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

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内部学生向け(物工教務室)

応用物理学輪講 I

2020年7月3日(金)16:50〜 
Aグループ 
 座長:青野 快
Bグループ  
座長:WANG Tianchun,新井 滉
氏名: 関根 孝彦
指導教員名: 鹿野田 一司 教授
発表題目(英語): 13C NMR study on the ambient-pressure Dirac fermions in the organic conductor α-(BETS)2I3
要旨(英語): An organic conductor α-(BEDT-TTF)2I3 has been known as a model material to study the strongly correlated massless Dirac fermion (DF) system; it has a charge-localized (CL) state under ambient pressure due to Coulomb repulsion, and under high pressure realizes itinerant zero-effective-mass quasi-particle excitations around the Fermi energy. Its Se-analogue α-(BETS)2I3 was expected to have a similar electron system and indeed the measurement on the resistivity of this material showed metal-insulator transition such like the CL state. However, the recent researches have indicated that it is NOT a CL state but a new type of DF state, which attracts attention to examine the actual electrons state.
  In our study, 13C nuclear magnetic resonance (NMR) experiment was conducted on a single crystal sample of α-(BETS)2I3. NMR spectral shifts that are dependent on the electrons local spin susceptibilities were measured under various angles of the magnetic field and the distribution of the spin densities in the unit cell was revealed for the first time. We have concluded that the magnetic symmetry does not changed through insulating and the electron spins are not localized. In my presentation, I am going to talk about our experimental, the results, and the future vision of the study on α-(BETS)2I3.
発表言語: 日本語
氏名: 田島 陽平
指導教員名: 吉岡 孝高 准教授
発表題目(英語): Development of a Sub-microsecond Broadband Pulsed Laser for Cooling Positronium
要旨(英語): Bose-Einstein condensation (BEC) is a fascinating phenomenon where identical bosons occupy the quantum-mechanical ground state. Since 1995, BEC has been observed with several species of bosonic particles. However, BEC with an antimatter system has not been achieved. The realization of antimatter BEC is expected to provide clues to solutions of essential questions in fundamental physics, such as the mystery of the disappearance of antimatter from the universe. Detailed studies have concluded that Positronium (Ps), which is a bound state of an electron (matter) and a positron (antimatter), is the best candidate for antimatter BEC because its small mass allows the BEC phase transition to occur at high temperatures.
 One of the experimental challenges for the realization of BEC of Ps is to cool a gas of Ps rapidly. Ps annihilates into photons with a lifetime of 142 ns. Hence, it has been considered that laser cooling is required to cool Ps below the critical temperature in around the annihilation lifetime.
 Although laser cooling is a major technique to cool neutral atoms, there are some difficulties in cooling Ps in terms of the large Doppler broadening and the annihilation lifetime. We need a peculiar light source where the wide bandwidth and the long pulse duration are fulfilled simultaneously for cooling Ps efficiently.
 In this presentation, I will introduce a prototype of a cooling laser for Ps that we have successfully developed. I will show the design and features of the laser.
発表言語: 日本語
氏名: 曽根 和樹
指導教員名: 沙川 貴大 准教授
発表題目(英語): Exceptional non-Hermitian topological edge mode and its application to active matter
要旨(英語): Topological materials exhibit edge-localized scattering-free modes protected by their nontrivial bulk topology through the bulk-edge correspondence. Recent studies have also revealed the existence of topological edge modes in non-Hermitian systems. Since non-Hermitian Hamiltonians can describe the dynamics of both classical and quantum nonpreserving systems, the non-Hermitian band topology should enrich the application of topological edge modes. While the classification of non-Hermitian Hamiltonians has been explored in terms of bulk band topology [1], its bulk-edge correspondence is still unclear as some studies have discussed non-Hermitian models that seem to break the bulk-edge correspondence [2].
 In our work [3], we reveal a mechanism for realizing robust gapless edge modes independently of the conventional bulk topology of non-Hermitian Hamiltonians. The gapless edge modes found here owe their robustness to the gapless points called exceptional points, which are unique to non-Hermitian systems and protected by the symmetry and topology of the edge band structure. We term the edge modes as "exceptional edge modes" and analyze their robustness by using numerical calculations. We propose their application to laser devices where lasing wave packets propagate along the edge of the sample. We also discuss the existence of exceptional edge modes in chiral active matter, a collection of self-rotating particles like bacteria.

