応用物理学輪講 I
7月11日
[注意事項]
発表の10日前までに　office[at]ap.t.u-tokyo.ac.jp　宛てに「氏名」「指導教員」「発表題目（英語）」「要旨（英語）」「発表言語（英語または日本語）」を送付して下さい。
発表日
2025年7月11日(金) 16：50～18：50

Aグループ

座長
池田　理玖
指導
教員名
吉岡　孝高　准教授
発表者名 松本　滉永
指導教員名 木村　剛　教授
発表題目（英語） Electric field-induced nonreciprocal directional dichroism in antiferromagnets with broken time-reversal symmetry
要旨（英語） Antiferromagnets with broken time-reversal symmetry have attracted attention for exhibiting ferromagnet-like behaviors (e.g., anomalous Hall effect and magneto-optical Kerr effect) despite the absence of net magnetization. Their unconventional spin splitting is also of great interest under the concept of altermagnetism. To access their time-reversal-odd property, we investigated a novel phenomenon, the diagonal electrotoroidic (ET) effect, where the application of an electric field E induces a magnetic toroidal moment T along E. While the off-diagonal response (T⊥E) of the ET effect is known to occur in ferromagnets, the diagonal effect (T || E) has not been explored.

In this presentation, we show our observation of E-induced T in time-reversal-broken antiferromagnets Co2SiO4 and ErCrO3. For this observation, we measured nonreciprocal directional dichroism (NDD), a representative phenomenon in ferrotoroidic systems, under applied electric fields. The spatial mapping of this phenomenon enabled the observation of two-dimensional maps of antiferromagnetic domain structures. Furthermore, we report its photon-energy dependence of the electric field-induced NDD in the visible to near-infrared range.
発表言語 日本語
発表者名 三澤　遼
指導教員名 Max Hirschberger 准教授
発表題目（英語） Search for loop-current order by resonant X-ray scattering
要旨（英語） Multiferroics are systems in which both inversion and time-reversal symmetries are broken, and they are characterized by an order parameter known as the toroidal moment, defined as the cross product of spin and position. Extending beyond spin-based multiferroics, loop current order, involving toroidal moments arising from orbital currents, has been proposed as a hidden order in the pseudogap phase of cuprate superconductors. This order is termed “hidden” because its associated order parameter often evades detection by conventional probes.
A powerful technique to investigate such hidden orders is resonant elastic X-ray scattering (REXS). Its element- and orbital-specific selectivity, as well as resonant enhancement of signals, enable, in principle, the detection of arbitrary multipole orders in solids. In this talk, I review the use of REXS in identifying toroidal moments in multiferroic systems, and then discuss its application to cuprates, highlighting the experimental challenges involved. I conclude by exploring the prospects of detecting loop current order in kagome metals via REXS.
発表言語 英語
発表者名 八木　春樹
指導教員名 Gong Zongping 准教授
発表題目（英語） Random Field Trips / Applications of Random Matrices and Topological Recursion to Quantum Field Theories
要旨（英語） Random matrix theory is a framework that is deeply connected to various areas of mathematical physics. While random matrix theory is often discussed in the context of quantum chaos from the perspective of applied physics, the purpose of this talk is to introduce the original contexts in which random matrix theory has been studied.

The content of the talk is as follows. First, I will briefly explain what a random matrix is. Next, I will describe the two-dimensional “quantum gravity theory” that emerges from random matrices and whose partition function is expressed as a genus expansion. Finally, I will introduce the loop equations and topological recursion as the structures underlying such genus expansions, and explain how these structures are governed by conformal field theory.
発表言語 日本語

Bグループ

座長
石井　智博
指導
教員名
井手上　敏也　准教授
発表者名 山内　健聖
指導教員名 関　真一郎　准教授
発表題目（英語） X-ray Magnetic Circular Dichroism induced by collinear antiparallel spin arrangement in altermagnets
要旨（英語）  Altermagnet is a new classification of magnetic materials, characterized by a collinear antiferromagnetic spin order with broken time-reversal symmetry, and a wave-vector-dependent anisotropic spin-split band structure. Although the spins form an antiparallel order and there is no net magnetization, altermagnets can exhibit some ferromagnet-like properties, such as the anomalous Hall effect, due to fictitious magnetic fields. 
 In this presentation, I will explain X-ray Magnetic Circular Dichroism (XMCD) in altermagnets, a phenomenon previously thought to be fundamentally dependent on magnetization. I will discuss some examples of XMCD induced by antiferromagnetic order, and present some of the results from our experiments.
発表言語 日本語
発表者名 山岡　良太
指導教員名 中島　多朗　准教授
発表題目（英語） Study of Magnetic Excitations in the van der Waals Magnet CrNb₄Se₈ by Inelastic Neutron Scattering
要旨（英語） In recent years, van der Waals (vdW) magnets have been actively studied as next-generation materials for spintronics. In this study, we focus on CrNb₄Se₈, a vdW material in which Cr ions form a triangular lattice between NbSe₂ layers stacked along the c axis. Our previous neutron diffraction experiments have revealed that the Cr magnetic moments form a 120°-type spin order characterized by a magnetic propagation wave vector of (1/3, 1/3, 0). This suggests that the in-plane and out-of-plane magnetic interactions are predominantly antiferromagnetic and ferromagnetic, respectively. However, we could not quantitatively estimate the magnitudes of the magnetic interactions. To address this issue, in the present study, we performed inelastic neutron scattering measurements on bulk samples using the high-resolution chopper spectrometer HRC at J-PARC. By comparing the observed magnon dispersions with calculations by a linear spin wave model based on a Hamiltonian that includes only nearest-neighbor interactions, we revealed that the out-of-plane magnetic interaction, which would be expected to be small for vdW magnets, is comparably strong as compared to the in-plane nearest neighbor antiferromagnetic interaction.
発表言語 日本語
発表者名 山下　涼介
指導教員名 森本　高裕　准教授
発表題目（英語） Nonlinear optical response of strongly correlated materials: Photocurrent generation mediated by spin excitations
要旨（英語） The development of laser technology has led to recent progress in the theoretical and experimental research on nonlinear optical phenomena. In particular, the second-order DC response of noncentrosymmetric materials has attracted attention as the bulk photovoltaic effect (BPVE). While the nonlinear optical responses of free electronic systems are well understood by band theory, those of strongly correlated electron systems, which are mainly mediated by their collective excitations, remain unexplored. Spin excitation is one of the typical collective excitations, and several theoretical methods have been established to capture the excitation structure, dynamics and optical responses. These methods provide insights into the nonlinear optical responses of multiferroic magnets mediated by spin excitations.
In this talk, I will introduce the application of the infinite Time-Evolving Block Decimation (iTEBD), a numerical method for one-dimensional quantum systems using tensor network formalism, to the nonlinear optical response of 1D multiferroic magnets. Through non-perturbative calculations by iTEBD, we can obtain nonlinear optical responses that reflect the properties of the ground state and excitation structures of quantum spin chains.
発表言語 日本語
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