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2026.05.01

Magnon thermal Hall effect in antiferromagnetic insulators

講師：川野雅敬助教（東京大学大学院総合文化研究科）
日時：令和8年5月18日（月）13:30-
場所：本館2階 290 物理学系輪講室

　The Hall effect is traditionally associated with the motion of charged particles subjected to an external magnetic field via the Lorentz force. Beyond this classical picture, Hall responses can also arise without an external magnetic field. In magnetic insulators, magnons, charge-neutral bosonic quasiparticles, can exhibit the thermal Hall effect through an emergent gauge field, without relying on the Lorentz force [1]. The thermal Hall effect is of fundamental importance as it provides a powerful probe of charge-neutral carriers and emergent gauge fields in quantum magnets.
In this seminar, I will discuss the realization of the magnon thermal Hall effect in two classes of systems where it was previously thought to be absent or strongly suppressed.
First, I will discuss the realization of this effect in edge-shared lattices, such as square and triangular lattices. The conventional U(1) gauge-field picture imposes a no-go condition that precludes the thermal Hall effect in these geometries [2,3]. We overcome this limitation by introducing a non-Abelian gauge-field picture, in which the noncommutativity of gauge fields generates an additional emergent magnetic flux [4,5]. This flux breaks effective time-reversal symmetry and enables the magnon thermal Hall effect even in edge-shared lattices.
Second, I will discuss the thermal Hall effect in a spin-gapped system: 1/3-plateau phase of a kagome antiferromagnet. Spin-gapped systems are generally expected to suppress low-energy transport due to the absence of mobile excitations. We demonstrate that this picture breaks down in the presence of strong geometric frustration. We observe the coexistence of two distinct types of quasiparticles: localized, magnetically neutral modes that contribute to longitudinal heat transport, and mobile magnetic excitations with topologically nontrivial bands, giving rise to a sizable thermal Hall effect [6].

References
[1] Y. Onose et al., Science 329, 297 (2010)
[2] H. Katsura et al., Phys. Rev. Lett. 104, 066403 (2010)
[3] T. Ideue et al., Phys. Rev. B 85, 134411 (2012)
[4] H. Takeda, M. Kawano et al., Nat. Commun. 15, 566 (2024)
[5] M. Kawano, Phys. Rev. B 112, L060403 (2025)
[6] H. Takeda, M. Kawano et al., submitted

連絡教員：物理学系　藤井　啓資（内線2136）

https://www.phys.sci.isct.ac.jp/wp/wp-content/uploads/2026/05/118tokubetsu.pdf

セミナー

2026.05.01

Optical renormalization of collective magnetic excitations in quantum materials

講師：Dr. Davide Bossini（University of Konstanz, Germany）
日時：令和8年6月1日（月）16:30-
場所：南5号館1階 103B第2会議室

　It has been proposed that magnetic waves in solids, i.e. spin waves or magnons, are promising information carriers for future information technology, enabling the processing of data at THz rates with limited energy dissipations. In this talk, I will briefly discuss how these excitations can be coupled to charges, highlighting recent progress involving processes at terahertz frequencies [1-2].
The main part of the talk will address the optical manipulation of magnons. I will show how resonant excitation of specific magnetic and electronic transitions drives the system into non-equilibrium states in which magnon modes, that are not directly excited, become activated and substantially modified. Two distinct physical scenarios will be discussed. In the first, optical excitation of electronic transitions modifies the magnetic anisotropy in a 20-nm-thick magnetic film, leading not only to the generation of coherent magnons but also to an on-demand frequency renormalization [3]. Both redshifts and blueshifts of the magnon frequency are achieved, reaching up to 40% of its equilibrium value at room temperature. In the second scenario, I will present an approach based on high-momentum magnons with wave vectors near the edges of the Brillouin zone, which can be resonantly driven using mid-infrared laser pulses. This excitation pathway activates distinct zone-center modes whose amplitudes and frequencies are strongly renormalized compared to their equilibrium values [4]. I will conclude by outlining future perspectives of this research direction, with the long-term goal of achieving arbitrary optical control over magnon dispersion relations in quantum materials.

References
[1] T. Mezger et al., Physical Review Letters 135, 076702 (2025).
[2] M. Cimander et al., Nature Communications 17, 1480 (2026).
[3] V. Wiechert et al., Nature Communications 17, 145 (2026).
[4] C. Schoenfeld et al., Science Advances 11, 25 (2025).

