6月3日 | 応用物理学輪講 I | 輪講 | 東京大学工学部物理工学科・大学院工学系研究科物理工学専攻

工学部物理工学科

応用物理学輪講 I

6月3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: 2022年6月3日（金）16：50～18：50

Aグループ

座長: 矢田　季寛
指導 教員名: 沙川　貴大　教授

発表者名	大竹　雄太郎
指導教員名	求　幸年　教授
発表題目（英語）	Inside deep neural network: insight from statistical physics
要旨（英語）	Machine learning using deep neural networks (DNNs), which can extract essential features from large data sets, has been useful in many areas such as speech recognition, visual object recognition, and object detection [1]. Despite these successes, DNNs are like black boxes: it is still unclear why they have high generalization performance. Recently, a statistical mechanical approach based on the replica method was developed to understand the inside of DNNs [2]. In this approach, DNNs were considered as spin systems, and the spin glass order parameters were examined for each layer of the DNNs. It was shown that the layers near the input and output boundaries are glassy, but those in the bulk remain liquid like. Although a such phase structure appears to play an important role, further studies are desired to clarify the mechanism of learning by DNNs from the microscopic viewpoint. In this study, we propose a way to improve DNNs by introducing the optimization of temperature in terms of the spin systems. Specifically, we introduce temperature for each layer and optimize it through the learning process. We show that this procedure can achieve higher accuracy than the constant temperature throughout the system. We find that the distribution of temperature becomes asymmetric in the DNN because of back propagation in the learning process, and the optimized temperature is relatively low in some layers close to the output, leading to peculiar phase structure. In the presentation, we show the detailed analyses while changing many parameters in the DNNs and discuss the possibility to further improvement. [1] Y. LeCun, Y. Bengio and G. Hinton, Deep learning, Nature 521, 436 (2015) [2] H. Yoshino, SciPost Phys. Core 2, 005 (2020)
発表言語	日本語

発表者名	沖上　和希
指導教員名	速水　賢　講師
発表題目（英語）	Engineering skyrmion crystals in ferromagnetic/antiferromagnetic bilayers free from the DM interaction
要旨（英語）	A magnetic skyrmion crystal (SkX) is a nanoscale vortex-like spin texture and has some topological features. As a spin texture, the SkX is specified by the integer topological number, and it also has nontrivial electromagnetic phenomena, such as the giant topological Hall effect. Owing to these features, the SkX is expected to be a next-generation device, and thus it is an important issue to reveal the stabilization mechanisms. One of the stabilization mechanisms of the SkX is the Dzyaloshinskii-Moriya (DM) interaction in noncentrosymmetric systems [1]. On the other hand, other mechanisms to stabilize the SkX have been clarified through intensive studies, such as short-range competing exchange interactions [2] and effective long-range exchange interactions mediated by itinerant electrons [3]. Since these mechanisms are applicable to centrosymmetric lattice structures without the DM interaction, they might give a deep understanding of the origins of the SkXs discovered in centrosymmetric magnets, such as Gd2PdSi3 [4] and GdRu2Si2 [5]. In the present study, we investigate another scenario to stabilize the SkX by considering the role of the layered structure in the systems free from the DM interaction. Specifically, we consider a bilayer triangular lattice system with different types of interactions: one is characterized by the ferromagnetic exchange interaction and the other is the antiferromagnetic exchange interaction. By performing the variational calculations and Monte Carlo simulations, we find that such a bilayer system can host the SkX in the ground state under an external magnetic field and an easy-axis anisotropy depending on the strength of the interlayer exchange interaction. We discuss the ground-state magnetic phase diagram by changing the temperature and interlayer exchange interaction. [1] N. Nagaosa and Y. Tokura, Nat. Nanotech. 8, 899 (2013). [2] T. Okubo et al., Phys. Rev. Lett. 108, 017206 (2012). [3] S. Hayami et al., Phys. Rev. B 95, 224424 (2017). [4] T. Kurumaji et al., Science 365, 914 (2019). [5] N. D. Khanh et al., Nat. Nanotechnol. 15, 444 (2020).
発表言語	日本語

発表者名	黒川　亮
指導教員名	十倉　好紀　卓越教授
発表題目（英語）	Magnetotransport properties in half-Heusler RAuSn.
要旨（英語）	A huge magnetoresistance of 10 to the 4th power was observed in the half- Heusler type HoAuSn. I am interested in the origin of this huge magnetoresistance. Half-Heusler compounds are ternary compounds with a formula of XYZ, consist of 3 atoms. As I will discuss in the presentation it is an ideal platform to explore topological quantum states. In this talk, I will introduce the material system and its physical properties and introduce the progress of my research.
発表言語	日本語

