# Synopsis: Supercurrents in a junction—it’s all about the timing

Microwave-frequency excitation of a superconductor-metal-superconductor junction is used to resolve the dynamic mechanisms behind the temporal response.

Superconductor–normal-metal–superconductor (SNS) junctions are used in a variety of sensor applications, but also provide a way to study the fundamentals of superconductivity. Two current values determine the response of an SNS junction when excited by ac signals: critical current, the maximum supercurrent that can flow through the junction, and retrapping current, below which the junction switches from a resistive state back to a superconducting state. What actually causes the time response of these currents is a longstanding puzzle and the picture is more complicated in junctions than in all-superconductor samples.

In a paper published in Physical Review Letters, Francesca Chiodi, Marco Aprili, and Bertrand Reulet, at the Université Paris-Sud, Orsay, France, have performed conductivity experiments on $\text{Nb}$$\text{Al}$-based junctions. By varying the excitation frequency and measuring the current through the junctions, Chiodi et al. have identified the characteristic time scales of the critical and retrapping currents, and associate the timescales of these currents with elastic and inelastic electron-phonon scattering, respectively, giving further insight into the complicated dynamics of SNS junctions. The understanding of the dynamics of SNS junctions may offer new routes to design fast devices, such as low impedance detectors. – Daniel Ucko

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