Far-infrared frequency-dependence of the AC Josephson effect in niobium point contacts

Citation:

Weitz, D. A. ; Skocpol, W. J. ; Tinkham, M. Far-infrared frequency-dependence of the AC Josephson effect in niobium point contacts. Physical Review B 1978, 18, 3282-3292. Copy at http://www.tinyurl.com/y2vm8dwt
weitz1978.pdf1.65 MB

Abstract:

We have measured the far-infrared frequency dependence of the strength of the ac Josephson effect in Nb cat-whisker point contacts, which have consistent and reproducible behavior and minimal extrinsic high-frequency limitations due to capacitance and heating. We monitor the constant-voltage Josephson steps induced on the dc I−V curves by an optically pumped far-infrared laser at fundamental frequencies corresponding to voltages from ∼0.2 to ∼2 times the energy-gap voltage. At all the frequencies studied, we find that the shape of the power dependence of the step amplitudes is fit reasonably well by Werthamer's frequency-dependent theory for tunnel junctions in the voltage-bias approximation. However, the observed magnitude of the steps is considerably less than predicted by the theory. By fitting to the I−V curves of the steps, we find that some of this discrepancy can be accounted for by heating-enhanced noise rounding. The remaining discrepancies (of the order of a factor of 2) are attributed to departures from a voltage bias at low frequencies and, tentatively, to the effects of the Ginzburg-Landau relaxation time at higher voltages. Our data confirm the expected intrinsic roll off of the strength of the ac Josephson effect above the energy gap.

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Last updated on 07/09/2021