Bグループ

座長: 山本　孝
指導 教員名: 岩佐　義宏　教授

発表者名	大島　久典
指導教員名	渡辺　悠樹　准教授
発表題目（英語）	Charge fluctuation and charge-resolved entanglement in monitored quantum circuit with U(1) symmetry
要旨（英語）	Recent attention has focused on the fact that critical phenomena can also occur in far-from-equilibrium systems, which share some properties of those in equilibrium systems. A pioneering example of those is the measurement-induced phase transition, stemming from the competition between the scrambling effect of the unitary dynamics and the disentangling effect of measurements[1,2]. Such phase transition occurs in a stochastic monitored system, and shows a conformal field theory (CFT)-like criticality as is the case in equilibrium critical systems . We numerically investigate this phase transition in terms of charge fluctuation[3] in a subsystem and charge-resolved entanglement[4] under U(1) symmetry associated with the total spin, or total particle number. We find that the late-time charge fluctuation exhibits logarithmic scaling at the transition and that the decay exponent of the two-point charge correlator is 2 for half-filling systems, which implies a free boson CFT, or a Tomonaga-Luttinger-liquid (TLL)-like universality. Furthermore, we compute the charge-resolved moment and find the results are well consistent with the TLL prediction, which supports the TLL-like universality of the measurement-induced phase transition under U(1) symmetry. [1] Y. Li, X. Chen, & M. P. A. Fisher, Phys. Rev. B 98, 205136 (2018) [2] B. Skinner, J. Ruhman, & A. Nahum, Phys. Rev. X 9, 031009 (2019) [3] H. F. Song, et al., Phys. Rev. B 85, 035409 (2012) [4] J. C. Xavier, F. C. Alcaraz, and G. Sierra, Phys. Rev. B 98,041106(R) (2018)
発表言語	日本語

発表者名	大西　嘉祐
指導教員名	川﨑　雅司　教授
発表題目（英語）	Fabrication and characterization of pyrochlore oxide hetero-interfaces
要旨（英語）	The pyrochlore structure has a tetrahedral network and has been studied eagerly to elucidate the effect of geometrical frustration on the properties of localized spin and itinerant electron systems. In this work, Pb2Ru2O6.5/ Dy2Ti2O7 hetero-interfacial thin films were fabricated on YSZ (111) substrate by pulsed laser deposition method to measure the transport properties of the insulator material Dy2Ti2O7.Dy2Ti2O7 is a ferromagnetic insulator that exhibits 2-in/2-out and 3-in/1-out states at low temperatures, which are called “spin ice” materials. These 2-in/2-out and 3-in/1-out states have scalar spin chirality and are expected to exhibit the topological Hall effect. On the other hand, Pb2Ru2O6.5 is a paramagnetic metal that retains its metallicity down to low temperatures. In this presentation, we will discuss the respective physical properties of Pb2Ru2O6.5 and Dy2Ti2O7, the results of transport properties and the optimization of the thin film fabrication.
発表言語	日本語

発表者名	梶原　駿
指導教員名	岩佐　義宏　教授
発表題目（英語）	Controlling magnetic properties of a two-dimensional magnetic material Cr3Te4 by ion gating
要旨（英語）	Transition metal dichalcogenides (TMDCs) are layered materials that exhibit a variety of physical properties depending on the combination of transition metals and chalcogens. Cr3Te4 can be regarded as a material in which Cr is self-intercalated between the layers of CrTe2 belonging to TMDCs, and is known to exhibit room temperature ferromagnetism in bulk [1].Our group has succeeded in preparing epitaxial thin films of Cr3Te4 by molecular beam epitaxy (MBE) and found that magnetic properties such as ferromagnetic transition temperature and magnetic anisotropy can be modulated by adjusting the thin film preparation conditions. In addition, we found that the magnetic properties can also be significantly modulated by electrochemically intercalating Li into the epitaxial Cr3Te4 thin film samples. These results open up the possibility of applying gating technology to nanoscale spintronics devices. [1] E. F. Bertaut et al., J. Phys. 25, 582 (1964).
発表言語	日本語

応用物理学輪講 I一覧ページへ戻る