Directional-dependent Berezinskii–Kosterlitz–Thouless transition at EuO/KTaO3(111) interfaces

In two dimensions, a phase-coherent superconducting state is established via a Berezinskii-Kosterlitz-Thouless (BKT) transition, whose critical temperature TBKT is determined by the global superfluid stiffness in uniform superconducting systems. We report that at the interface between (111)-oriented KTaO3 and ferromagnetic EuO, the two-dimensional superconducting state exhibits a BKT transition relying on the direction of in-plane bias current. The highest TBKT occurs when current is applied along one of the [11$$\bar{2}$$] axes of KTaO3, underscoring a spontaneous breaking of the threefold lattice rotational symmetry. Such directional dependence of TBKT 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 TBKT 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. The superconducting transition temperature is generally believed to be independent of the direction of applied small bias currents. Here, the authors demonstrate the occurrence of directional-dependent Berezinskii-Kosterlitz-Thouless transition at the two-dimensional superconducting interface EuO/KTaO3(111), which hints at emergent superconducting textures engendered by ferromagnetic proximity.