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応用物理学輪講 Ⅰ

        
2017年12月1日(金)16:50~ 
62号講義室(2F) 
 座長:鈴木 啓史、関 靖秀
64号講義室(2F)  
座長: 住谷 達哉
氏名: 中村  駿甫
指導教員名: 沙川 貴大 准教授
発表題目(英語): Algebraic Bethe ansatz approach to out-of-time order correlators
要旨(英語): Quantum chaos plays a crucial role to understand the mechanism of relaxation in quantum many-body systems. In recent years, the concept of out-of-time ordered correlator (OTOC) has attracted much attention as an indicator of quantum chaos. The OTOC represents the sensitivity of chaotic dynamics to initial conditions, which provides a quantum analogue of the Lyapunov exponent. It has recently been proved that the Lyapunov exponent of the OTOC has a fundamental upper bound, which is saturated by the Sachdev-Ye-Kitaev model.  It has also been argued that quantum criticality is the key to saturate the bound, leading to the so-called fast scrambling.
   In this presentation, I focus on the XXZ model of the one-dimensional quantum spin chain, which exhibits quantum criticality. I obtained an analytical expression of the OTOC by using the quantum transfer matrix method of the algebraic Bethe ansatz, where the OTOC is averaged over the local operator basis.  I show that the OTOC in the thermodynamic limit can be represented by the largest eigenvalue and the corresponding eigenstate of the quantum transfer matrix.  I also discuss my ongoing numerical calculation to obtain the eigenvalue and the eigenstate.
発表言語: 日本語
氏名: 場本 圭 一
指導教員名: 小林 洋平 准教授
発表題目(英語):  The Optimization of Femtosecond Laser Processing Using Reinforcement Learning
要旨(英語):  Laser processing technology can perform high-quality and high-precision cutting of many kinds of material including metals, plastics, wood, cloth, CFRP, and sapphire. Moreover, various kinds of processing such as drilling, welding, and surface reforming, could be performed by a single machine in future.
 This ‘can do everything’ characteristic of laser processing is owing to its vast parameter space. There are a huge number of parameters in laser processing including wavelength, pulse energy, pulse duration, and repetition frequency, and the optimizations of these parameters are of key importance to achieving high-quality processing. Currently, the optimization of these parameters relies on ‘Experience and Intuition,’ which is also the case for the evaluation of outcomes. The parameter space is so vast that one cannot achieve the best optimization by grid searching.
 Recently, we succeeded in-situ monitoring of a removed volume during laser processing from scattering light using a home-made supervised deep learning system. The real-time monitoring is one key ingredient for an efficient optimization, and the other is a strategist.
 In our study, we are developing a reinforcement learning system that changes processing parameters according to the information from the in-situ monitoring system. The system is consist of an automatic laser processing system with the reinforcement learning engine, and the system will search the optimized parameters efficiently.
 I took two parameters, fluence and numerical aperture, as a starting point for the reinforcement learning system. I constructed a laser processing system with an adjustable laser fluence and numerical aperture. The system can measure the processing time and plasma intensity during the processing. In the presentation, I’ll report the result obtained from this experiment.
発表言語: 日本語 
氏名: 長屋  勇輝
指導教員名: 福谷 克之 教授
発表題目(英語): Development of Spin-Polarized Atomic H source
要旨(英語):  The hydrogen atom is composed of an electron and a proton, each of which has a spin 1/2. Whereas the hydrogen atom exists in spin singlet and triplet states under zero magnetic field, the degeneracy is lifted with a magnetic field. By using a hexapole magnet, the hydrogen atom spin can be polarized, which has been developed in our grope. We have been developing the experimental apparatus in which Spin-Polarized Atomic H source (SPH) can be generated with a microwave cavity, a hexapole magnet, a stern gerlach magnet, a skimmer and apertures and detected by resonance-enhanced multiphoton ionization. The purpose of the present study is to develop SPH and conduct a SPH scattering experiment at magnetic surfaces, where the change in the spin polarization is measured. For example, the oxygen molecule has a magnetic moment and the magnetic structure of the O2 layer on Ag(111) is predicted to change depending on the O2 coverage. However, there is almost no way to measure the topmost surface magnetisms. We therefore aim to develop the measurement technique to detect the surface magnetisms such as O2 on Ag(111). Furthermore, SPH will not only allow us to investigate the surface magnetisms but also perform the research about interactions between the nuclear spin of hydrogen atoms and surfaces, which will provide deeper understanding of mechanisms, such as hydrogen molecules ortho-para conversion on surfaces. In the presentation I will to show you how to generate and detect SPH, and the current status and the future plans of the experiments.
発表言語: 日本語 
氏名: 林  悠大
指導教員名: 高 橋 陽太郎 准教授
発表題目(英語):  Terahertz dynamics of conduction electrons in skyrmion-hosting B20-alloy thin films
要旨(英語):   The magnetic skyrmions whose the constituent spins point to all directions wrapping up a unit sphere has been discovered in various noncentrosymmetric magnets.  Recently the skyrmions have been attracting much attention from the view point of emergent electromagnetic responses originating from their topological spin texture: Conduction electrons acquire quantum Berry phase proportional to the skyrmion number when traversing the skyrmions, leading to unique transport phenomena such as the topological Hall effect, the ultralow current-driven motion as well as the skyrmion Hall effect.  These transport phenomena have been mainly investigated by the dc electrical transport measurements thus far, whereas these phenomena can be viewed as the low-frequency limit of the resonance structures in the finite frequency region.  Therefore, the optical measurement, in particular, low energy optics can be an effective tool for the investigation of the novel topological phenomena of the conduction electrons.
 In this study, we investigate terahertz dynamics of conduction electrons in the series of B20-type alloys, which are the most prototypal skyrmion-hosting materials.  In this presentation we will focus on terahertz magnetoconductivity of MnSi thin film in which large DC negative magnetoresistance is observed near the magnetic transition temperature.  The terahertz conductivity spectra are well fitted by the Drude model even under magnetic fields, which give the quantitative measure of the scattering rate and the effective masses.  Our results indicate that the negative magnetoresistance is attributed to the change in the scattering rate rather than that in the effective masses.
発表言語: 日本語 
氏名: 橋本 知 明
指導教員名: 為ヶ井 強 准教授
発表題目(英語): Trapping Large Magnetic Field by Suppression of Thermomagnetic Instability in Coated Conductor Stacks
要旨(英語): Coated conductors (CCs)  is used for an alternative method to generate a large magnetic field like bulk materials of high temperature superconductors. However, when one attempts to magnetize such bulk materials, one faces problems of mechanical strength and thermomagnetic instability. In order to solve such problems of the bulk magnet, stacking short segments of CCs has been proposed and a modest field has been successfully trapped. In the present study, we aimed at improving the trapped magnetic field by modeling the bulk magnet using GdBCO CCs with better JcH characteristics. For that purpose, GdBCO CCs produced by Fujikura were irradiated by 800 MeV Xe with Bφ(matching magnetic field) = 4 T. Two stacks of each 40 pieces of GdBCO CCs were placed next to each other and miniature Hall probes for measuring the trapped field were placed at the center of the stacks. In this condition, we have succeeded in trapping 7.95 T at the center of the stacks. However, it was also found that as the sweep rate of the magnetic field was increased, flux jumps occurred and the trapped magnetic field was strongly suppressed. We also report the evaluation of the local magnetic characterizations of the GdBCO CCs used for trapping magnetic fields, and discuss thermomagnetic instability due to changes in the number of CC stacked and sweep rate of the magnetic field.
発表言語: 日本語
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