Observation of multiple Andreev-re ection assisted cooling eects in a nano superconducting Josephson junctions*

We demonstrate the cooling eect induced by multiple Andreev-reflection (MAR) in the thermal hysteresis of superconducting constrictions in a clean and short limit. We measure the current-voltage (I - V ) characteristics of a single constriction and micro-SQUID device, and observe the voltage-step jumps above the critical current (I_c) and below the gap (Δ). Our analysis shows that the voltage-steps taken place over Ic are related to the dynamic phase-slip, but within Δ are induced by MAR. To demonstrate the cooling effect, we show the co-existence of MAR and quantum interference in the retrapping currents in a micro-SQUID device. Our findings provide a unified picture to understanding nonequilibrium superconductivity, and have strong implications in developing superconducting ballistics devices.