Phase-dependent dissipation and supercurrent of a graphene-superconductor ring under microwave irradiation

TL;DR

This study investigates how microwave irradiation affects phase-dependent dissipation and supercurrent in a graphene-superconductor ring, revealing nonequilibrium effects and photon-assisted transitions.

Contribution

It demonstrates the deviation from adiabatic Josephson behavior under microwave irradiation and introduces a new method to study photon-assisted physics in superconducting systems.

Findings

Supercurrent response deviates from adiabatic Josephson effect at high frequencies.

Dissipation is enhanced at phase 0 when irradiation frequency exceeds the minigap.

Phase-dependent dissipation is more sensitive to microwave irradiation than supercurrent.

Abstract

A junction with two superconductors coupled by a normal metal hosts Andreev bound states whose energy spectrum is phase-dependent and exhibits a minigap, resulting in a periodic supercurrent. Phase-dependent dissipation also appears at finite frequency due to relaxation of Andreev bound states. While dissipation and supercurrent versus phase have previously been measured near thermal equilibrium, their behavior in nonequilibrium is still elusive. By measuring the ac susceptibility of a graphene-superconductor junction under microwave irradiation, we find supercurrent response deviates from adiabatic ac Josephson effect as irradiation frequency is larger than relaxation rate. Notably, when irradiation frequency further increases above the minigap, the dissipation is enhanced at phase 0 where the minigap is largest and dissipation is minimum in equilibrium. We argue that this is evidence of the nonequilibrium distribution function which allows additional level transitions on the same side of the minigap. These results reveal that phase-dependent dissipation is more sensitive than supercurrent to microwave irradiation, and suggest a new method to investigate photon-assisted physics in proximitized superconducting system.