Spring 2025 Schedule
Feb. 18, 2025
Daniel Wang
UMass Amherst
[Canceled] Exploring the Galactic Core: The mysteries of Sagittarius A* - our supermassive black hole
Host: Ting-Wen Lan
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
The presence of supermassive black holes (SMBHs) in galaxies is well known. But why most of them remain silent in today's Universe is poorly understood. Sgr A* at the center of our Milky Way Galaxy is an asymptotic example of such a low-luminosity SMBH. The proximity of Sgr A* provides a unique opportunity to observe and understand the dynamics of black hole activity and its interplay with its Galactic nuclear environment. Based primarily on deep X-ray observations and computer simulations, I will discuss what Sgr A* has been doing recently and how this interplay may have determined the life cycle of black hole activity and other galactic nuclear processes that profoundly affect the structure and evolution of our Galaxy. Such studies, complemented by observations of other nearby SMBHs and their environments, provide insights into the functioning of galactic ecosystems and astrophysical processes under extreme conditions.
Brief Bio
Daniel Wang is a professor in the Astronomy Department at University of Massachusetts Amherst. He received his Ph.D. in astronomy in 1990 from Columbia University and was awarded the 1992 ASP Robert J. Trumpler Award for Outstanding North American Ph.D Dissertation Research in Astronomy. He was then a NASA Edwin P. Hubble Postdoctoral Fellow at the University of Colorado and later a Lindheimer Fellow at Northwestern University. He was also a member of the Institute for Advanced Study at Princeton and a Raymond and Beverley Sackler Distinguished Visiting Astronomer at the University of Cambridge. He was honored first as the Siyuan Visiting Professor and later as the Yixing Visiting Chair Professor in the School of Astronomy and Space Science at Nanjing University, later as a US Fulbright Scholar and a visiting professor at Pontificia Universidad Catolica de Chile, and as a visiting professor at Tsung-Dao Lee Institute and Shanghai Jiao Tong University. He is currently a visiting scientist at ASIAA. His research is primarily on high-energy astrophysics, covering a broad range of topics: compact objects, stellar feedback, and the hot circumgalactic/intergalactic media, as well as galactic nuclei and their environments.
Feb. 25, 2025
J. Xavier Prochaska
UC Santa Cruz
Three Decades of Science in Silhouette
Host: Ting-Wen Lan
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
I will discuss a perspective on counting and locating the majority of our Universe's baryons from the scant signatures imprinted on the light and signals of distant sources (quasars and fast radio bursts). These data offer what I consider the most robust measurement of a cosmological parameter (the baryonic mass density) and I'll describe previous and ongoing efforts to establish where the majority of this matter resides. If time permits, I'll briefly detail my new adventure to measure – again in silhouette – the constituents of our Earth's oceans (e.g. phytoplankton).
Brief Bio
J. Xavier Prochaska is a distinguished professor at the University of California Santa Cruz. He is also an associated member of the Kavli IPMU at the University of Tokyo and the National Observatory of Japan. He received his PhD from UC San Diego in 1998 and after that he joined the Carnegie Observatories as a Carnegie and Hubble fellow. In 2002, he became a faculty member at UCSC. He has been working on probing the distribution of baryons in various forms, diffuse gas, molecules in and out of galaxies across cosmic time by utilizing both the collecting power of large telescopes, including the Hubble Space Telescope, the Keck telescope in optical, and ALMA in radio, as well as the statistical power provided by large sky surveys, including the SDSS and DESI. He is one of the pioneers in applying machine learning (ML) techniques to astronomical datasets. Several years ago, he established a team to study the nature of fast radio bursts and search for their host galaxies. With such information, along with his collaborators, Prof. Prochaska resolved the so-called missing baryon problem. In addition to astrophysics, he has been working on physical oceanography and is currently an affiliate faculty member of the Ocean Sciences Department at UCSC. In 2023, he received the Simons Pivot Fellowship to support his exploration in oceanography.
Mar. 04, 2025
Sinya Aoki
YITP, Kyoto
Conservation laws and gravity
Host: Jiunn-Wei Chen
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
In this colloquium, I discuss conservation laws in a presence of gravity including general relativity. After an introduction, contents of my colloquium are
I. Energy non-conservation in general relativity
I-1. Why is energy conserved?
I-2. Energy non-conservation in general relativity: Expanding Universe
I-3. Cases for energy conservation in a curved spacetime
II. Gravitational fields carry energy? Noether’s 2nd theorem and general relativity
II-1. Energy of gravitational field?
II-2. Noether’s 2nd theorem
II-3. Noether’s 2nd theorem in general relativity
III. Conserved charge in curved spacetime (including GR)
III-1. Proposal for a conserved charge
III-2. Perfect fluid and Stefan-Boltzmann law
III-3. A simple model of expanding Universe (scalar+radiation)
IV. Summary
Questions and comments during my presentation are very welcome.
