Semester 2/2025
14.01.2026
Speaker: Edvin Idrisov
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21.01.2026
Speaker: D.K. Efimkin
Time: 13:00-14:00
Venue: online
Title: Exciton-Polarons in Doped Monolayer Semiconductors
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The dynamics of a quasiparticle can be profoundly altered by interactions with the surrounding quantum environment. This general phenomenon—where the particle becomes dressed by excitations of the medium—is broadly referred to as a polaronic effect, a concept introduced by L. Landau and S. Pekar. Historically, the study of Bose polarons, where the medium consists of bosonic excitations such as phonons or magnons, played a foundational role in the development of path integral methods and advanced numerical tools in quantum many-body physics. More recently, the focus has shifted to Fermi polarons, where a mobile impurity interacts with a fermionic bath. This problem has become particularly relevant due to its realization in ultracold atomic gases, enabling precise experimental control and sparking substantial theoretical progress over the past decade.
In this talk, I will present how the exciton-polaron framework successfully captures the optical response of two-dimensional semiconductors under moderate doping. In these systems, photoexcited excitons interact with a Fermi sea formed by excess electrons or holes, leading to the formation of two distinct quasiparticle branches: the attractive and repulsive exciton-polarons. These manifest experimentally as two well-separated peaks in the absorption spectrum. I will discuss how the doping dependence of their resonant energies, spectral weights, and linewidths aligns remarkably well with recent experimental measurements. This agreement high-lights the power of the exciton-polaron theory as a unifying language for describing many-body effects in doped semiconductors, and paves the way for a deeper understanding of quasiparticle behavior in strongly interacting, low-dimensional materials.
28.01.2026
Speaker: Peng Rao (TUM)
Time: 15:00-16:00
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04.02.2026
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11.02.2026
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18.02.2026
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25.02.2026
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04.03.2026
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11.03.2026
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18.03.2026
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25.03.2026
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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
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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.
21.10.2025 (Exp.)
Speaker: DongKeun Ki (Hongkong)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
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22.10.2025 (Theory)
Speaker: Aaron Hui (Brown University)
Time: 09:00-10:00 (Thailand time)
Venue: online
Title: The curious case of current noise in electron hydrodynamic flow
Abstract: Experimental innovations in the past decade have uncovered a new mode of electron transport: strongly-interacting hydrodynamic flow. In this regime, the familiar spatial locality of Ohm’s law is replaced by the non-local, viscous flow dictated by the Stokes equations. Here, I consider the implications of this non-locality for noise thermometry, a technique by which random fluctuations of current are used to infer thermal properties of materials. I lay out our generalized theory of Johnson noise thermometry, showing how the hydrodynamic regime enables sensitivity beyond what is allowed by Ohm’s law. I also discuss ongoing experiments that use our theory to observe previously unnoticed features of hydrodynamic flow.
29.10.2025 (Exp.)
Speaker: Koichi Oyanaki (Iwate)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title:
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12.11.2025 (Theo.)
Speaker: Ryuishi Shindou (Peking University)
Time: 13:00-14:00 (Thailand time)
Venue: online
Title: Quasi-localization phenomena in chiral symmetry classes
Abstract: Transport phenomena lie at the heart of physics, and transports of disordered topological systems have been extensively studied for last decades [1]. Recent numerical works clarified that the metal-insulator transition in chiral symmetry classes becomes two-step phase transitions in the presence of the 1D weak band topology, where a new thermodynamic phase, dubbed as “quasi-localized” phase, universally emerges between metal and Anderson localized phases in phase diagrams [2,3]. In the quasi-localized phase, an exponential localization length along a spatial direction with the 1D weak topology (“topological direction”) is divergent, while the localization length along the other directions is finite. In the talk, I will argue the universal emergence of the quasilocalization phenomena from a lens of topological field theory [3,4].
