Giant Magnetochiral Anisotropy in Weyl-semimetal WTe2 Induced by Diverging Berry Curvature

The concept of Berry curvature is essential for various transport phenomena. However, an effect of the Berry curvature on magnetochiral anisotropy, i.e. nonreciprocal magneto-transport, is still elusive. Here, we report the Berry curvature originates the large magnetochiral anisotropy. In Weyl-semimetal WTe2, we observed the strong enhancement of the magnetochiral anisotropy when the Fermi level is located near the Weyl points. Notably, the maximal figure of merit $\bar{\gamma}$ reaches $1.2\,{\times}10^{-6} \rm{m^2T^{-1}A^{-1}}$, which is the largest ever reported in bulk materials. Our semiclassical calculation shows that the diverging Berry curvature at the Weyl points strongly enhances the magnetochiral anisotropy.Comment: 26 page

MnSiにおけるスキルミオンおよびカイラルスピン構造の磁気輸送特性

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 十倉 好紀, 東京大学教授 永長 直人, 東京大学教授 川﨑 雅司, 東京大学教授 大谷 義近, 東京大学准教授 賀川 史敬University of Tokyo(東京大学

Trapping and manipulating skyrmions in two-dimensional films by surface acoustic waves

Abstract Skyrmions, topologically stable spin structures with particle-like properties, are promising for spintronics applications such as skyrmion racetrack memory. Though reliable control of skyrmion motion is essential for the operation of spintronics devices, the straight motion of skyrmions along the driving force is in general difficult due to an inevitable transverse force originating from their topology. Here, we propose a method of precise manipulation of skyrmions based on surface acoustic waves (SAWs) propagating in two dimensions. Using two standing SAWs, saddle-shape local potentials like quadrupole ion traps are created to trap skyrmions robustly. Furthermore, by tuning the frequencies of the SAWs, we show that trapped skyrmions not only move in straight lines but also move precisely in any direction in a two-dimensional thin film. These results could be helpful for the future design of spintronics devices based on skyrmions

Lighting Control System using an Actor -Critic type Learning Algorithm

Abstract-A novel lighting control system using the Actor -Critic algorithm was developed, in which users can set the brightness of the system through sensory operation, such as "much brighter" or "slightly darker". During development, this system must learn two states, i.e., the demands of the user and the brightness around the user. The Actor -Critic algorithm was applied for this purpose, and a simplified algorithm was developed. The effectiveness and usefulness of the proposed algorithm are discussed here through numerical simulations

Amorphous Ferromagnetic Metal in van der Waals Materials

Abstract Amorphous solids are non‐equilibrium states of matter that lack long‐range structural order, and are recognized as key materials in electronics. Although 2D van der Waals materials have been intensively studied in the wide field of materials science, their amorphous states have been less studied experimentally. Here, a van der Waals ferromagnetic semiconductor CrGeTe3 transforms into an amorphous ferromagnetic metal upon irradiation with high‐energy Xe ions. Notably, the Curie temperature of the amorphous state reaches 200 K, which is three times higher than that of the crystalline phase. The anomalous Hall conductivity in the amorphous phase is governed by the extrinsic skew‐scattering mechanism, although conventional theory predicts that skew scattering is dominant only in ultra‐clean ferromagnetic metals. The present results call for a new theory of the anomalous Hall effect in highly disordered ferromagnetic conductors. Moreover, the unique change in magnetic and transport properties due to amorphization of the van der Waals material is expected to open up a new research field in materials science