Finite momentum superconductivity in superconducting hybrids: Orbital mechanism

TL;DR

This paper demonstrates both theoretically and experimentally that superconducting/normal metal hybrids can host finite momentum superconducting states at zero current due to orbital effects, revealing nonreciprocal properties.

Contribution

It introduces the orbital mechanism for finite momentum superconductivity in hybrid structures, supported by experimental evidence and theoretical analysis.

Findings

Finite momentum state exists at zero current in SN hybrids under in-plane magnetic field.

The state exhibits nonreciprocal kinetic inductance and asymmetric depairing currents.

Orbital effects from electron density gradients drive this finite momentum superconductivity.

Abstract

Normally in superconductors, as in conductors, in the state with zero current $I$ the momentum of superconducting electrons $\hbar q =0$. Here we demonstrate theoretically and present experimental evidences that in superconducting/normal metal (SN) hybrid strip placed in in-plane magnetic field $B_{in}$ finite momentum state ($\hbar q \neq 0$) is realized when $I=0$. This state is characterized by current-momentum dependence $I(q)\neq -I(-q)$, nonreciprocal kinetic inductance $L_k(I) \neq L_k(-I)$ and different values of depairing currents $I_{dep}^{\pm}$ flowing along the SN strip in opposite directions. Found properties have {\it orbital} nature and are originated from gradient of density of superconducting electrons $\nabla n$ across the thickness of SN strip and field induced Meissner currents. We argue that this type of finite momentum state should be rather general phenomena in superconducting structures with artificial or intrinsic inhomogeneities.