Topological properties and functionalities in oxide thin films and interfaces

Abstract

As symbolized by the Nobel Prize in Physics 2016, "topology" has been recognized as an essential standpoint to understand and control the physics of condensed matter. This concept may be spreading even into application areas such as novel electronics. In this trend, there has been reported a number of study for the oxide films and heterostructures with topologically non-trivial electronic or magnetic states. In this review, we overview the trends of new topological properties and functionalities in oxide materials with sorting out a number of examples. The technological advances in oxide film growth achieved over the last few decades are now opening the door for harnessing novel topological properties. two-dimensional chiral p-wave superconductivity, a topological superconducting state equivalent to the Pfaffian state, the Majorana zero modes are expected to localize in vortex cores. Their non-abelian braiding has also attracted recent interest due to its possible application to the topological quantum computation. 2-2. Properties and functionalities of topological spin structuresA striking example of the topologically non-trivial spin structure is magnetic skyrmion [21]. In a typical structure called Bloch-type skyrmion, spins in two dimensions gradually rotate towards the center in perpendicular to the radius directions, as illustrated in Fig. 1(b) [22,23]. The topological invariant concerned is integer skyrmion number (0, ±1, ±2, ...), defined as integral of solid angle subtended by constituent spins in real space. The single skyrmion behaves as a topologically protected spin object, thus attracting rising attention as a robust information carrier in solids. Here, for stabilizing canting of neighboring spins, asymmetric Dzyaloshinskii-Moriya (DM) interaction derived from the spin-orbit coupling is a key parameter. The skyrmion and its lattice structure have been directly observed in chiral metals such as B20type transition-metal silicides and germanides [22,24] and a chiral insulator [25] by some special transmission electron microscopy techniques.Recent studies show promise for the application of the skyrmion to non-volatile magnetic memory, which has the advantages of low driving current and high memory density over the magnetic bubble and racetrack memory devices [26,27]. The non-coplanar spin alignment in the skyrmion produces emergent magnetic field to conduction electrons, resulting in giant Hall effect that is already applied for detecting the skyrmions [28][29][30][31][32][33]. Lately, in addition to the Bloch-type skyrmion, more variety of topological spin structures have been discovered such as Néel-type skyrmion [34,35] and anti-skyrmion [36,37]. More effort will focus on engineering skyrmionic materials in order to realize stable manipulation of skyrmions above room temperature. 3-1. Topological electronic states in oxide filmsCompared to selenides and tellurides forming a group of topological insulator materials, reports of threedimensional topological insulator in oxides have been limited to theo...

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