The 9th QLC young colloquium
Date & Time : Monday, February 20, 2023. 13：30～14：30
Speakers：Daiki SEKINE（Tohoku University）, Masahiro HORI（Tokyo University of Science）
Place：online using “Zoom”
*If you wish to join this seminar, please register at this site by noon, February 20.
*Zoom meeting ID information will be sent by the day of this colloquium to those who registered.
Speaker：Daiki SEKINE（Tohoku University）
Title：Real space imaging of multipolar domains in MnTiO3
Multipoles, which describe the distributions of the electric charge and magnetic moment in materials, have recently attracted much attention because of their potential to describe various physical phenomena in a uniform way from the microscopic point of view. While theoretical studies are exploring the relation between the multipoles and physical phenomena, experimental methods to directly observe most of the multipoles are not established at the moment. As development of method to detect each multipole has intensively been done in recent years, there is a strong requirement to establish the methods which directly detect the multipoles.
In the context of these circumstances, we have worked on the detection of the multipoles by optical second harmonic generation (SHG), which is sensitive to the symmetry . Under the electric dipole approximation, it is well known that the SHG emerges only in noncentrosymmetric systems. However, utilizing the SHG process beyond the electric dipole approximation, such as magnetic dipole transition, SHG can emerge even in centrosymmetric systems. Furthermore, since SHG is also sensitive to the breaking of time reversal symmetry, SHG has been used as a probe for magnetism. Therefore, SHG has a potential to detect the odd-/even-parity multipoles and the electric/magnetic multipoles.
In this study, we tried to detect the multipoles and their domains in MnTiO3 by SHG. MnTiO3 is expected to have various multipoles simultaneously in terms of the symmetry. Considering the symmetry of its crystal structure, the existence of electric toroidal dipole is allowed. It is related to ferroaxial order, which has been intensively studied in recent years . Furthermore, MnTiO3 has an antiferromagnetic transition at the Néel temperature 65 K, and shows the antiferromagnetic order breaking both space inversion and time reversal symmetries. The antiferromagnetic order is decomposed into magnetic monopole, magnetic quadrupole, and magnetic toroidal dipole, and these multipoles show the fascinating physical phenomena in previous studies [3, 4]. Here, we performed the SHG imaging measurement in MnTiO3, and succeeded in visualizing the electric toroidal and magnetic (toroidal) domains. In this presentation, I will discuss the detail and recent progress.
 T. Hayashida, Y. Uemura, K. Kimura, S. Matsuoka, D. Morikawa, S. Hirose, K. Tsuda, T. Hasegawa, and T. Kimura, Nat. Commun. 11, 4582 (2020).
 N. Mufti, G. R. Blake, M. Mostovoy, S. Riyadi, A. A. Nugroho, and T. T. M. Palstra, Phys. Rev. B 83, 104416 (2011).
 T. Sato, N. Abe, S. Kimura, Y. Tokunaga, and T. Arima, Phys. Rev. Lett. 124, 217402 (2020).
Speaker：Masahiro HORI (Tokyo University of Science）
Title：Topological Superconducting States in Two-Dimensional Quasiperiodic Systems
Topological superconductivity (TSC) results in a novel superconducting state characterized by a nonzero topological invariant in the bulk. Theoretically, so far TSC has mainly been studied in periodic crystals such as square lattice systems. In such systems with translational symmetry, the superconducting order parameter is uniformly distributed. Meanwhile, quasicrystals (QCs) are characterized by long-range order without periodicity.
We examine an s-wave TSC in two-dimensional Penrose and Ammann-Beenker QCs with Rashba spin-orbit coupling and Zeeman field by solving the Bogoliubov-de Gennes equations [1,2]. The mean-field approximation is applied and the superconducting order parameter as well as the spin-dependent Hartree potential are obtained self-consistently .
We find that the self-consistently obtained mean fields are spatially inhomogeneous in both QCs. We demonstrate how the underlying aperiodic structure of a QC is reflected in the superconducting order parameter. We also calculate the Bott index as the topological invariant of the system, which is equivalent to the first Chern number in the presence of translational symmetry. Our results confirm the existence of a stable TSC state in QCs and the appearance of a Majorana zero mode along the edges of a QC, despite the lack of translational symmetry.
 M. Hori et al., to be submitted.
 S. L. Goertzen, K. Tanaka, N. Nagai, Phys. Rev. B 95, 064509 (2017).
Committee Chair：Hiroki WADATI（University of Hyogo）