発表者名 
澤村 駿 
指導教員名 
高橋 陽太郎 准教授 
発表題目（英語） 
Photovoltaic effects by phonon excitation in ferroelectric semiconductor SbSI 
要旨（英語） 
Recently, nonlinear electromagnetics in quantum materials has been studied because it provides the means to probe and control new phase of matter. Above all, "shift current", which can be accounted for in terms of geometric phase, has been attracting much attention. As apposed to the previous photovoltaic effect at the interface, shift current has defect tolerance, and its direction can be reversed by the rotation of the polarization of light. These features lead to new mechanism for generation of electricity.
We focused on a ferroelectric semiconductor SbSI to measure bulk photovoltaic effect by soft phonon excitation under the radiation of terahertz pulses. In this presentation, I will explain the backgrounds of bulk photovoltaic effect including shift current. Then, the property of SbSI and the method of this research are introduced. After that, the results, which indicate large efficiency of the photo current at the resonant frequency, are shown. Finally, future plans for more precise computation of physical quantity are discussed. 
発表言語 
日本語 

発表者名 
正力 健太郎 
指導教員名 
高橋 陽太郎 准教授 
発表題目（英語） 
Magnetooptical response of massive Dirac fermions in kagome magnets 
要旨（英語） 
Light combines with magnetism to produce magnetooptical effects such as the magnetooptical Kerr effect and the Faraday effect. This effect has been applied to technologies such as optical isolators, magnetic domain imaging, and MO disks. Under these circumstances, in the field of material science, the concept of topology, which deals with invariant properties due to continuous deformation, has been attracting attention in recent years, and topological electrons such as Dirac/Weyl fermion have been found in the band structure of kspace.
In recent years, it has been recognized that this Dirac Weyl fermion plays an important role in nonlinear optical responses such as highorder harmonic generation and photocurrent [1]. Topological electronic structure can be regarded as having a magnetic monopole in the kspace, and it is expected that it will be strongly coupled with the magnetooptical response and new charge dynamics will be generated [2].
In this presentation, we will describe the characteristic magnetooptical responses of RMn6Sn6 (R = Tb,Dy), kagome lattice magnets with massive Dirac fermions from lattice geometry.
[1] L. Wu, *et al., *Nat. Phys. 13*,* 350 (2017).
[2] W. K. Tse, & A. H. MacDonald, Phys. Rev. Lett. 105, 057401 (2010). 
発表言語 
日本語 

発表者名 
高木 翼 
指導教員名 
石坂 香子 教授・小川 直毅 委嘱教授 
発表題目（英語） 
Photocurrent spectroscopy in magnetic topological materials 
要旨（英語） 
There are a variety of nonlinear optical(NLO) effects in solid including bulk photovoltaic effect(BPVE), harmonic generations, optical rectification, and nonlinear Kerr rotations. They are typically realized by light irradiation on noncentrosymmetric materials that results in nonlinear polarizations of electron systems. Recent studies have revealed that the topology of the electronic wave function in crystals, especially their phase factors, can describe remarkable physical phenomena such as NLO effects.
Shift current and injection current are known to dominant mechanisms of BPVE. These photocurrents are originated from such phase factors; therefore they could be enhanced in the topological materials with Weyl nodes where Berry curvature diverges. Investigation of relation between photocurrent and phase factors is of great importance in studying topologyrelated physics as well as for applications in optical devices such as infraredphotodetection sensor or solar cells. In this presentation, result of shiftand injection current spectroscopy on magnetic topological materials, which are (Ge,Mn)Te, MnBi2Te4 are CBST will be given. 
発表言語 
日本語 
