Photoresponse and nonequilibrium properties of a superconductor–normal-metal point contact

Abstract

We report measurements of the photoresponse of superconductor–normal-metal point contacts (SNPC’s) between electrochemically etched W whiskers (tip radius 10–200 nm) and bulk superconducting Ta immersed in superfluid He at 1.8 K. These observations are shown to be well described by a model in which the incident light induces an increase in the effective electron temperature of the irradiated W tip, and a nonequilibrium quasiparticle density in the Ta resulting in a decrease in the superconducting gap. The resulting optically induced current increment through the contact manifests itself as a photoresponse voltage across the contact, and can be computed from the BTK conductance theory of ballistic low-barrier SNPC’s. The effect is used in this work to probe the increase in effective electron temperature of the nanoscale metal tip induced by the incident light; this temperature is shown to vary with laser power as Teff3−THe3=aIlaser, an effect that follows from a previously unobserved heat conduction mechanism in which the nanoscale dimension of the tip plays an important role. The photoresponse also provides a direct observation of destruction of superconductivity in the vicinity of the contact at sufficiently large currents through the contact.

Source-reported materials — not catalogue approval

Similar papers