Emergence of spin-orbit coupled ferromagnetic surface state derived from Zak phase in a nonmagnetic insulator FeSi

A chiral compound FeSi is a prototypical example of the strongly-correlated d-electron insulators. Its peculiar charge and spin dynamics have provoked many important physical concepts such as d-electron Kondo insulator and insulator-to-metal transition due to on-site strong Coulomb interaction. Triggered by recent new insights into topological aspects of correlated insulators, FeSi is attracting renewed attention. However, it remains highly nontrivial that compounds made of light (and usually common) elements, like FeSi, can bear topological characteristics.

In this study, we demonstrate a novel surface state hosting metallic conduction and ferromagnetic order in FeSi with insulating and nonmagnetic bulk state. We identify that the surface state is not categorized into the class of topological insulators but can be described by the central concept in modern theory of polarization, i.e., Zak phase. Owing to the unique dipolar charge distribution of Zak-phase origin, the surface state produces the strong SOC properties despite the absence of heavy-metal elements. By taking advantage of the emergent SOC, we also realize spintronic functionalities related to current-induced spin-dynamics. (e.g., magnetization switching). Our results shed light on compounds made of common elements and demonstrate another route for SOC-based spin manipulation [1].

[1] Y. Ohtsuka, N. Kanazawa et al., Science Advances in press.