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.

Publisher's VersionAbstractWe 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.

weitz1978.pdf Weitz, D. A. ; Skocpol, W. J. ; Tinkham, M. High-frequency behavior of ideal superconducting point contacts.

Physical Review Letters 1978,

40, 253-256.

Publisher's VersionAbstractWe have studied niobium point contacts which are very consistent and reproducible from junction to junction, in both their dc and high-frequency behavior. We find a strong correlation between the sharpness of the gap structure and the ac Josephson effect, and we present the first quantitative measurements of the far-infrared frequency dependence of the Josephson effect above the energy gap. The measured I−V curves are also compared with available theoretical models.

weitz1978.pdf Weitz, D. A. ; Skocpol, W. J. ; Tinkham, M. Properties of Josephson point-contact far-infrared detectors.

Infrared Physics 1978,

18, 647-656.

Publisher's VersionAbstractThe far-infrared (FIR) properties of niobium cat-whisker point contacts were studied using radiation from an optically pumped FIR laser. The reproducible behaviour of junctions with excellent high-frequency performance allowed a measurement of the FIR frequency dependence of the strength of the a.c. Josephson effect. The shape of the laser-induced steps was used to measure the effective noise temperatures, which increase with bias voltage in agreement with a heating model of metallic constrictions. The high-quality junctions were tested as frequency-selective, incoherent FIR detectors, with the d.c. bias in the vicinity of the incipient laser step. The response was found to be linear in the laser power, and the best measured responsivity at 604 GHz was 2 × 105 V/W, while the best noise equivalent power was 10p−13 W/Hz, with a 450 Hz chopping frequency. The NEP is limited by the voltage noise in the junction, which was found to have an approximately frequency dependence. The detector performance is degraded considerably at higher FIR frequencies. Also studied was the low-laser-power behaviour of the *I–V* curves near the critical current, which may be of importance for mixing applications with external local oscillators.

weitz1978.pdf