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Technical Physics

Effect of microwave radiation on the voltage-current characteristics of a variable-thickness Josephson microbridge

Vladimir N. Gubankov, V. P. Koshelets, G. A. Ovsiannikov

Published Apr 1, 1979• 0 citations
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Abstract

The temperature and field variation of the photoresponse of tin Josephson film microbridges are studied in experiments in which the microbridges are irradiated in the centimeter and millimeter ranges. Experiments have been carried out under conditions that correspond to narrow-band and wide-band detection. Certain features of the response which are due to stimulation of superconductivity in the microbridges by the microwave radiation and a direct current are studied. In heterodyne detection these features disappear at a high power level of the heterodyne oscillator, and a substantial increase is observed in the response, this is caused by a higher differential resistance of the microbridge. The maximum sensitivity (approx.10/sup 4/ V/W) is achieved with selective detection.

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Amplification, down-conversion and detection of the microwaves in thin-film Josephson variable thickness bridges of submicron dimensions have been investigated experimentally. The results for indium and tin bridges are discussed taking into account the influence of the nonequilibrium electrons in a weak link region. The same experiments were carried out with SNS variable thickness Nb-Al-Nb bridges. They showed that the properties of SNS bridges agreed better with the theoretical predictions of the resistively shunted junction model in a wide temperature range. Moreover the parameters of the Nb-Al-Nb bridges have possessed better stability and reliability.

IEEE Transactions on Magnetics•Jan 1, 1979•6 citations
The temperature and field variation of the photoresponse of tin Josephson film microbridges are studied in experiments in which the microbridges are irradiated in the centimeter and millimeter ranges. Experiments have been carried out under conditions that correspond to narrow-band and wide-band detection. Certain features of the response which are due to stimulation of superconductivity in the microbridges by the microwave radiation and a direct current are studied. In heterodyne detection these features disappear at a high power level of the heterodyne oscillator, and a substantial increase is observed in the response, this is caused by a higher differential resistance of the microbridge. The maximum sensitivity (approx.10/sup 4/ V/W) is achieved with selective detection.
Measurements are reported of the Josephson radiation from microbridges coupled to an X-band receiver via a nonresonant microwave transformer. Although closely coupled to the bridge, the transformer does not modify the bridge properties. We find that near the transition temperature the radiation linewidth is proportional to the square of the dynamic resistance, which is also predicted by the resistively shunted junction model. The effective noise temperature determined by the linewidth is 19±3 K and the measured maximum integral power is 5×10−12 W.
We report on the characteristics of thin film microbridge Josephson junctions fabricated using electron beam lithography and discuss the techniques used to fabricate these junctions, which are typically 0.3μ square. Extensive measurements have been made on both individual junctions of tin and indium and on small arrays of these junctions. Synchronization of current biased series arrays by an applied microwave field has been achieved. This synchronization appears as a zero differential resistance across the entire array at a voltage of MVJwhere M is the number of elements in series and VJis the Josephson voltage hv/2e. Furthermore we have observed a systematic variation with microbridge dimension of the microwave enhancement of the critical current.
The intrinsic response of Josephson junctions to microwave and far-infrared radiation is studied. Junctions are formed by pressing a Nb or Nb-Ti pointed wire onto a Nb thin film which in turn is pressed against the open end of a waveguide. Electromagnetic radiation is applied through the back of the film so that only the wave magnetic field is coupled to the junction. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22 to 105 GHz. The dependence of the responsivity of these currents on film thickness over a range from 50 to 500 Å is studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies. The observed responsivity of a 50-Å film junction at 22 GHz is (6±3) ×10−2 V/W. To facilitate comparison of our results with other experiments, a more conventional coupling configuration in which the wave electric field rather than the wave magnetic field is coupled to the junction is studied. The results are comparable with other experiments with the responsivity in the E field coupling case 106 times larger than in the B field coupling configuration. We demonstrate that for successful use of simple Josephson junction detectors in the microwave and millimeter region it seems crucial to impedance match the junctions to the radiation source, probably by making them a part of a resonant structure.
Amplification, down-conversion and detection of the microwaves in thin-film Josephson variable thickness bridges of submicron dimensions have been investigated experimentally. The results for indium and tin bridges are discussed taking into account the influence of the nonequilibrium electrons in a weak link region. The same experiments were carried out with SNS variable thickness Nb-Al-Nb bridges. They showed that the properties of SNS bridges agreed better with the theoretical predictions of the resistively shunted junction model in a wide temperature range. Moreover the parameters of the Nb-Al-Nb bridges have possessed better stability and reliability.