連絡教員：物理学系　佐藤　琢哉（内線2716）

https://www.phys.sci.isct.ac.jp/wp/wp-content/uploads/2026/05/441.pdf

セミナー

2026.05.01

Terahertz-driven phonon angular momentum in perovskites

講師：Professor Martina Basini（Department of Physics, ETH Zurich, Switzerland）
日時：令和8年5月22日（金）16:30-
場所：本館2階 290 物理学系輪講室

　In the solid state, processes involving angular momentum and its transfer have underpinned numerous foundational phenomena in physics, including magnetism, the conventional and anomalous quantum Hall effects, and orbital magnetism. The advent of laser sources in the terahertz frequency range has opened new avenues for the coherent excitation of phonons that carry angular momentum and enable the exploration of angular momentum transfer among different subsystems. This emerging field has revealed a range of novel phenomena-such as the phonon Hall effect, ultrafast Einstein-de Haas effect, phonon Faraday effect, and phonon Zeeman effect [1, 2], highlighting the role of phonons in angular momentum dynamics.
　In this seminar, I will present different methods for preparing an orbital angular momentum state for both IR-active and Raman-active phonons in centrosymmetric perovskites and discuss its coupling to macroscopic properties. Particularly, I will focus on (i) a phonon-associated, magnetic-like contribution generated in SrTiO₃ and KTaO₃ by a strong circularly polarised terahertz field, resonant to its soft phonon [3] and, on (ii) evidence of terahertz-driven strain in LaAlO₃, revealed by an unconventional decay in the angular momentum dynamics [4].

References
[1] M. Basini et al. Nature 628, 534 (2024).
[2] C. S. Davies et al. Nature 628, 540 (2024).
[3] In preparation, THz field-induced magnetic-like response in the quantum paraelectric diamagnet KTaO₃.
[4] M. Basini et al. Phys. Rev. Lett. 136, 156902 (2026).

連絡教員：物理学系　佐藤　琢哉（内線2716）

https://www.phys.sci.isct.ac.jp/wp/wp-content/uploads/2026/05/440.pdf

お知らせ教員公募
2026.05.01

理学院物理学系助教公募（物性実験）
令和8年6月15日（月曜日）日本時間必着

日本語：https://www.phys.sci.isct.ac.jp/wp/wp-content/uploads/2026/05/koubo_jk_jp_0501.pdf

English：https://www.phys.sci.isct.ac.jp/wp/wp-content/uploads/2026/05/koubo_jk_en_0501.pdf

セミナー

2026.04.10

Material Characterization at the Nanoscale viaPlasmon-Enhanced Nanospectroscopy & Sensing

講師：早澤紀彦氏（理化学研究所開拓研究所及び光量子工学研究センター）
日時：令和8年5月7日（木）14:00-
場所：南5号館5階 503CD 大会議室

The realization of optical microscopy capable of true nanoscale observation has long been a central goal in science and engineering. With the rapid development of nanomaterials and nanodevices in recent years, analytical techniques that can address increasing structural and functional complexity have become essential. In this context, optical microscopy offers unique advantages, including operation under ambient conditions without the need for cryogenic temperatures or high vacuum, as well as non-contact and non-invasive measurement capabilities.
Our research focuses on the development of advanced nanospectroscopy and sensing methodologies based on plasmonic resonances. The key advantages of plasmonic systems arise from two fundamental properties: the strong enhancement and the extreme localization of electromagnetic fields. These properties are critical for nanoscale measurements, as field localization improves spatial resolution, while field enhancement compensates for inherently weak signals originating from small volumes of material or limited numbers of molecules.
Plasmonic resonances can be broadly classified into two categories: (1) localized surface plasmon resonance (LSPR) and (2) propagating surface plasmon resonance (SPR). In this presentation, I will introduce representative nanospectroscopic techniques based on LSPR, such as tip-enhanced Raman spectroscopy (TERS), as well as sensing approaches based on SPR, including Goos–Hänchen and Imbert–Fedorov shifts (GHS and IFS) and the photonic spin Hall effect (PSHE).
Particular emphasis will be placed on the extension of these techniques to diverse experimental environments, enabling broader applicability to a wide range of target materials. Furthermore, beyond the pursuit of high spatial resolution, recent efforts toward achieving high temporal resolution and rapid sensing—particularly through terahertz (THz) spectroscopy—will also be discussed.

連絡教員：物理学系　佐藤　琢哉（内線2716）

https://www.phys.sci.isct.ac.jp/wp/wp-content/uploads/2026/04/439.pdf

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