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Japanese Journal of Applied Physics

Rf Power Dependence of AC Josephson Current in Point-Contacts of BaY(Tm)CuO Ceramics

Tsutomu Yamashita, Akira Kawakami, Takaharu Nishihara, Masasuke Takata, Kohji Kishio

Published May 1, 1987• 31 citations
Publisher

Abstract

Using BaYCuO and BaTmCuO ceramics, point-contact Josephson junctions were fabricated to observe ac Josephson effect. Clear Shapiro steps were observed at 4.2 K. The rf power dependence of Josephson currents were qualitatively fitted to Bessel functions.
DOI
10.1143/jjap.26.l671

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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 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.