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Soviet Journal of Quantum Electronics

Nonlinear properties of Nb3Sn superconducting point contacts

M. V. Danileǐko, V. E. Derkach, A. M. Kostyshin, M. T. Shpak

Published Aug 31, 1981• 2 citations
Publisher

Abstract

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.

DOI
10.1070/qe1981v011n08abeh008032

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IEEE Transactions on Magnetics•Jan 1, 1977•2 citations
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.
Recently there has been a marked upsurge in interest in the construction of multipliers and mixers for the SHF and IR regions. Of special interest in this respect are Josephson junctions fabricated from high-temperature superconductors. The present paper reports the results of experimental investigations of superconducting point junctions (SPJ) fabricated from the high-temperature superconductor Nb3Sn, and operating in regimes of frequency multiplication and mixing in the SHF spectral region. A controllable Nb3Sn Nb3 Sn SPJ that could be adjusted after being immersed in liquid helium was used in the experiments. The method of preparation of the SPJ was the same as the one that was employed in Ref. 1. The assembled experimental setup allowed the SPJ to be irradiated with three-centimeter and four-millimeter microwaves separately and simultaneously. The current-voltage characteristics (CVO) of the (tuned) SPJ were registered on an oscillograph and then recorded automatically at different DHF-power levels. The differential resistance RD = dU/dl of the junction as a function of mixing voltage UO was recorded with the aid of a lock-in amplifier2 co nstructed by us. All the measurements were carried out at the liquid-helium temperature. The SPJ in question had the following parameters: the critical current Ic = 0.1–2.5 rnA and the resistance in the normal state Rn = 0.1–10 Ω. When the SPJ was subjected to 8.9- and 87-GHz radiation separately, the CVC exhibited current steps at voltages corresponding to the fundamental frequency and the harmonics of the external radiation, as well as a sub harmonic structure. The multiplication of the fundamental frequency to 1 THz was recorded in the case of the irradiation of the SPJ by an 8.9–GHz signal and the registration of the dependence of RD on the mixing voltage U0. This differential CVC exhibited a subharmonic structure near the fundamental frequency at low SHF-power levels and near this frequency and its harmonics at higher levels. When the SPJ was irradiated by 87-GHz signals, the multiplication of the fundamental frequency of the signals to 2.2 THz was recorded on the differential CVC. The SHF-power dependence of the harmonics and subharmonics had an oscillatory character. The mechanism underlying the formation of the sub harmonic structure on the CVC of the SPJ can be explained if we take account of the effect of the action of the Josephson self-radiation on the junction itself.3 It should be noted that the manifestation of the Josephson properties in the frequency range ∼1 · 1012 Hz was observed for a SPJ with high Ic and Rn ⩾ 1 Ω. When the Nb3Sn–Nb3Sn SPJ was irradiated by two signals with frequencies 87 and 8.9 GHz simultaneously, there appeared near the steps for the 87-GHz signal , as the power of the 8.9-GHz signal was increased, steps corresponding to signals with frequencies equal to combinations of the two frequencies, i.e., (87m ± 8.9k)-Ghz combination steps appeared on the eve. Thus, the first experimental investigations of the multiplication and mixing of signals by Nb3Sn-Nb3Sn SPJ have shown that such SPJ operate quite well in regimes of mixing and multiplication of external SHF radiation.
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.
Current-voltage characteristics of NbN-NbN Josephson point contacts have been examined. The point contacts were formed by NbN thin films deposited on Nb or W whiskers and Nb posts. The current-voltage characteristics of those contacts were investigated with and without irradiation of microwaves and far-infrared light. High values of the I c R n product and the energy gap structure were observed, which corresponded to the high-energy gap of NbN. Shapiro steps for far-infrared light up to 170 µm were observed and the possibility of replacing Nb Josephson point contacts with NbN ones is indicated.
An investigation was made of the mechanism of generation of oscillation modes in a resonator excited by a Josephson point junction or bridge. On the basis of a system of Maxwell's equations and material equations in the Fokker–Planck approximation expressions were derived for the amplitudes of the fundamental-frequency harmonics and for the number of harmonics with comparable amplitudes. It was shown that with an appropriate choice of junction parameters and excitation frequency, the number of harmonics may be ~103, the harmonics in this case falling within the optical band. Detailed experimental investigations were made of the nonlinear high-frequency properties of a superconducting niobium–niobium point junction in an external microwave field. Different operating conditions with frequency multiplication by the point junction were studied. Theory and experiment were compared. It was found that for frequency multiplication in the submillimeter band, a superconducting point junction operating under nonequilibrium conditions is potentially more useful than a conventional Josephson point junction.
Properties of a niobium-niobium Josephson superconducting point contact (SPC) used as a mixer-multiplier have been investigated experimentally. Conversion of a microwave signal with frequency fs= 127 GHz into a signal with the intermediate frequency fif= 8.6 GHz has been obtained using heterodyne generators with frequencies fh= 136.68, 45, and 6.8 GHz, so that fif= nfh1,2,3 ....20- fswhere n = 1.2, 3.20 are heterodyne harmonics produced in the SPC. Dependences of the conversion efficiency (ηn) on the bias voltage, the heterodyne power and the heterodyne harmonics have been measured. In the case of the second and the twentieth harmonics, the best value of η20/η2was 0.25. This value is essentially higher than that obtained for the usual (conventional) semiconducting mixer. Performed experiments have demonstrated the perspective of utilization of SPC as a mixer-multiplier in millimeter and submillimeter wavebands.