Semester 1/2025
06.08.2025 (Theor.)
Speaker: Amaresh Jaiswal (NISER)
Time: 13:00-14:00 (Thailand time)
Venue: Online
Title: Spin-hydrodynamics and its applications: from high energy physics to condensed matter systems
Abstract:
Spin hydrodynamics is an emerging interdisciplinary framework that extends conventional hydrodynamics to include spin degrees of freedom in systems with intrinsic angular momentum. Originally motivated by experimental discoveries of spin polarization in relativistic heavy-ion collisions, spin hydrodynamics has rapidly evolved to become a powerful theoretical tool with applications across high-energy nuclear physics and condensed matter systems.
In this talk, I will introduce the basic principles of relativistic spin hydrodynamics, including its formulation from kinetic theory and its applications in condensed matter systems, including electron hydrodynamics in graphene. Emphasis will be placed on the unifying aspects of spin hydrodynamics across physical domains and potential future directions in both theory and experiment. The talk is aimed at a broad audience with interests ranging from high-energy nuclear physics to condensed matter physics.
13.08.2025 (Exp.)
Speaker: Shuichi Iwakiri (NIMS)
Time: 13:00-14:00 (Thailand time)
Venue: Online
Title: Non-reciprocal response of a two-dimensional electron gas in the quantum Hall regime
Abstract:
Breaking of inversion symmetry leads to nonlinear and nonreciprocal electron transport, in which the voltage response does not invert with the reversal of the current direction. Various types of nonreciprocal phenomena have been discovered such as the nonlinear Hall effect and superconducting diode effect and attracting attention recently. In this talk, we will see that even a strikingly simple system, a two-dimensional electron gas with a back gate shows such nonreciprocal behavior in the quantum Hall regime. In this system, the inversion symmetry is broken due to the presence of the back gate and magnetic field, and our phenomenological model provides a qualitative explanation of the experimental data. Our results suggest a universal mechanism that gives rise to nonreciprocal behavior in gated samples and call for careful analysis of such phenomena.
20.08.2025 (Theor.)
Speaker: Ryotaro Sano (University of Tokyo)
Time: 13:00-14:00 (Thailand time)
Venue: Online
Title: Surface acoustic waves-driven magnon spin Hall effect in atomically thin van der Waals antiferromagnets
Abstract:
Intrinsic magnetism in two-dimensional (2D) materials had long been believed to hardly survive due to the enhanced thermal fluctuations. However, the recent discovery of exfoliated van der Waals (vdW) magnets has opened up a new avenue for 2D magnetism at finite temperatures [1,2]. Especially, transition metal phosphorus trichalcogenides are a family of easily exfoliatable vdW antiferromagnets [3]. These materials share the same honeycomb structure, but the bulk antiferromagnetic (AFM) phase varies depending on the magnetic elements. Furthermore, antiferromagnets exhibit ultrafast dynamics, null stray field, and robustness against external fields. Therefore, the investigation of these materials paves the way toward not only the understanding of 2D magnetism, but also future AFM spintronic devices.
Standard methods such as magnetization measurements and neutron diffraction, which could only access macroscopic magnetic properties, are not suitable for the study of atomically thin magnets. Especially, antiferromagnets do not have net magnetization, magneto-optical Kerr effect is not available either. Although recent studies have focused on Raman spectroscopy [4] and second-harmonic generation [5] to detect crystal symmetry lowering associated with the AFM transition, these signals do not provide clear identification in the monolayer limit. Therefore, an inclusive method which suits for exploring 2D antiferromagnets
is highly desired.
Here, we propose a magnon spin Hall current driven by the surface-acoustic waves (SAWs) as a novel probe for such 2D vdW antiferromagnets [6]. Owing to extremely large mechanical flexibility of 2D materials, SAWs are ideally suited for fundamental research of them. A modulation of exchange energies due to strain mimics the role of gauge fields for magnons. The strain gauge fields work at two valley points in the opposite direction, leading to the activation of the valley degrees of freedom (DOF). Therefore, the valley DOF with the use of SAWs is a promising concept for detection of the magnetic order in 2D vdW antiferromagnets.