The Anderson transition in 2D chiral symmetry classes is instigated by spatial proliferation of vortex excitations associated with a U(1) subgroup symmetry of a field variable in a non-linear sigma model [5], while the 1D weak topology confers a quantal (``Berry'') phase upon the vortex excitations, dependent on their spatial polarization relative to the topological direction. Due to quantum interference induced by the Berry phase, a partition function of the metal phase near their mobility edge is dominated by vortex excitations polarized along the topological direction. The proliferation of the polarized vortex excitations naturally renders correlations of the field variable to be weakly and strongly disordered along the topological direction and the other direction(s), respectively, yielding the universal emergence of the quasi-localized phase next to the metal phase. To uphold this physical picture, we use dual representations of the chiral nonlinear sigma models [3,4]. A renormalization group (RG) analysis of the 2D dual model shows that a RG phase diagram in the presence of the 1D weak topology has a weak coupling fixed point with vanishing vortex fugacity and finite conductivities (for critical metal phase), and a strong coupling fixed point with divergence vortex fugacity and finite/zero conductivity along the topological/the other direction (for the quasilocalized phase). A phase transition between these two phases are characterized by a saddle-fixed point with a universal critical exponent, that also takes a consistent value to the 2D numerical result [3]. For the 3D dual theory, we employ its analogy to 3D magnetostatics of type-II superconductors under an external magnetic field, and argue that the quasilocalized phase is a spatially inhomegeneous phase similar to mixed ('vortex lines') phase in the 3D type-II superconductors. Thereby, a 3D localized bulk is permeated by an array of 1D conducting regions aligned along the topological directions [4].
[1] "Anderson transitions" F. Evers, and A. D. Mirlin, Rev. Mod. Phys. 80, 1355 (2008).
[2] “Anisotropic Topological Anderson Transitions in Chiral Symmetry Classes”, Z. Xiao, K. Kawabata, X. Luo, T. Ohtsuki, and R. Shindou, Phys. Rev. Lett. 131, 056301 (2023).
[3] “Topological effect on Anderson Transition in Chiral Symmetry Classes”, P. Zhao, Z. Xiao, Y. Zhang, and R. Shindou, Phys. Rev. Lett. 133, 226601 (2024).
[4] "Theory of the Anderson transition in three-dimensional chiral symmetry classes: Connection to type-II superconductors", P. Zhao, and R. Shindou, arXiv:2506.21050, to appear in Phys. Rev. B.
[5] “Metal-insulator transition in two-dimensional random fermion systems of chiral symmetry classes”, E. J. Koenig, P. M. Ostrovsky, I. V. Protopopov, and A. D. Mirlin, Phys. Rev. B 85, 195130 (2012).
18.11.2025 (Expt.)
Speaker: Alexey Berdyugin (NUS)
Time: 13:00-14:00 (Thailand time)
Venue: hybrid
Title: Making the Cleanest Graphene Ever!
Abstract: The electronic quality of graphene has improved significantly over the past two decades, revealing novel phenomena. However, even state-of-the-art devices exhibit substantial spatial charge fluctuations originating from charged defects inside the encapsulating crystals, limiting their performance. Here, we overcome this issue by assembling devices in which graphene is encapsulated by other graphene layers while remaining electronically decoupled from them via a large twist angle (~10–30°). Doping of the encapsulating graphene layer introduces strong Coulomb screening, maximized by the sub-nanometer distance between the layers, and reduces the inhomogeneity in the adjacent layer to just a few carriers per square micrometre. The enhanced quality manifests in Landau quantization emerging at magnetic fields as low as ~5 milli-Tesla and enables resolution of a small energy gap at the Dirac point.
Beyond large-angle graphene encapsulation, similar screening can be achieved using a graphite layer separated from graphene by an ultrathin three-layer hBN spacer. In such devices, we observe quantum oscillations appearing already at 1 mT, carrier inhomogeneity suppressed to 10⁷ cm⁻², and transport mobilities exceeding 6 × 10⁷ cm² V⁻¹ s⁻¹—surpassing those of state-of-the-art GaAs 2DEGs and establishing graphene devices with screening as the highest-mobility material ever created.
Finally, I will discuss moiré physics in twisted devices incorporating such screening layers. In magic-angle twisted bilayer graphene decoupled from the screening layer by a large twist, we observe a controllable suppression of superconductivity with increasing screening strength, pointing towards an unconventional origin of superconductivity in this system.
26.11.2025 (Theo.)
Speaker: Chen Chuan (LZU)
Time: 13:00-14:00 (Thailand time)
Venue: online
Title: Kitaev Spin Liquid as the Mother of Competing Phases in Extended Kitaev Models
Abstract: The Kitaev honeycomb model hosts a quantum spin liquid with fractionalized anyons, but real materials include additional Γ and Γ′ interactions that drive rich magnetic behavior. I present a quasiparticle-based framework showing how gap closings of visons, fermions, and local bosons naturally organize transitions into zigzag, stripy, 120°, and incommensurate spiral phases. A notable outcome is that an antiferromagnetic Kitaev model with a ferromagnetic Γ term yields a phase that combines magnetic order with the fractionalization pattern of the Kitaev spin liquid. Depending on the sign of Γ′, the resulting order becomes either stripy or incommensurate. This viewpoint offers a unified way to understand many ordered phases in extended Kitaev models as descendants of the Kitaev spin liquid.