応用物理学輪講 I
7月2日
[注意事項]
発表の10日前までに宛てに「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
2021年7月9日(金)16:50〜

Aグループ

座長
伊藤 宏陽
指導
教員名
川﨑 雅司 教授
座長
上野 祥広
指導
教員名
鹿野田 一司 教授
発表者名 藤本 健
指導教員名 齊藤 英治 教授
発表題目(英語) Physics Simulation with Graph Neural Network
要旨(英語) Computation time is one of the hardest challenge many physicist have ever faced with. Especially in fluid simulation, solving Navier Stokes e.q. for a plenty number of particles is quite arduous procedure from the perspective of computational time. Recent research, however, succeeded in reducing computational time drastically by applying Graph Neural Network to calculation of multi particles dynamics.
 Graph Neural Network(GNN) has been one of the most prospective method of deep learning algorithm for last 5 years, which takes graph structure data as input and output target value. By focusing on the connection of data, which is often ignored in simulation or theoretical analysis of Physics, it is shown that computationally reasonable dynamics calculation can be achieved and directly obtain the new types of Hamiltonian expression in complex systems.
 In this presentation, starting from the basic explanation of GNN and its application, I’ll give some detailed contents, evaluation of GNN based simulator method, and some amazing results of simulation.
発表言語 日本語
発表者名 古山 昂樹
指導教員名 志村 努 教授
発表題目(英語) 1D computer generated holographic memory
要旨(英語) Currently a holographic memory is attracting attention as a next generation optical memory. Holographic memories have large capacity and high data transfer rate. We have proposed a time series signal holographic memory using a volume hologram, and have investigated its write/read characteristics and theoretical recording density limit. The conventional volume holographic memory has a very large recording density, but it has some disadvantages. One is the influence of the thermal expansion/shrinkage of the hologram. The Bragg condition is not maintained when the temperature changes, and the signal disappears due to the thermal expansion of the hologram. Second is the issue of reproducibility. Volume holographic memories cannot be duplicated in the entire disc at once, such as by injection molding like ROM type optical disks.
 Thus, we have proposed a holographic memory using a surface hologram [1]. Since it uses the Raman-Nath diffraction, the signal is reconstructed even if the hologram expands or shrinks. Though the recording density is almost the same as that of conventional optical discs, holographic memory discs can be produced at single process by injection molding, etc. and the data transfer rate is comparable to that of volume holographic memory.
 Now, we propose a 1D surface holographic memory and are analyzing its write/read characteristics. In this system, strips of holograms are recorded on the hologram disc and read out sequentially. Because each holograms do not overlapped spatially, unlike conventional holographic memory systems, the crosstalk noise between pages will be reduced and it has a possibility to improve the recording density. The hologram to be recorded is obtained by using the Computer Generated Hologram method and we plan to make it by metasurfaces. In this presentation, I will show the simulation results of 1D surface holographic memory and talk fabrication process with metasurface.
[1] S. Hirayama et al., Photonics, 6(2), 70 (2019).
発表言語 日本語
発表者名 増木 亮太
指導教員名 有田 亮太郎 教授
発表題目(英語) Validity and Range of Applicability of Quasiharmonic Approximation on Calculation of Thermal Expansion and Phonon Frequency Shift
要旨(英語) The thermal expansion is one of the most fundamental properties of solids. Because the volume-change of materials cause problems in various situations, the way to control the thermal expansion is intensely studied. Therefore, it is of great importance to clarify the physical mechanism of thermal expansion in detail.
 Quasiharmonic approximation(QHA), which neglects all the effect of the phonon anharmonicity except the volume-dependence of the phonon frequencies, is the most widely used method for the first-principles calculation of the thermal expansion. It is empirically known that QHA gives accurate results for the thermal expansion coefficient. However, QHA often fails to correctly describe the temperature-dependent shift of the phonon frequencies even for simple materials like silicon. This is quite
paradoxical and casts doubt to the validity of QHA because the thermal expansion coefficient is closely related to the Gruneisen parameter, which is linked to the phonon frequency shift.
 In this research, we formulate a theory of the thermal expansion based on the self-consistent phonon(SCP) theory, which takes into account the anharmonic effect in a non-perturbative way, and resolve the paradox. We prove that the Gruneisen formula also holds within SCP theory. By using the perturbation expansion, we show that the phonon anharmonicity gives a small correction to the theramal expansion coefficient. On the other hand, we show that the phonon anharmonicity gives a dominant correction to the T-dependent shift of the phonon frequencies. We perform numerical calculations on some materials to show that our theory is correct.
発表言語 英語
発表者名 松井 彬
指導教員名 有田 亮太郎 教授
発表題目(英語) Numerical study on the transport phenomena in the antiferromagnetic skyrmion system
要旨(英語) The antiferromagnetic (AFM) skyrmion has been attracting attention owing to its promising application to memory devices; the AFM skyrmion is free from the stray fields and the skyrmion Hall effect, which has been preventing the application of the ferromagnetic skyrmion. [1] Despite the recent observation of the AFM skyrmion phase in a bulk system [2], the basic properties of the AFM skyrmion, such as its stability and the transport phenomena, are yet to elucidate.
 In this study, we numerically calculate the topological charge and spin Hall conductivity for the AFM skyrmion system. Exploiting the state-of-the-art real-space calculation method, we investigate the transport phenomena of the AFM skyrmion system for various skyrmion sizes and coupling strengths. We reveal that the transport phenomena in the AFM skyrmion system can significantly deviate from that in the conventional ferromagnetic skyrmion, which holds great promise in future applications to devices.
[1] B. Göbel et al., Physics Reports 895, 1-28 (2021)
[2] S.Gao et al., Nature 586, 37-41 (2020)
発表言語 英語