Brief Bio
Professor Sinya Aoki is the director of the Yukawa Institute for Theoretical Physics at Kyoto University. He received his PhD in 1987 from the University of Tokyo. After postdoctoral terms at Brookhaven National Laboratory and the State University of New York, Stony Brook, in the USA, he joined the faculty of the University of Tsukuba in 1991. He then moved to the Yukawa Institute for Theoretical Physics at Kyoto University in 2013 and later served as the Director of the Institute.
Professor Aoki is an expert in theoretical high-energy physics, especially in Lattice Quantum Chromodynamics (LQCD). A new phase structure he discovered as a graduate student is now commonly referred to as "Aoki fingers." His research on Nuclear Force from Lattice QCD with Hatsuda and Ishii was awarded the prestigious Nishina Memorial Prize in 2012 and the MEXT Commendation for Science and Technology in 2014.
Mar. 11, 2025
Jiadong Zang
University of New Hampshire
An artificial intelligence era of magnetism
Host: Danru Qu / Shao-Yu Chen
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
Magnetic materials play a crucial role in numerous aspects of daily life, yet their choice remains limited, and discovering new ones is highly challenging. In recent years, the emergence of machine learning and artificial intelligence has revolutionized materials discovery, offering new hope for identifying novel functional magnetic materials. However, a comprehensive database of magnetic materials is still lacking. In this talk, we address this challenge by leveraging advanced large language models to extract material properties from experimental data reported in peer-reviewed journal articles. The database currently includes more than 30,000 magnetic materials and still keeps growing. Our database is highly inclusive and also encompasses superconductors and thermoelectric materials. We hope this resource accelerates materials discovery and paves the way for a new era in magnetism. No background on magnetism and condensed matter is required for this talk.
Brief Bio
Prof. Jiadong Zang received bachelor’s degree in 2007 and PhD degree in 2012, both from Fudan University. He was a postdoctoral fellow in the Institute of Quantum Matter at the Johns Hopkins University during 2012-2015. In 2015, he joined the Department of Physics at the University of New Hampshire (UNH) as an assistant professor. He was promoted to associate professor in 2020, and then to the full professor in 2023. His research field is theoretical condensed matter physics with a focus on many aspects of magnetism, including topological magnetism, quantum transport, and functional magnetic materials. Prof. Zang was recipient of IUPAP Young Scientist Prize in the field of magnetism and the Alexander von Humboldt Fellowship for Experienced Researchers. He is currently the chair of the APS New England Section.
Mar. 18, 2025
C.-J. David Lin
National Yang Ming Chiao Tung University
Parton physics from lattice gauge theory: past, present and future
Host: Chia-Hsien Shen
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
Understanding hadron structure and relevant aspects of quantum chromodynamics is one of the key topics in contemporary physics. Given the non-perturbative nature of such problems, lattice gauge theory is the most reliable tool for making progress. However, conventional lattice calculations are normally performed in Euclidean space, making it challenging to access light-cone dynamics that is crucial for parton physics. In this talk, I will briefly introduce this subject, and give an overview of current lattice methods for tackling this issue. Furthermore, a tensor-network approach will be discussed. This approach allows for direct lattice computations in Minkowski space and is applicable for future calculations carried out on quantum computers. I will show our numerical results of meson parton distribution functions in the Schwinger model from this tensor-network method.
Brief Bio
C.-J. David Lin received his PhD degree from the University of Edinburgh in January 1999. He held postdoctoral positions at the University of Kentucky (February to September 1999), the University of Southampton (October 1999 to August 2003) and the University of Washington in Seattle (September 2003 to August 2006), as well as a visiting scholarship at the University of Cambridge (October 2006 to January 2007), before joining the faculty of National Chiao-Tung University (now National Yang Ming Chiao Tung University) in February 2007. As a lattice field theorist, his research interests span a wide variety of topics in particle and nuclear physics, including composite Higgs model, two-Higgs doublet model, hadron structure, flavour physics, and lower-dimensional quantum field theories.
Mar. 25, 2025
Wei-Li Lee
AS Institute of Physics
Charge transport signatures in topological phases of matter
Host: Cheng-Tien Chiang
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
Topological phases of matter are an emerging and intriguing field, exhibiting remarkable new physical phases and quantum phenomena. In a topological system, nontrivial bulk band topology is accompanied by the presence of unusual surface states, forming a special bulk-surface correspondence. Several surface-sensitive tools have been utilized with great success to reveal these unusual surface states in various topological systems. However, the experimental demonstration of distinct charge transport signatures for surface states in topological systems remains a grand challenge due to the overwhelming contributions from bulk and impurity states, which is particularly detrimental to topological nodal systems.
