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6月26日(金)16:50〜 
Aグループ 
 座長:真島 裕貴、SAIKA Bruno Kenichi
Bグループ  
座長:元 志喜
氏名: 小林 優斗
指導教員名: 為ヶ井 強 准教授
発表題目(英語): Enhancement of critical current density in terms of pinning centers introduced by Co doping and heavy-ion irradiation in 1144 type iron-based superconductor CaKFe4As4
要旨(英語): Introduction of defects to superconductors enhances their critical current density (*J*c). Recently, a new iron-based superconductor, CaKFe4As4, with a new type of structure was found [1], and its *J*c was evaluated to
be ~2 MA/cm2 at 2 K and self-field [2].
 To enhance *J*c in CaKFe4As4, we introduced defects by chemical and physical methods. In the chemical method, we have grown high-quality single crystals in which a part of Fe is replaced by Co up to 9 %. Co-doping is believed to make the inherently overdoped CaKFe4As4 closer to optimally doped one. A relatively strong magnetic field dependence of *J*c in the pristine CaKFe4As4 is weakened by modest Co-doping (0.03 < *x* <0.07), leading to large *J*c at high fields. It clearly demonstrates that the introduced Co atoms work as point defects.
 In the physical method, 200 MeV Au ions or 2.6 GeV U ions are irradiated into CaKFe4As4, which are known to produce columnar defects. In order to compare effects of two different kinds of defects on *J*c and get some insight into the effect of coexisting point defects, the pristine and 3% Co-doped crystals are irradiated. *J*c of the pristine crystals is enhanced by the introduction of defects by both 2.6 GeV U and 200 MeV Au up to BΦ = 4 T. *J*c of Co-doped crystals is enhanced by the introduction of defects by 2.6 GeV U up to BΦ = 4 T, and by 200 MeV Au up to BΦ = 2 T.
[1] A. Iyo *et al*., J. Am. Chem. Soc. *138*, 3410 (2016).
[2] S. Pyon *et al*., Phys. Rev. B *99*, 104506 (2019).
発表言語: 日本語
氏名: 清水 宏太郎
指導教員名: 求 幸年 教授
発表題目(英語): Phase transitions between helices, vortices, and hedgehogs by spatial anisotropy in chiral magnets
要旨(英語): Chiral magnets show a variety of magnetic textures, such as one-dimensional (1D) helices, two-dimensional (2D) vortices and skyrmions, and three-dimensional (3D) hedgehogs. One of the relevant parameters to control these magnetic textures is spatial dimension or anisotropy of the
system.For instance, it was experimentally observed that a 3D hedgehog lattice (HL) is deformed and changed to a quasi-2D structure by making the samples thinner [1]. While such a control is important for engineering the topological magnetic textures and associated transport phenomena, there remain many open problems on the effect of spatialdimension or anisotropy.
 In this talk, we report our theoretical results for the effect of spatial anisotropy on the topological magnetic textures in metallic chiral magnets by variational calculations for an effective spin model
of chiral magnets [2,3]. We find that the 3D HL characterized by three orthogonal wave vectors, which is stable for the isotropic case, changes into a 2D vortex crystal while increasing the in-plane interactions. Meanwhile, while increasing the out-of-plane interactions, a topological phase transition takes place by pair annihilation of hedgehogs and anti-hedgehogs, and later, the system changes to a 1D helical state. We also show that another 3D HL characterized by four wave vectors exhibits different types of transitions for the anisotropy. We discuss the details of the magnetic and topological transitions, with emphasis on the changes of the emergent magnetic field arising from the noncoplanar spin textures.
[1] N. Kanazawa et al., Phys. Rev. B 96, 220414 (2017).
[2] S. Hayami, R. Ozawa, and Y. Motome, Phys. Rev. B 95, 224424 (2017).
[3] S. Okumura, S. Hayami, Y. Kato, and Y. Motome, Phys. Rev. B 101, 144416 (2020).
発表言語: 英語
氏名: 佐藤 達郎
指導教員名: 小林 洋平 教授
発表題目(英語): High sensitivity Mid-IR comb spectroscopy with immersion grating
要旨(英語): As one of the most powerful methods to examine illness such as COVID-19, breath diagnosis is gathering attention. Although some types of breath diagnosis devices have already been developed, most of them only target a limited range of molecules. Generally, the absorption lines of molecules exist in the mid-infrared (MIR) region; therefore, diagnostics for various diseases are enabled by precise and broadband spectroscopy in this region.
