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Investigation of uncontrolled superconducting point contacts in a wide-band microwave system

L.F. Soloveychik, V. S. Étkin

Published Jun 1, 1975• 0 citations
OpenAlex

Abstract

At the present time there is considerable interest in the use of superconducting point contacts (SPC's) for various measurement purposes, in particular the measurement of millimeter and submillimeter band electromagnetic radiation. One problem facing the development of SPC detectors is ensuring their stability and resistance to external influences and noise and the ability to match them to the radiation source over a wide band. A design was developed with a stable uncontrolled SPC in an oversize quasi-optical waveguide for use within a wide band of wavelengths, and some of its characteristics in the 3 cm and 4 mm bands were investigated in the case of ''high resistance'' SPC's with an asymptotic resistance of 10 to 30 ..cap omega.. and critical currents of 5 to 40 ..mu..A.

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Soviet Journal of Quantum Electronics•Oct 31, 1976•1 citation
At the present time there is considerable interest in the use of superconducting point contacts (SPC's) for various measurement purposes, in particular the measurement of millimeter and submillimeter band electromagnetic radiation. One problem facing the development of SPC detectors is ensuring their stability and resistance to external influences and noise and the ability to match them to the radiation source over a wide band. A design was developed with a stable uncontrolled SPC in an oversize quasi-optical waveguide for use within a wide band of wavelengths, and some of its characteristics in the 3 cm and 4 mm bands were investigated in the case of ''high resistance'' SPC's with an asymptotic resistance of 10 to 30 ..cap omega.. and critical currents of 5 to 40 ..mu..A.
An experimental investigation was made of superconducting point contacts (SPC's), made of the hightemperature superconductor Nb 3 Sn, in external microwave fields. It was found that under nonequilibrium conditions these SPC's had a high degree of nonlinearity which made it possible to observe current steps corresponding to harmonics of the external radiation frequency at voltages exceeding the limiting value for these SPC's. Maximum frequency multiplication up to 4.2 × 10 12 Hz was recorded for an 8.9 × 10 9 Hz signal and up to 5 × 10 12 Hz for a 7.8 × 10 10 Hz signal. Investigations were made of the subharmonic structure of the current-voltage characteristics of SPC's in external microwave fields. Mixing of the microwave radiation with the contact self-radiation was observed experimentally at gap singularities.
Weakly connected superconductors can function as very low power sensors by utilizing the coherence properties of superconductors. One configuration which has produced significant success incorporates a superconducting point contact and a small resistance element in a low-inductance superconducting circuit. When the critical supercurrent (ic) of the point contact is of the order of φ0/L, (φ0=h/2e, L=inductance of the ring), that circuit exhibits coherent quantum behavior. Used as a parametric amplifier, it has been used to measure ultralow voltages, limited by the thermal noise fluctuations in the resistance element R. The voltage sensitivity is approximated by 8kTR/φ0, where k is Boltzmann's constant and T the absolute temperature of the resistance R. We have measured 4×10−16 V across 1.7×10−10 Ω at 4.2°K with a signal-to-noise ratio approaching 10. The operation of this sensor follows from the Josephson oscillation of voltage-biased superconducting point contacts. A dc voltage V0 across the resistance of the circuit produces an oscillating current at the angular frequency ωJ=2πV0/φ0 and an oscillating voltage across the point contact at ωJ. Upon supplying an rf or microwave current at frequency ωD, the point contact mixes ωJ and ωD. Using a homodyne detector at ωD of bandwidth larger than ωJ, we can retrieve the oscillation at ωJ. Direct measurement of this frequency ωJ yields V0. Such superconducting circuits may be useful as cryogenic thermometers, as thermal radiation sensors, and in low-level spectroscopy.
A superconducting point contact waveguide SQUID has been operated at 89 GHz. The performance is qualitatively the same as at lower frequencies in agreement with the SQUID model. With the Nb point located across a 0.15 mm high E band waveguide, reflection coefficient measurements were made as a function of both supercurrent phase as controlled by an externally applied magnetic field and millimeter wave power. Large parametric reactance variations were produced by changing the supercurrent phase. As the millimeter wave power is increased flux cycling was observed which involved up to four steps. This behavior corresponds to a flux transition time less than 10-12sec. Implications of the\cos \phiterm and negative resistance effects in the observed behavior of the reflection coefficient will be discussed.
A description is given of recent work at Berkeley on superconducting mixers and detectors for infrared and millimeter wavelengths. The first report is a review article which summarizes the status of development of superconducting components for infrared and millimeter wave receivers. The next report describes accurate measurements and also theoretical modeling of an SIS quasiparticle waveguide mixer for W-band which uses very high quality Ta junctions. The best mixer noise is only 1.3 times the quantum limit. Both the mixer gain and the noise are in quantitative agreement with the quantum theory. Next, a report is given on measurements and theoretical modeling of the absorptivity (surface resistance) of high quality epitaxial films of the high {Tc} superconductor YBCO from 750 GHz to 21 THz. Finally, there are reports on the design and experimental performance of two different types of high {Tc} bolometric detectors. One is a conventional bolometer with a gold-black absorber. The other is an antenna coupled microbolometer.