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Japanese Journal of Applied Physics

Correlation between Types of Junction and Submillimeter-Wave Responses in Point-Contact Josephson Junctions

Seizo Morita, Takashi Noguchi, Satoru Takaki, Shinichi Horii, Syozo Imai, Yosihisa Takeuti, Nobuo Mikoshiba

Published Jan 1, 1982• 6 citations

Abstract

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.

DOI: 10.1143/jjap.21.71

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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.

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Shapiro steps in the submillimeter-wave (SMMW) range were observed in junctions made of a high-T c superconductor, YBa2Cu3O x (YBaCuO), and niobium. Junctions were made by pressing a Nb needle on a sintered YBaCuO disk. Shapiro steps up to the fifth order were observed for SMMWs of wavelength 871.6 µm (frequency 344.0 GHz). The dependence of the Shapiro step width on the SMMW power is in accordance with the Bessel function dependence. For SMMWs of wavelength 570.6 µm (525.4 GHz), Shapiro steps up to the second order were observed.

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.

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).