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

TL;DR

This study reveals that at EuO/KTaO3(111) interfaces, the BKT transition temperature depends on current direction, indicating spontaneous symmetry breaking and quasi-one-dimensional phase textures affecting superconductivity.

Contribution

It demonstrates directional dependence of the BKT transition at a superconducting interface, linked to interfacial phase segregation and symmetry breaking.

Findings

Highest T_BKT occurs along [11̅2] axes, showing directional dependence.

Nonreciprocal signals reflect anisotropic superconducting fluctuations.

Interfacial phase segregation leads to quasi-one-dimensional textures.

Abstract

In two dimensions, a phase-coherent superconducting state is established via a Berezinskii-Kosterlitz-Thouless (BKT) transition, whose critical temperature $T_{\rm BKT}$ 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 $T_{\rm BKT}$ 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 $T_{\rm BKT}$ 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 $T_{\rm BKT}$ 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.