 We have developed a MIR comb spectroscopy system that is spectrally tunable between 3-5 um with a highly dispersive diffraction grating and a two-dimensional array detector. By improving the stability of the system and introducing a multi-pass cell, our system has potential to detect concentration levels of tens of ppb which is typical of some biomarkers in human breath.
 In my talk, I will introduce the motivation of this study and specifications of our spectrometer. In addition, I will demonstrate the potential of applying deep-learning techniques to detecting components in a mixed gas.
発表言語: 日本語
氏名: 杉本 宜陽
指導教員名: 齊藤 英治 教授
発表題目(英語): Measurement of local and nonlocal nuclear spin Seebeck effect
要旨(英語): Spintronics, which is applying spin degree of freedom of electrons for engineering, is receiving a lot of attention. Recently, electron spin has been actively researched. On the other hand, nuclear spin, which is spin degree of freedom of nuclears, has many unexplained parts.
 Our research topic is nuclear spin Seebeck effect. Spin Seebeck effect is the conversion effect of thermal current to spin current. In order to prove nuclear spin Seebeck effect, we need to distinguish electron spin Seebeck effect to nuclear spin Seebeck effect. One of the differences of these is temperature dependence of electron spin Seebeck effect and nuclear spin Seebeck effect. It is predicted that the peak of temperature dependence of nuclear spin Seebeck effect is lower than that of electron spin Seebeck effect because the Zeeman energy of a nuclear spin is smaller than that of an electron. We measured local nuclear spin Seebeck effect and observed electromotive force derived from nuclear spin Seebeck effect at 0.1 K. Now we plan to measure nonlocal nuclear spin Seebeck effect.
We made the device for measurement of nuclear spin Seebeck effect. The device consists of Pt film and MnCO3 substrate. Pt film was patterned by an electron beam lithography exposure and deposited by sputtering on MnCO3.
発表言語: 日本語 
氏名: 島崎 紘太
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 serected, which has been developed in our group. The purpose of the present study is to develop a Spin-Polarized Atomic H source (SPH) and conduct an SPH scattering experiment at magnetic surfaces, where the change in the spin polarization can be measured. Furthermore, SPH will also also reveal the mechanism how hydrogen molecules are formed on surfaces and interactions between the nuclear spin of hydrogen atoms and surfaces.
In the presentation I will show you how to generate and detect SPH, and the current status and the future plans of the experiment.
発表言語: 日本語
氏名: 角崎 正典
指導教員名: 小芦 雅斗 教授
発表題目(英語): Entanglement verification of light without state estimation
要旨(英語): Entanglement is a quantum correlation which cannot occur in classical physics, and is one of the properties which characterize quantum mechanics.
 When we apply quantum mechanics to quantum computer, quantum communication, quantum key distribution, and so on, entanglement takes an important role.
 Especially in quantum communication, entanglement is implemented by using light. However, estimating states of light requires infinite types of observations because Hilbert space has infinite dimension in the case of light. This makes entanglement verification difficult.
 In this talk, I would explain what is entanglement briefly, introduce a method of entanglement verification in the case of spin of electron, and expand this method in the case of light.
発表言語: 日本語  
氏名: 清水 康司
指導教員名: 鹿野田 一司 教授
発表題目(英語): Attempt to observe quantum oscillation in spin liquid candidate κ-(ET)2 Cu2(CN)3
要旨(英語): Sqin liquid is material in which spins do not form long-distance order.κ-(ET)2 Cu2(CN)3 is shown to be spin liquid by several NMR measurements. It is suggested by the measurements of both specific heat and resistivity under pressure that the ground state ofκ-(ET)2 Cu2(CN)3 is described by spinon fermi surface. An observation of quantum oscillation in spin liquid can be a strong evidence of spinon fermi surface. In the first half, I will introduce the result of magnetoresistance measurement under pressure to pursue quantum oscillation. Magnetoresistance data suggest that small pieces of metal phase are mixed in the insulator phase. So, in the latter half, analysis which attempt to remove effects of metal mixed in the insulator phase.
発表言語: 日本語
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