27.08.2025 (Theor.)
Speaker: Kamal Das (Penn State)
Time: 10:00-11:00 (Thailand time)
Venue: Online
Title: Quantum Geometry and Nonlinear Transport in the Layered Antiferromagnet
CrSBr
Abstract:
Quantum geometry in Bloch bands describes how the wave-like nature of electrons is shaped by the underlying crystal structure of materials. It includes two key ingredients: Berry curvature, which measure the “geometric phase” a Bloch state acquires when moving through the momentum space and the quantum metric, that quantities how “far apart” quantum states in the Hilbert space move from their original configuration during such motion. While the Berry curvature linked to the anomalous Hall effects and topological phenomena has been widely studied, the quantum metric is now gaining attention, especially because it shows up in unusual electrical signals that don’t follow the standard (linear) Ohm’s law, such as currents that depend on the square of the applied field.
In the first part of my talk, I will introduce the ideas behind quantum geometry. I’ll explain how these geometric features arise from the band structure of electrons in crystals, and why they naturally appear in the response to electric fields. I will highlight their contributions to second-order effects and demonstrate how the quantum metric induced second order transport can be used to probe hidden magnetic order, such as the Néel vector in antiferromagnets. In the second part, I will present our recent work on CrSBr, a layered 2D magnet that is stable in air and has a relatively high magnetic ordering temperature. Unlike well-known topological magnets, CrSBr is topologically trivial at first glance. However, we find that it shows strong nonlinear transport signals—thanks to hidden band crossings that become active when the material is slightly doped. Surprisingly, these effects are mostly controlled by the outermost atomic layers, even though there are no topological surface states involved. I will discuss what drives this surface-dominated behaviour and what it means for future applications in quantum devices based on 2D magnets.
03.09.2025 (Exp.)
Speaker: Curtis Vidura Mcdowell (Chula)
Time: 13:00-14:00 (Thailand time)
Venue: online
Title: Fabry–Pérot Interferometry in Large Angle Twisted Bilayer Graphene
Abstract: In this talk, I will discuss the electronic properties of large-angle twisted bilayer graphene (tBLG). Since the Dirac cones are widely separated in k-space, the two layers are effectively decoupled, acting like two monolayer graphene sheets, despite being atomically close. This enables layer-independent Fabry–Pérot cavities which results in resistance fringes in bipolar regions. In our device, we also observe additional, multiple unexpected oscillations that either depend on carrier density and the global back-gate voltage. I will present a model that captures many of these features by incorporating a simple smooth doping profile, perhaps arising from disorder, and I will show that some of these oscillations are further enhanced at small, nonzero magnetic fields.
17.09.2025 (Exp.)
Speaker: Jonah Waissman (The Hebrew University of Jerusalem)
Time: 13:00-14:00 (Thailand time)
Venue: online
Title: Observation of Electronic Viscous Dissipation in Graphene Magneto-thermal Transport
Abstract: Hydrodynamics describes the collective transport of strongly-interacting particles. Due to enhanced electron-electron interactions at elevated temperatures, the behavior of electrons in clean graphene can be depicted as a hydrodynamic flow of charge. In this new regime, the well-known rules of Ohmic transport no longer apply, necessitating the consideration of collective electron dynamics. In particular, the hydrodynamic analogues of Joule heating and thermal transport require consideration of the electronic viscosity, but remain unexplored. In this work, we probe graphene via thermal transport measurement in small magnetic fields and find an unexpected enhancement of cooling in Corbino geometries. We construct a theory that identifies the origin of this effect in viscous dissipation of the electron fluid, enabling a new measurement of the electronic viscosity and underlying microscopic thermal and electrical conductivities. This reveals the Lorenz ratio of the graphene electronic fluid, which is shown to be strongly suppressed compared to the Wiedemann-Franz value, in agreement with longstanding expectations for the hydrodynamic regime. Our results mark the first observation of viscous electronic heating in an electron fluid, offering a new, transport-based methodology for identifying hydrodynamic states in other material systems, and providing insight for thermal management in electronic hydrodynamic devices.
