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Japanese Journal of Applied Physics

Frequency Dependence of AC Josephson Effect in Point-Contact (Y, Er) BaCuO/Nb

Sadamu Kita, Hisao Tanabe, Tetsuro Takahashi, Masayoshi Tonouchi, Yasufumi Fujiwara, Takeshi Kobayashi

Published Aug 1, 1987• 6 citations
Publisher

Abstract

Microwave and millimeterwave dependences of ac Josephson effect were investigated in YBaCuO ceramic superconductors. We observed very clear constant-voltage steps (Shapiro steps) on the current-voltage curve of YBaCuO/Nb junctions at signals of 14.1,25.5 and 35.0 GHz. The observed step voltages were frequency dependent in accordance with the (h/2e)f rule. This strongly suggests that pair-electrons serve as the superconducting particle even in ceramic materials. In addition, the ac Josephson effect was also observed in point-contact ErBaCuO ceramics containing a magnetic element Er.

DOI
10.1143/jjap.26.l1353

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Journal of Applied Physics•Dec 15, 1994•1 citation
Microwave and millimeterwave dependences of ac Josephson effect were investigated in YBaCuO ceramic superconductors. We observed very clear constant-voltage steps (Shapiro steps) on the current-voltage curve of YBaCuO/Nb junctions at signals of 14.1,25.5 and 35.0 GHz. The observed step voltages were frequency dependent in accordance with the (h/2e)f rule. This strongly suggests that pair-electrons serve as the superconducting particle even in ceramic materials. In addition, the ac Josephson effect was also observed in point-contact ErBaCuO ceramics containing a magnetic element Er.
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.
The ac Josephson effects in Nb point contacts were measured using optically pumped sub-millimeter-wave laser. Laser-induced constant voltage steps (Shapiro steps) were observed on the dc I–V curves of the hysteretic junction as well as non-hysteretic junctions. We found that the height of the fundamental step normalized to the critical current at no irradiation in hysteretic junctions was considerably larger than that previously reported. The enhancement of the step observed can be explained by taking the effect of the shunt capacitance into consideration.
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).
The rf-power dependence of the ac-Josephson steps is measured at 720 GHz, using small area Nb tunnel junctions. This frequency is well above the gap frequency of Nb. The junction is placed in a waveguide, and connected to a superconducting stripline, which effectively tunes out the junction capacitance and facilitates the coupling of the radiation to the junction. We observe three Josephson steps, and the first step crosses the zero current axis over a considerable range of rf-power. This indicates the possible application of THz Josephson steps in voltage standards. The data are compared to the theory and we find clear evidence for the predicted intrinsic roll-off of the Josephson current amplitude above the gap frequency.