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Photoresponse and conductance mechanisms in nanoscale superconductor normal-metal point contacts

This work presents measurements of the dynamic resistance and photoresponse of high impedance (10-1000) point contacts (SNPCs) between bulk superconducting tantalum and an electrochemically etched ( 10-200nm radius at tip) tungsten whisker immersed in superfluid He at 1.8K. The dynamic resistance of these nanoscale contacts is well described by the ballistic BTK conductance model which includes the effects of Andreev scattering at the Normal-Superconductor interface. Deviations from the BTK conductance theory are also observed. The contacts show resistance peaks at symmetric bias voltages much larger than the superconducting gap (I V I>10 A). The voltage at which these peaks occur is found to be linear in the square root of the measured contact resistance, strongly supporting a model in which the resistance peaks result from the destruction of superconductivity by the magnetic field produced by current flow through the ballistic contact. The model is in quantitative agreement with observation and implies that each resistance peak corresponds to the appearance of a single flux quantum at the contact. Measurements of the photoresponse at 860nm of the Ta-W point contacts are reported. A model which accurately describes these measurements is presented: The incident light induces an increase in the effective electron temperature of the irradiated W tip and a non-equilibrium quasi-particle density in the Ta resulting in a decrease in the superconducting gap. The photoresponse signal is then proportional to the current increment resulting from these changes and can be computed from the BTK theory applied to an SNPC in the presence of incident light. Three novel effects and applications follow from this model: 1) In several contacts, photoresponse measurements showed that the W metal electron temperature varied with laser power as Teff 3 -THe3=aPIser, implying heat conduction from the W to the superfluid He via inelastic scattering of electrons off the W surface oxide. 2) The photoresponse is used in a direct observation of the destruction of superconductivity at the contact when a finite bias resistance peak occurs. 3) The quasi-particle recombination time is measured from the roll-off of the photoresponse signal (proportional to the induced quasi-particle density) as laser intensity modulation frequency is increased. The thesis introduces a technique of measuring this roll-off even when the roll-off frequency is beyond the front-end detection bandwidth of the point contact. The measurement yields a value of zR=50ns, implying an average phonon trapping enhancement factor of 10. Thesis Supervisor: Ali Javan Title: Francis Wright Davis Professor of Physics