Bグループ

座長
海老原 豪
指導
教員名
吉岡 孝高 准教授
座長
余 同桦
指導
教員名
有田 亮太郎 教授
発表者名 古澤 千晶
指導教員名 賀川 史敬 准教授
発表題目(英語) Study of the instability of the metastable skyrmion during the current-driven motion in B-20 MnSi
要旨(英語) Magnetic skyrmions, nanometer-sized vortex-like swirling spin textures, have attracted attention with their unique physical properties rooted by their topological nature. In particular, the current-driven motion of skyrmions is an important topic in terms of application to information carriers for high-density, highly efficient magnetic storage taking advantage of skyrmions' stability and controllability with low current density. However, the metastable skyrmions are not stable and often decay during the current-driven motion. The microscopic origin of these phenomena is not fully understood.
 In this study, focusing on a FIB-fabricated B-20 type MnSi microcrystal, a typical skyrmion-hosting material, we investigated skyrmions' decay during the current-driven skyrmion motion using micromagnetic simulations and experiments. In this talk, I will briefly review the decay during the current-driven motion of skyrmions and present our latest experimental and numerical results. With this updated knowledge, I will also discuss previous studies, such as the magnetic field dependence of the metastablity of the skyrmions [1] and the decay phenomena during the current-driven motion observed in bulk MnSi crystal [2].
[1] H. Oike et al., Nat. Phys. 12, 62 (2016).
[2] F. Kagawa et al., Nat. Commun. 8, 1332 (2017).
発表言語 日本語
発表者名 堀 智洋
指導教員名 十倉 好紀 卓越教授
発表題目(英語) Engineering of ferromagnetic-metal surface state in FeSi
要旨(英語) Spin-orbit coupling (SOC) is the relativistic interaction between the spin and momentum degree of freedom of electrons, and produces various interesting phenomena in condensed matter systems. Especially, at the surface or interface SOC is greatly enhanced and spin polarized 2D state can be produced. These surface/interface with large SOC and spin-momentum locked state can be applied to a great variety of next generation devices [1].
 A chiral compound FeSi is a nonmagnetic narrow-gap insulator in bulk state and has attracted attention for its unique charge and spin dynamics. Recently, it has been found that FeSi has a ferromagnetic-metal surface state in which polarized electronic states originating from the bulk band topology are accumulated [2]. In addition, resulting “floating” surface electrons produce a large spin-orbit interaction due to the surface potential gradient, which produces a Rashba-type spin polarization. This new type of surface electron state is expected to be applied to noble-metal-free SOC-based spin manipulation instead of the conventional one using heavy metals.
 In this presentation, I will explain the interesting surface state of FeSi and introduce the application of this surface state based on the control of SOC.
[1] A. Soumyanarayanan et al., Nature 539, 509 (2016).
[2] Y. Ohtsuka et al., arXiv:2104.12438 (2021).
発表言語 日本語
発表者名 松澤 郁也
指導教員名 福谷 克之 教授
発表題目(英語) Metal-insulator transition in hydrogenated RNiO3
要旨(英語) It is known that some of perovskite nickelates RNiO3 with rare earths as R exhibits a metal-insulator transition (MIT) due to temperature changes [1], and there are many materials in which physical properties such as magnetism and electric conductivity change due to hydrogen doping [2, 3]. Recent studies have suggested that doping RNiO3 with hydrogen induces a new insulating state, and it has been reported that the resistance of SmNiO3(SNO) changes significantly at room temperature due to hydrogen doping [4]. However, the mechanism of hydrogen-induced MIT is not well understood.
 The purpose of my research is to elucidate the mechanism of hydrogen-induced MIT in RNiO3 by experimentally measuring the relation between the hydrogen concentration and electronic structure.
 In the present study, we prepared RNiO3(R=Sm, Nd, La) thin films on a substrate, and measured the changes in the electric resistance of the sample due to hydrogenation. We also measured the hydrogen concentration in the sample by using nuclear reaction analysis(NRA). We discuss the MIT mechanism due to hydrogenation.
[1]J. B. Torrance, et.al., Phys. Rev. B 45, 8209 (1992)
[2]G. Srinivas, et.al., J. Mater. Chem., 19, 5239-5243 (2009)
[3]R.K.Singhal, et.al.,Journal of Alloys and Compounds Volume 496, Issues 1–2, 324-330 (2010)
[4]Jian Shi, You Zhou & Shriram Ramanathan, Nature Communications volume 5,4860 (2014)
発表言語 日本語