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AIP conference proceedings

Josephson effects in granular NbN microbridges

Edward J. Cukauskas, J. H. Claassen, Marty Nisenoff, C. H. Galfo, R. L. Steiner, Bascom S. Deaver

Published Jan 1, 1980• 1 citation
Publisher

Abstract

High resistance granular NbN microbridges with submicron dimensions have been fabricated and investigated. Devices with critical currents less than 5 microamps and bridge resistances of the order of 50 ohms have been studied as a function of temperature at both zero and finite voltages. The current‐voltage characteristics were measured with various levels of applied radiation in the frequency range 11–40 GHz. For these frequencies, constant voltage‐current steps were observed at the Josephson frequency, 2eV/h for low applied voltages, at higher voltages the radiation induced step spacing deviated from that of ideal Josephson behavior. Additional current steps were also observed at temperatures just above the bridge Tc where no finite critical current could be dected. These effects and other characteristics observed on the I‐V curves will be reported and discussed.

DOI
10.1063/1.32146

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IEEE Transactions on Applied Superconductivity•Jun 1, 2005•1 citation
High resistance granular NbN microbridges with submicron dimensions have been fabricated and investigated. Devices with critical currents less than 5 microamps and bridge resistances of the order of 50 ohms have been studied as a function of temperature at both zero and finite voltages. The current‐voltage characteristics were measured with various levels of applied radiation in the frequency range 11–40 GHz. For these frequencies, constant voltage‐current steps were observed at the Josephson frequency, 2eV/h for low applied voltages, at higher voltages the radiation induced step spacing deviated from that of ideal Josephson behavior. Additional current steps were also observed at temperatures just above the bridge Tc where no finite critical current could be dected. These effects and other characteristics observed on the I‐V curves will be reported and discussed.
The output noise of a granular NbN microbridge has been measured at frequencies near 25 MHz as a function of bias voltage, temperature, and rf bias at 70 GHz. It is generally greater than predicted by the resistively shunted junction model, but is significantly lower than has previously been observed in clean superconducting microbridges with comparable RIc products. An estimate of the performance of the device as a Josephson mixer at 70 GHz is made on the basis of these measurements. At an operating temperature of 10 K a double sideband mixer noise temperature ∼200 K, with conversion loss of ∼3 dB should be attainable. Using the noise as a probe of the average temperature within the microbridge, it is found that simple self-heating is not a likely explanation for the disappearance of Josephson effects at high dc or microwave bias.
The current‐voltage characteristics and Josephson radiation have been studied for a number of indium bridges as a function of temperatures. Our results show strong deviations from the predictions of the RSJ model, even in the temperature regime where the bridge size is smaller than the coherence length. We ascribe this to non‐equilibrium effects in the around the microbridge.In order to improve the applicability of the Josephson effect, we have investigated coherent arrays of 2, 4 and 10 microbridges. The 10 microbridge array is a novel design which allows the bridges to be parallel‐biased at dc (equal voltages) and series‐biased at 9 GHz (high output impedance).
A substrate step technique has been used to fabricate superconducting NbN weak links of length ∼750 Å, width ∼1.5 μm. The film in the weak-link region is less than 100 Å thick, and shows evidence of being granular. These devices have very high resistance (as much as 30 Ω), are quite rugged, and show almost ideal Josephson response to microwave radiation at 70 GHz. A vortex flow model is expected to apply because of the small coherence length in the weak-link material. However, the well-known resistively shunted junction model is found to be a significantly better predictor of device behavior.
Microbridge-type Josephson junctions made of granular bulk YBCO superconductors were experimentally studied at microwave frequencies. Although the bridge included many grain boundaries, clear Shapiro steps were induced at every hf/2e voltage for microwave frequencies, and step height dependence for microwave power similar to that of a single weak link bridge was observed. A heterodyne mixing experiment of microwaves at about 20 GHz was also performed using the detector. The IF signal showed peak amplitudes corresponding to the peak dynamic resistance due to nonlinearity resulting from the Josephson effect. It was confirmed that the detector operates stably for the Josephson-effect heterodyne mixer at 60 K or below. The detector has potential for high sensitivity and good impedance matching due to its large dynamic resistance and high normal state resistance, respectively.
Abstract To reduce the self‐heating effect and thermal noise in bridge‐type Josephson mixers, all‐NbN nanobridges with an Al “direct shunt” resistor were fabricated. This paper describes the millimeter‐wave response and mixing properties of the NbN(g)/Al nanobridge‐type Josephson mixers in the 100‐GHz frequency. The effective noise temperature TNeff of the nanobridges was decreased to the bath temperature (liquid helium temperature, 4.2 K) by the Al “direct shunt” resistor and the millimeter‐wave induced steps were clearly observed at higher voltage (∼4 mV) than in the nanobridge without the Al “direct shunt” resistor. The millimeter‐wave current dependence of the height of the millimeter‐wave induced steps was analyzed quantitatively, taking into account the thermal noise in the RSCJ model. The simulated result agreed with the experimental result very well. The systematic decrease of the induced step height with an increase of millimeter‐wave current was explained as the increase of effective noise temperature due to absorption of millimeter‐wave power. In the 100‐GHz Josephson mixing experiment, a large IF power output was obtained at a voltage up to about 1.6 mV. The effective noise power NEP and conversion efficiency were about 3 × 10−20 W/Hz and 0.2, respectively.
To investigate the temperature dependence of microwave-induced characteristics of a NbN Josephson junction array, the surface resistance, R/sub s/, of NbN films was measured using the dielectric resonator method at 22 GHz and temperatures from 5.8 K to 14.8 K. From the measured R/sub s/ values, the attenuation of 16 GHz microwaves in a NbN coplanar waveguide was calculated and constant-voltage-step amplitude for a NbN junction array biased with a DC current and 16 GHz microwaves was calculated as a function of temperature, resulting in a theoretical temperature dependence of constant-voltage-step amplitude.