In this presentation, we will review our experimental efforts in demonstrating charge transport signatures in several topological systems. The first is the quantum anomalous Hall effect in a magnetic topological insulator, which arises from unique chiral edge modes. The second system is the observation of unique Weyl-orbit quantum oscillations in Weyl metal SrRuO₃ thin films, originating from Fermi-arc surface states. Finally, I will briefly present our recent observations of nonlinear and nonreciprocal transport effects in Weyl metal SrRuO₃ thin films, supporting for the existence of the Berry curvature dipole and zero-energy chiral edge modes in a topological Weyl semimetal. These results were made possible only in thin films with ultra-low defect levels, grown using the molecular beam epitaxy technique. The demonstration of charge transport signatures forms an important foundation for future quantum technologies based on topological phases of matter.
Brief Bio
Dr. Wei-Li Lee is a research fellow at the Institute of Physics, Academia Sinica. Dr. Lee received his Ph.D. from Princeton University in 2005. After that, he joined Johns Hopkins University as a postdoctoral researcher until 2006. In 2007, he became a faculty member at the Institute of Physics, Academia Sinica. His research focus is on the charge and thermal transport properties in quantum materials, including complex oxides and their interface, strongly correlated electronics systems, topological systems, graphene-based electronics and 2D materials and devices, etc.
Apr. 01, 2025
Wolfgang Kuch
Freie Universität Berlin
Ultrafast magnetization dynamics in antiferromagnet / ferromagnet layered systems
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Abstract
Antiferromagnetic films have certain advantages over ferromagnetic ones such as the absence of stray fields and a faster magnetization dynamics. They also allow to tune the magnetic properties of adjacent ferromagnetic films, clusters, or adatoms. While the static interaction between ferromagnetic and antiferromagnetic layers has been thoroughly studied in many systems in the past, the dynamic response of ferromagnetic/antiferromagnetic layered systems is lacking investigation, mainly due to the difficulty to detect antiferromagnetic spin order on short timescales.
Taking advantage of magnetic circular and linear dichroisms in resonant reflectivity of soft x rays is an invaluable tool for probing the magnetic response of untransparent ferromagnetic/antiferromagnetic bilayers. By combining ultrashort soft-x-ray probe pulses with near-infrared pump pulses in a pump–probe experiment allows to follow the ultrafast magnetic response of such systems in an element-resolved way.
Using this method, we observe that the presence of antiferromagnetic order in NiMn layers speeds up the demagnetization of an adjacent metallic ferromagnetic layer, which is attributed to bidirectional laser-induced superdiffusive spin currents between the ferromagnetic and the antiferromagnetic layers [1]. The elemental resolution of the method allows to separately detect the ultrafast response of antiferromagnetic Mn and ferromagnetic Co in [Mn/Co] multilayers, where optically induced intersite spin transfer between Co and Mn layers leads to an ultrashort transient magnetization in the elemental antiferromagnet Mn [2]. The ultrafast reduction of magnetic linear dichroism from antiferromagnetic CoO in Fe/CoO bilayers can be compared to the one in magnetic circular dichroism of the ferromagnetic Fe layer. In both layers, the magnetic order is quenched in less than 200 fs upon laser excitation [3]. To our knowledge, this is the first experiment directly detecting the time-resolved evolution of antiferromagnetic order by magnetic linear dichroism in the soft-x-ray regime.
Work done in collaboration with C. S. Awsaf, E. Golias, I. Kumberg, S. Thakur, J. Gördes, C. Schüßler-Langeheine, N. Pontius, P. M. Oppeneer, M. Weißenhofer, S. Sharma, J. K. Dewhurst, I. Gelen, R. Hosseinifar, Q. Guillet, K. Frischmuth, T. Shinwari, M. Walter.
[1] I. Kumberg et al., Phys. Rev. B 102, 214418 (2020).
[2] E. Golias et al., Phys. Rev. Lett. 126, 107202 (2021).
[3] C. S. Awsaf et al., arXiv:2408.14360 (2024).
Brief Bio
Wolfgang Kuch has been a full professor at Freie Universität Berlin (Germany) since 2004. He received his diploma degree in 1989 from the Goethe-Universität Frankfurt (Germany) and his Ph.D. degree from Universität Stuttgart (Germany) in 1993. Before joining Freie Universität Berlin, he was a staff scientist at the Max-Planck-Institut for Microstructure Physics in Halle (Germany). His research interest is on magnetic films, surfaces, and adsorbed molecules, including the dynamic behavior on ultrashort time scales.
Apr. 08, 2025
Midterm exam
Apr. 15, 2025
Ting-Wan Chen
National Central University
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Apr. 22, 2025
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Apr. 29, 2025
Yuan-Ron Ma
National Dong Hwa University
Host: Cheng-Tien Chiang
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
May 06, 2025
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
May 13, 2025
Wen-Bin Jian
National Yang Ming Chiao Tung University
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
May 20, 2025
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
May 27, 2025
Sut-Ieng Tam
National Yang Ming Chiao Tung University
Host:
Time: 2:20 pm
Place: Room 104, CCMS-New Phys. building
Joint Colloquia