Directional-dependent Berezinskii-Kosterlitz-Thouless transition at EuO/KTaO${}_3$(111) interfaces

Abstract

In two dimensions, a phase-coherent superconducting state is established via a Berezinskii-Kosterlitz-Thouless (BKT) transition, whose critical temperature $T_{\mathrm{BKT}}$ is determined by the global superfluid stiffness in uniform superconducting systems. We report that at the interface between (111)-oriented KTaO${}_3$ and ferromagnetic EuO, the two-dimensional superconducting state exhibits a BKT transition relying on the direction of in-plane bias current. The highest $T_{\mathrm{BKT}}$ occurs when current is applied along one of the [11$\bar{2}$] axes of KTaO${}_3$, underscoring a spontaneous breaking of the threefold lattice rotational symmetry. Such directional dependence of $T_{\mathrm{BKT}}$ is consistently reflected in the nonreciprocal signals stemming from superconducting fluctuations above the transition. We attribute this phenomenon to an interfacial phase segregation; the phase with higher $T_{\mathrm{BKT}}$ self-organizes into quasi-one-dimensional textures that stretch along one of the [11$\bar{2}$] directions. Our results point toward the emergence of exotic phases of matter beyond the description of conventional BKT physics at a superconducting interface that is subjected to ferromagnetic proximity.

A combined file including main text (21 pages, 4 figures) and Supplementary Information (29 pages, 11 figures). Accepted by Communications Physics