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Japanese Journal of Applied Physics

I-V Characteristics of NbN-NbN Josephson Point Contacts

Yuji Ohno, Yukinobu Miki, Toshinari Goto, Ko Hara

Published Apr 1, 1988• 0 citations
Publisher

Abstract

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.

DOI
10.1143/jjap.27.l677

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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.
It has been found that point-contact Josephson junctions can be clearly classified into bridge-type and tunnel-type by introducing new parameters characterizing their I-V curves. Nearly-ideal tunnel-type junctions with Nb–SuO x –Sn (Film) point-contacts were obtained. Tunnel-type junctions showed larger gap energies than bridge-type junctions in Nb–SnO x –Sn and Nb–NbO x –NbN point-contacts. The submillimeter-wave responses were measured in Nb–Nb point-contacts, and it was found that the contacts with the best performance were bridge-type junctions with a sharp gap structure; tunnel-type junctions showed rather bad performance. On irradiation with laser light, the center of the Shapiro step moved to the lower-current side in bridge-type junctions but to the higher-current side in tunnel-type junctions. These differences can be explained by differences in the heating effects between bridge-type and tunnel-type junctions.
This paper compares the measured characteristics of a three-dimensional Nb thin-film point-contact junction with those predicted by the Stewart-McCumber model. The erratic microwave-induced I-V characteristic we observed at low microwave power levels was also evident in a digital simulation –based on the Stewart-McCumber model–of the time dependence of the junction voltage and the phase space trajectory. Subharmonic steps V=(n/m)(h f/2e) were observed when a 70 GHz signal was applied, but the usual condition ωC R n≃1 for substeps was not fulfilled. Substeps were also observed in the digital simulation even when the current-phase relationship was purely sinusoidal.
All NbN Josephson junctions with polycrystalline NbN films for both electrodes and barriers have been fabricated and their microwave characteristics for a quantum voltage standard were investigated. A superconductor and a normal metal were prepared by changing the partial pressure of N2during the rf sputter deposition of NbN films. For the Josephson junctions normal resistance was controlled by changing the partial pressure of N2during the NbNxbarrier deposition, and critical current was controlled by changing barrier thickness. As a result, it was found that the product of current and normal resistance can be optimized to maximize the current range of the constant-voltage step for quantum voltage standards.
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.
The response of Josephson junctions to microwave and far infrared radiation was 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 junctions. The change of the dc Josephson current is monitored at different levels of microwave power over a frequency range from 22GHz to 105GHz. The dependence of the responsivity of these current on film thickness over a range from 50 A to 500 A was studied. The observed responsivity agrees very well with that predicted by theory. Higher responsivity is observed for thinner films and lower microwave frequencies.
The Shapiro steps in the submillimeter wave (SMMW) region were measured in point-contact Josephson junctions made of a Nb strip with a sharpened edge and a YBaCuO thin film. The thin film was prepared by the chemical vapor deposition (CVD) method. The dependence of the Shapiro step width on the SMMW power was measured for the wavelength of 570.6 µm (525.4 GHz). The dependence was similar to that found in microbridges made of low T c superconductors. The first-order Shapiro step was observed for the wavelength of 232.9 µm (1.287 THz).