Publications

1978
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 VersionAbstract
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.
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 VersionAbstract
We 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 VersionAbstract
The 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
1977
Weitz, D. A. ; Skocpol, W. J. ; Tinkham, M. Niobium point-contact Josephson-junction behavior at 604 gHz. Applied Physics Letters 1977, 31, 227-229. Publisher's VersionAbstract

We have measured the ac Josephson effect in Nb point contactsat 604 GHz (496 μm). We find the coupling of the far‐infrared radiation to the point contact to depend in a simple manner on the resistance of the contact. The behavior of the high‐resistance point contacts (50⩽R⩽200 Ω) is very reproducible, allowing a quantitative comparison of the data to the frequency‐dependent Werthamer theory. We also account for the effects of noise and heating and compare these to Tinkham’s heating theory.

weitz1977.pdf
1974
Johns, J. W. C. ; Mckellar, A. R. ; Weitz, D. Wavelength Measurements of C-13 O-16 Laser Transitions. Journal of Molecular Spectroscopy 1974, 51, 539-545. Publisher's VersionAbstract

Wavelength measurements of 160 13C16O laser lines in the range from 1500 to 1900 cm−1 have been made with a conventional grating spectrometer. The accuracy of the measurements is about ±0.003 cm−1. Although more precise measurements of these lines will ultimately be made using microwave beating techniques, the present results should be of interest for the determination of improved molecular constants for CO and especially for the various spectroscopic applications of the CO laser.

johns1974.pdf
1973
Bradley, L. L. T. ; Schwob, H. P. ; Weitz, D. A. ; Williams, D. F. Delayed Electroluminescence Quenching in Anthracene. Molecular Crystals and Liquid Crystals 1973, 23, 271-282. Publisher's VersionAbstract

Observations of the transient behaviour of electroluminescence in sublimation flakes have shown that quenching of the delayed electro-luminescence component by the carriers themselves becomes important at high currents. The results show quenching is of the same order as observed for carrier quenching of optically produced triplet excitons. No quenching was observed in thick crystals though carrier densities of the same order are present. This is presumably due to the difference in recombination zone width which may be larger in these thick crystals.

bradley1973.pdf
Schwob, H. P. ; Weitz, D. A. ; Williams, D. F. The Variation of the Carrier Recombination Region with Carrier Density in Anthracene Crystals. Molecular Crystals and Liquid Crystals 1973, 24, 271-282. Publisher's VersionAbstract

The time dependence of slow electroluminescence has been used as an experimental probe for studying the physical characteristics of the recombination region in anthracene. Under conditions where two injecting contacts are used, and neither carrier is trapped to a large extent, the recombination zone is found to occupy approximately 0.3 of the crystal volume for current densities of 10−5-10−8 amps cm−2. When forced hole injection is used, the recombination volume at low currents decreases, particularly at low temperatures. This decrease in recombination volume is discussed in terms of carrier trapping and filament formation.

schwob1973.pdf

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