Phase Asymmetry of Andreev Spectra From Cooper-Pair Momentum

TL;DR

This paper demonstrates a phase asymmetry in Andreev spectra in a Josephson junction, caused by finite-momentum Cooper pairing under an in-plane magnetic field, revealing a novel nonreciprocal superconducting response.

Contribution

It introduces a new type of Josephson diode effect based on phase asymmetry of Andreev states, driven by orbital effects rather than Zeeman or spin-orbit interactions.

Findings

Maximum phase asymmetry at 0.15 T magnetic field

Asymmetry absent without in-plane magnetic field

Effect attributed to finite-momentum Cooper pairing

Abstract

In analogy to conventional semiconductor diodes, the Josephson diode exhibits superconducting properties that are asymmetric in applied bias. The effect has been investigated in number of systems recently, and requires a combination of broken time-reversal and inversion symmetries. We demonstrate a dual of the usual Josephson diode effect, a nonreciprocal response of Andreev bound states to a superconducting phase difference across the normal region of a superconductor-normal-superconductor Josephson junction, fabricated using an epitaxial InAs/Al heterostructure. Phase asymmetry of the subgap Andreev spectrum is absent in the absence of in-plane magnetic field and reaches a maximum at 0.15 T applied in the plane of the junction transverse to the current direction. We interpret the phase diode effect in this system as resulting from finite-momentum Cooper pairing due to orbital coupling to the in-plane magnetic field, without invoking Zeeman or spin-orbit coupling.