24.09.2025 (Theor.)
Speaker: Joji Nasu (Tohoku)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title: Observation and control of fractional quasiparticles in Kitaev spin liquids
Abstract:
The Kitaev quantum spin liquid has recently attracted great interest due to its fractional quasiparticles: Majorana fermions and visons. Intensive efforts have been made to identify these quasiparticles in candidate materials with dominant Kitaev-type interactions. Notably, the recent observation of the half-quantized thermal Hall effect in α-RuCl3 under magnetic fields provides possible evidence for the existence of topological Majorana edge modes. When such modes emerge, each vison hosts a Majorana zero mode, forming a composite quasiparticle that behaves as a non-Abelian anyon, an essential building block for topological quantum computation based on anyon braiding. However, the creation, detection, and manipulation of these quasiparticles remain highly challenging, even within the ideal Kitaev model. In this talk, we propose a theoretical framework for observing Majorana fermions and spatiotemporal manipulation of visons by locally applied magnetic fields in the Kitaev quantum spin liquid. For the detection of Majorana fermions, we focus on spin transport arising from their itinerant nature: a magnetic pulse excites itinerant Majorana fermions, which propagate through the bulk without altering the spin but induce a magnetization signal at the opposite edge. This striking phenomenon directly reflects the presence of fractional quasiparticles. We further demonstrate that time-dependent magnetic fields can control vison motion, accompanied by a Majorana zero mode. In addition, we show that visons can be created and annihilated through the application of a local magnetic field. Our results demonstrate the possibility of the spatiotemporal creation and manipulation of non-Abelian anyons, providing a potential pathway toward practical implementations of topological quantum computation.
01.10.2025 (Exp.)
Speaker: Chang-woo Cho (Chungnam National University)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title: Microscopic Magnetic Parameters of van deer Waals CrSBr Probed by Microwave Absorption
Abstract:
The emergence of van der Waals (vdW) magnets has opened up new opportunities for studying low-dimensional magnetism and spintronic functionalities in layered systems. Among these, layered antiferromagnets such as CrI₃, CrCl₃, and CrSBr exhibit intriguing low-temperature magnetic behavior arising from alternating spin alignments between adjacent layers. In particular, CrSBr hosts a pronounced biaxial magnetic anisotropy, which not only stabilizes in-plane moment orientation but also makes it an attractive candidate for integration into spin-based electronic applications.
In this seminar, I will present our study of low-energy magnon excitations in bulk CrSBr, probed via microwave absorption spectroscopy. We investigate the magnetic field dependence of two distinct resonance modes along the three principal crystallographic axes, including fields beyond the saturation point. This reveal detailed information on the anisotropic magnetic behavior, magnetic transitions, and interlayer exchange couplings. To interpret the observed spectra, we develop a microscopic spin Hamiltonian incorporating biaxial single-ion anisotropy and interlayer exchange interactions. The model successfully reproduces the experimental magnon features and enables a quantitative determination of the key microscopic parameters governing the magnetic ground state of CrSBr.
22.10.2025 (Exp.)
Speaker: DongKeun Ki (Hongkong)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title:
Abstract:
29.10.2025 (Exp.)
Speaker: Koichi Oyanaki (Iwate)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title:
Abstract:
04.11.2025 (Exp.)
Speaker: Aveek Bid (IISC, india)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title:
Abstract:
12.11.2025 (Theo.)
Speaker: Ryoishi Shindou (Peking University)
Time: 13:00-14:00 (Thailand time)
Venue: online
Title:
Abstract: