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Applied Physics Letters

Submillimeter wave response of a high T c superconducting microbridge

Jun-ichi Takeya, Etsuo Kawate, Chan Hoon Park, Toshinari Goto

Published Nov 23, 1992• 6 citations
Publisher

Abstract

We observed the ac Josephson effect, the ‘‘Shapiro step,’’ at the high frequency of 584 GHz in an YBa2Cu3O7−δ grain boundary Josephson junction at 4.2 K for the first time. This Shapiro step grew to about 8% of the critical current without irradiation. We also noted that the critical current was gradually suppressed with increasing power. The critical current with irradiation was suppressed to about 97% of the critical current without irradiation. Detailed investigation revealed the consistency between the above two phenomena.

DOI
10.1063/1.108140

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Journal of Physics Conference Series•Jun 1, 2006•0 citations
We observed the ac Josephson effect, the ‘‘Shapiro step,’’ at the high frequency of 584 GHz in an YBa2Cu3O7−δ grain boundary Josephson junction at 4.2 K for the first time. This Shapiro step grew to about 8% of the critical current without irradiation. We also noted that the critical current was gradually suppressed with increasing power. The critical current with irradiation was suppressed to about 97% of the critical current without irradiation. Detailed investigation revealed the consistency between the above two phenomena.
High Tc Josephson junctions have IcRn products as large as a few mV at liquid helium temperature, and hence, the frequency normalized by IcRn∕Φ0 becomes on the order of 10−3 for a gigahertz (GHz) microwave signal. (Here, Ic, Rn, and Φ0 are the critical current, the normal resistance, and a flux quantum, respectively.) It is well known that a microwave-current-driven Josephson junction does not exhibit clear Shapiro steps under such low frequency conditions. Instead of Shapiro steps, however, we often observe a current-step-like structure in the current–voltage curve of YBa2Cu3O7−δ grain boundary Josephson junctions coupled to a GHz signal. In this article, we present a model analysis for such current-step-like structures. Two types of device models are employed: a simple resistively and capacitively shunted junction model and a rf-field-driven two-junction superconducting quantum interference device model. Numerical results of the latter model agree with the experimental results, which indicates that the envelope of small Shapiro steps, including the effects of microwave-induced motion of flux quanta, appears as a current-step-like structure in a high Tc Josephson device coupled to a GHz microwave.
We have fabricated step edge superconductor-normal metal-superconductor microbridges using YBa2Cu3O7−x(YBCO) and noble metals with critical current-normal resistance (IcRN) products as high as 10 mV and normal resistances up to 38 Ω. Our fabrication process achieves high values of the IcRN product by exploiting the anisotropy in the properties of epitaxial YBCO films, allowing contact only between normal metal and superconductor through the crystalline axes which support the largest Josephson coupling. This results in a dramatic increase in the normal resistance of a junction without decreasing its critical current. We discuss the role of the superconductor-normal metal boundary resistance on the junction electrical properties. We have coupled submillimeter wave rf currents quasioptically into junctions integrated at the feeds of noble metal planar log periodic antennas and have induced up to 7 Shapiro steps in the current-voltage characteristics with a 760 GHz beam from a far infrared laser.
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 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).
We demonstrate current-step-like structures observed in a YBa 2 Cu 3 O 7−δ (YBCO) grain boundary Josephson junction (GBJJ) coupled to a gigahertz (GHz) signal. It is well known that a microwave-current-driven Josephson junction does not exhibit clear Shapiro steps under a low frequency condition of f << I c R n /ϕ 0 , where f , I c , R n , and ϕ 0 are the microwave frequency, the critical current of the junction, the normal resistance of the junction, and a flux quantum, respectively. Instead of Shapiro steps, however, we often observe an anomalous upturn in the current-voltage curve of YBCO GBJJ's coupled to a GHz signal. We refer to the upturn as a current-step-like structure. Numerical analysis using an rf-field-driven two-junction superconducting quantum interference device (SQUID) model indicates that the envelope of small Shapiro steps, including the effects of microwave-induced motion of flux quanta, appears as a current-step-like structure.