時空間反転対称な磁性体における非線形電流生成の理論的研究

京都大学新制・課程博士博士(理学)甲第22991号理博第4668号新制||理||1670(附属図書館)京都大学大学院理学研究科物理学・宇宙物理学専攻(主査)教授 柳瀬 陽一, 教授 川上 則雄, 教授 石田 憲二学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDFA

Chiral photocurrent in parity-violating magnet and enhanced response in topological antiferromagnet

Rectified electric current induced by irradiating light, so-called photocurrent, is an established phenomenon in optoelectronic physics. In this paper, we present a comprehensive classification of the photocurrent response arising from the parity violation in bulk systems. We clarify the contrasting role of $\mathcal{T}$- and $\mathcal{PT}$-symmetries and consequently find a new type of photocurrent phenomena characteristic of parity-violating magnets, intrinsic Fermi surface effect and gyration current. Especially, the gyration current is induced by the circularly-polarized light and it is the counterpart of the shift current caused by the linearly-polarized light. This photocurrent adds a new functionality of materials studied in various fields of condensed matter physics such as multiferroics and spintronics. A list of materials is provided. Furthermore, we show that the gyration current is strongly enhanced by topologically nontrivial band dispersion. On the basis of the microscopic analysis of Dirac models, we demonstrate the divergent photocurrent response and elucidate the importance of tilting of Dirac cones.Comment: 28 pages, 8 figures, 4 table

Nonlinear optical responses in superconductors under magnetic fields: quantum geometry and topological superconductivity

Noncentrosymmetric superconductors offer fascinating phenomena of quantum transport and optics such as nonreciprocal and nonlinear responses. Time-reversal symmetry breaking often plays an essential role in the emergence and enhancement of nonreciprocal transport. In this paper, we show the nonreciprocal optical responses in noncentrosymmetric superconductors arising from time-reversal symmetry breaking by demonstrating them in $s$-wave superconductors with a Rashba spin-orbit coupling and a magnetic field. Numerical results reveal the superconductivity-induced bulk photocurrent and second harmonic generation, which are forbidden at the zero magnetic field. We discuss the properties and mechanisms of the superconducting nonlinear responses emerging under the magnetic field. In particular, we investigate the magnetic-field dependence of the photocurrent conductivity and clarify the essential ingredients which give a contribution unique to superconductors under the magnetic field. This contribution is dominant in the low carrier density regime although the corresponding joint density of states is tiny. We attribute the enhancement to the quantum geometry. Moreover, the nonlinear conductivity shows peculiar sign reversal at the transition to the topological superconducting state. We propose a bulk probe of topological transition and quantum geometry in superconductors.Comment: 36 pages, 21 figure