3:20 pm

Place: Chin-Pao Yang Lecture Hall, R104, CCMS-New Phys. building

Abstract

If artificial fixation schemes are to be successful reducing the atmospheric carbon dioxide concentration, they must operate at rates which far exceed those of current natural sinks. Photocatalytic and electrocatalytic CO2 reduction will be described in this context. Semiconductor-based photocatalyst CO2 reduction is conceptually simple but has struggled to exceed natural photosynthesis in conversion efficiency. I will show that optimization of mass transfer and control of the catalytic microenvironment addresses this challenge by increasing conversion rates by over an order of magnitude. New sulfide-based photoelectrocatalytic materials are showing promise as CO2 reduction photocathodes. Finally, I will show that the chemical reaction network of electrocatalytic CO2 reduction is highly dynamic and can have many of the functionalities found in biological networks such as inhibition/activation, cascades, and substrate channeling/pre-concentration.

Brief Bio

Joel W. Ager is a Senior Scientist in the Materials and Chemical Sciences Divisions of Lawrence Berkeley National Laboratory and an Adjunct Professor in the Materials Science and Engineering Department, UC Berkeley. He graduated from Harvard College in 1982 with an A.B in Chemistry and from the University of Colorado in 1986 with a PhD in Chemical Physics. After a post-doctoral fellowship at the University of Heidelberg, he joined Lawrence Berkeley National Laboratory in 1989. His research interests include the discovery of new photoelectrochemical and electrochemical catalysts for solar to chemical energy conversion and the fundamental electronic and transport properties of semiconducting materials. He is a frequent invited speaker at international conferences and has published about 400 papers in refereed journals with over 54,000 total citations and an H-index of 117 till date. Professor Ager is a Fellow of the Royal Society of Chemistry.

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