[1] K. Kawabata, K. Shiozaki, M. Ueda, and M. Sato, Phys. Rev. X 9, 041015 (2019).
[2] S. Yao and Z. Wang, Phys. Rev. Lett. 121, 086803 (2018).
[3] K. Sone, Y. Ashida, and T. Sagawa, arXiv:1912.09055 (2019).
発表言語: 日本語
氏名: 田中 佑磨
指導教員名: 石坂 香子 教授
発表題目(英語): Observation of the layer dependent electronic structures in atomically thin WTe2 flakes
要旨(英語): Development of scotch tape methods on graphene and related van der Waals (vdW) materials has triggered intensive researches on novel properties and phenomena realized in atomically thin two-dimensional (2D) crystals, e.g. the valley-induced circular dichroism, 2D superconductivity, and so on. More recently, to explore a wider variety of quantum phases and yet unknown functions, vdW heterostructures [1] obtained by stacking 2D micro-flake crystals have been attracting much attentions. Here the electronic structures tend to become complicated due to the coupling of stacked 2D layers and their relative stacking angles, which makes it difficult to be investigated by simple transport measuerements and band calculations.
 In our study, by using micro-focused laser angle-resolved photoemission spectroscopy (ARPES) in combination with the 2D materials manufacturing system (2DMMS) that can freely stack atomic layers by image recognition, machine learning, and autonomous robots[2,3], we developed a high-throughput procedure for investigating the band dispersions of atomically thin micro-flakes. We prepared 2D WTe2 flake samples for ARPES by using the graphite / h-BN as a substrate and by encapsulating with graphene [4,5]. We successfully observed the thickness-dependent band structures peculiar to WTe2. The 2D sample fabrication procedure used in this study should be applicable to a wide range of micro-flakes, heterostructures and twisted materials.

References
[1] A.K. Geim & I. V. Grigorieva, Nature 499, 419(2013).
[2] S. Masubuchi, et al., Nat. Commun. 9, 1413(2018).
[3] S. Masubuchi, et al., Npj 2D Mater. Appl. 3, 4 (2019).
[4] N. R. Wilson, et al., Sci. Adv. 3, e1601832(2017).
[5] I. Cucci, et al., Nano Lett. 19, 554(2019).
発表言語: 日本語 
氏名: 高橋 知宏
指導教員名: 芦原 聡 教授
発表題目(英語): Determination of crystal orientation by high harmonic generation
要旨(英語): In an intense laser pulse, a target emits high-order harmonics (HHG), which is the multiplication of photon energy (above five times) because of nonlinear interaction between light and matter. HHG from gases or solid-state materials is expected to be a light source of extreme ultraviolet (EUV) and attosecond pulses.
  The mechanisms of HHG from solid materials is fundamentally different from that from gases because of higher density of electrons and periodic structure. Although several theoretical models have been proposed, the HHG mechanism of solids is still under debate.
  It is true that the HHG mechanism of solids is unclear, but some characteristics were reported. It is known that an HHG from solid-state materials reflects the band structure and crystal symmetry. For example, it was reported that the band reconstruction of ZnO was achieved by analyzing the HHG, and an incident polarization dependence of HHG from MgO reflects the crystal structure [1][2].
  Based on these characteristics of HHG from solid-state materials, we propose the new method of crystal orientation analysis which uses HHG. In my presentation, I will talk about the physics of HHG and why HHG reflects crystal orientation. In addition, I will explain the experimental set up and recent progress.

[1] G. Vampa et. al., Phys. Rev. Lett. 115, 193603(2015)
[2] Y. S. You et al., Nat. Phys. 13, 345-349(2016)
発表言語: 日本語
氏名: 田宮 志郎
指導教員名: 小芦 雅斗 教授
発表題目(英語): Calculating nonadiabatic couplings by variational quantum eigensolver
要旨(英語): Recently, variational quantum algorithms have been attracted due to their potential for implementing near-term quantum devices. The variational quantum eigensolver(VQE) is one of the most promising variational quantum algorithms and is expected to be useful for investigating the properties of quantum many-body systems, which are intractable on classical computers.
The original proposal of the VQE was restricted to finding the ground energy and ground state of a given system Hamiltonian. Still, a variety of the extensions of the VQE has been proposed in recent years, finding the excited energies and the excited states, non-equilibrium steady states, and derivatives of energies with respect to external parameters of the system.
 The motivation of this work is developing new beneficial variational quantum algorithms based on the VQE for exploring the possibilities of near-term quantum devices. Here, we propose the VQE-based method for calculating nonadiabatic couplings in quantum chemistry[1]. Nonadiabatic couplings play an important role in simulating nonadiabatic molecular dynamics to study dynamical phenomena such as photochemical reactions. They are related to derivatives of eigenstates with respect to external parameters of the system. We show the evaluation of such quantities can be carried out by the measurements of the expectation value of observables, which is tractable on near-term quantum devices.
 In this talk, I will give a brief introduction to the VQE and explain the details of the results.

[1] Shiro Tamiya and Yuya O. Nakagawa, arXiv:2003.01706 (2020).
発表言語: 英語  
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氏名: 恒川 翔
指導教員名: 齊藤 英治 教授
発表題目(英語): Deep learning of mesoscopic electron systems
要旨(英語): In mesoscopic systems at low temperature, electrons transport coherently and quantum interference effect appears as fluctuation in magnetic conductance. Because the quantum interference results from the scattering by the sample edge or defects, the conductance fluctuation contains the sample geometric information. However, the data is too complex to be analyzed by the conventional methods, such as Fourier transformation.
 To tackle this problem, we have established a deep learning model, named quantum geometric decoder (QGD). Using numerical calculation data for 2 dimensional nanowire system, we have demonstrated decoding the conductance fluctuation into the geometric information, surprisingly including the wave function.
発表言語: 日本語