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

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 due to finite-time relaxation in Andreev levels. While current phase relation and dissipation 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 irradiation, we observe the nonequilibrium dissipation and supercurrent simultaneously for the first time. We find the supercurrent dependence on frequency and power deviates from adiabatic ac Josephson effect due to finite relaxation rate. Notably, with irradiation frequency larger than the minigap, the dissipation shows an enhancement 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 transitions between levels on the same side of the minigap. Our results demonstrate the ultra-sensitivity of the dissipation measurement for other proximitised superconducting systems where novel physics can be revealed by nonequilibrium-induced level transitions.