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応用物理学輪講 Ⅰ

        
2017年7月14日(金)16:50~ 
62号講義室(2F) 
 座長:井口 照悟
64号講義室(2F)  
座長:石田 智大、伊藤 望
氏名: 藤代  有絵子
指導教員名: 十倉 好紀 教授
発表題目(英語): Magnetic phase transition and transport properties in a new topological magnet MnSi1-xGex
要旨(英語): Topological spin textures in chiral magnets have attracted much attention because of their novel physical properties and potential application to energy-saving devices. MnSi has been the most typical material hosting “skyrmion” lattice whereas MnGe has been the only material hosting “hedgehog” lattice.
 Here I created a solid solution of MnSi1-xGex in order to study a magnetic phase transition between these two topological spin textures with different topology, dimensionality, and magnetic properties. A dramatic magnetic phase transition is identified at x ~ 0.3 accompanied by the sudden change in the “topology” of the spin texture. Furthermore, a non-trivial variation of topological Hall effect will be discussed in terms of both the real- and momentum-space contribution.
発表言語: 英語
氏名: 松岡  秀樹
指導教員名: 岩佐 義宏 教授
発表題目(英語):  Thin film growth of Transition Metal Dichalcogenide with 2D strongly correlated electron system
要旨(英語):  For physical property research, the target materials has been limited to bulk for a long time, but as a breakthrough there is development of thin film growth technology. By designing monolayer films, interfaces of
different substances, and gating devices using thin films, the field of the solid state physics spread widely.
 One important material family as a study target using thin film growth is Transition Metal Dichalcogenide (TMD). TMD is a layered material of abundant variety, and the layers are bonded by Van der Waals forces.
 This crystal structure with strong two-dimensional nature are expected to produce a high-quality monolayer thin film or emergence of the quantum phase at the clean interface between different TMD films. In the presentation, I will talk about the current status of TMD films growth by MBE (Molecular Beam Epitaxy) technique, especially focusing on
materials with 2D strongly correlated electron system.
発表言語: 日本語
氏名: 古川  頼誉
指導教員名: 押山 淳 教授
発表題目(英語): Determination of the densities of amorphous materials from first-principles calculations
要旨(英語):  Amorphous materials, which lack the long-range order while retaining the short-range order in the atomic arrangements, are indispensable for modern electronic devices. Their electronic and structural properties have been successfully revealed by first-principles calculations that are based on quantum theories e.g. the density functional theory (DFT). However, their densities have not ever been determined from first-principles calculations, despite its importance. This was due to the fact that because of the disordered nature of amorphous materials, their atomic configurations are not definite and thus there is no one-to-one correspondence between the densities and the total energies. To remedy this problem, we have devised a novel method which employs the DFT and the Car-Parrinello molecular dynamics method based on the DFT. We have applied it to amorphous silicon, which is a typical example of amorphous semiconductors, and found that the determined density and its bulk modulus nicely agree with the experiments. The procedure and the physical meaning of the method, and the calculated results are discussed in detail.
発表言語: 英語
氏名: 松ヶ 谷 明史
指導教員名: 今田 正俊 教授
発表題目(英語):  Introducing superconducting order parameter on the magnetic domain wall of the pyrochlore iridate/ Symmetry-based indicators of band topology in π-flux models
要旨(英語):   First topic is about the magnetic domain wall in the pyrochlore iridates. Pyrochlore iridates R2Ir2O7 (R: Y, Eu, Pd and so on) have large Coulomb correlations and spin-orbit interactions which lead the non-coplanar magnetic order called all-out-all-in (AOAI) order. The domain wall in such magnetic order is known to behave a 2D metallic states which can be driven by the external magnetic fields. The purpose of my research is determining whether these metallic domain wall states can become the topological states, especially the topological superconducting states by introducing superconducting order. Therefore, I take Hartree-Fock calculation with the particular superconducting order parameter and check the existence of localized edge states which are the evidence of topological sates.
 Second topic is about Lieb-Shultz-Mattis (LSM) theorem for symmetry protected topological (SPT) phase. LSM theorem and generalized one determine the constraint for filling (particle number per unit cell) to have symmetry short range entangled (sym-SRE) ground states which have a gap between excited states and no degeneracy. For example, ordinary band insulators or SPT phases such as a topological insulator have sym-SRE ground state. Original LSM theorem cannot distinguish ordinary phases and SPT phases, however recently, it was brought out that we can set the constraint for filling to get SPT phases under particular external magnetic field. Currently, we use only the magnetic translational symmetry to derive such a theorem so I trying to generalize it to the situation with space group symmetries.
発表言語: 英語 
氏名: 松浦  正俊
指導教員名: 香取 秀俊 教授
発表題目(英語): Stability improvement of mercury optical lattice clock  
要旨(英語): An optical lattice clock can realize small frequency instability because the clock transition spectroscopy of many atoms (N > 103) are performed at the same time, and a frequency ratio of optical lattice clocks can be measured with small statistical uncertainty in a short averaging time. High-stability frequency ratio of clocks with different atomic species can be used for a test of the constancy of the fine-structure constant α, and especially mercury (Hg) is sensitive to the variation of α.
 Our group has developed Hg optical lattice clock with the systematic uncertainty of 7×10-17 [1], and the frequency ratio of Hg and strontium (Sr) optical lattice clocks has been measured with the instability of 3×10-15/√(τ/s) for an averaging time of τ>10 s. To search the variation of α in a short time scale, Hg clock is required to be improved its stability, which is currently limited by the laser frequency noise and detection noise for spectroscopy.
 In my presentation, I will talk our efforts to improve the stability of the Hg optical lattice clock.
[1] K. Yamanaka et al., Phys. Rev. Lett. 114, 230801 (2015)
発表言語:  日本語
氏名: 三﨑  航
指導教員名: 永長 直人 教授 
発表題目(英語):  Effects of berry curvature on diffusive dynamics and electron-phonon system
要旨(英語):  Berry phase, which is now an indispensable concept for physicists, was originally defined as the adiabatic phase. However, it can also be regarded as the geometric phase induced by the projection in the Hilbert space on Brillouin zone. In that sense, because of this geometrical origin, this phase can appear not only in the slow process near equilibrium like Hall conductivity, but also in the ultra-fast phenomena following the creation of particle-hole pair/exciton in solid.
 Recently, due to the technological development, ultra-fast dynamics in solid following the irradiation of light, like an excitation of coherent phonon and resultant enhancement of Tc of "possible" superconductivity, can be detected by femto second time-resolved experiment, so clarifying the role of Berry phase in such an ultra-fast process is important also from the experimental perspective.
 In this talk, after briefly reviewing the concept of Berry phase on Brillouin zone, I will explain the qualitative effect of Berry phase on diffusive dynamics by showing the exact solution of Langevin equation for Bloch electrons in the presence of Berry curvatures, and then focus on more specific and experimentally realistic electron-phonon system.
発表言語: 日本語 

 
 

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