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Journal of Physics Conference Series

Current-step-like structure in a YBCO grain boundary Josephson junction coupled to a gigahertz signal

Yoshinao Mizugaki, Jian Chen

Published Jun 1, 2006• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1742-6596/43/1/313

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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.
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 demonstrate a zero-crossing Shapiro step in an asymmetric two-junction superconducting quantum interference device (SQUID) driven by one sinusoidal RF signal. The operation is based on the imbalanced evolution of the junction phases due to the tilted interference diagrams. A test circuit was fabricated using Nb/AlO x /Nb junction technology. When we applied an RF signal of 10 GHz, we observed a Shapiro step crossing the zero-current axis at the voltage position of the first order. The polarity of the step voltage was switched by a DC control flux to the SQUID loop. The experimental current–voltage curves were quantitatively reproduced by simulation.
The amplitudes of the first Shapiro steps for an external signal with frequencies of 72 and 265 GHz are measured as function of the temperature from 20 to 80 K for a 6 μm Josephson grain boundary junction fabricated by YBaCuO film deposition on an yttria-stabilized zirconia bicrystal substrate. Non-monotonic dependences of step heights for different external signal frequencies were found in the limit of a weak driving signal, with the maxima occurring at different points as function of the temperature. The step heights are in agreement with the calculations based on the resistively-capacitively shunted junction model and Bessel theory. The emergence of the receiving optima is explained by the mutual influence of the varying critical current and the characteristic frequency.
A zero-crossing Shapiro step (ZCSS) in an asymmetric and nonhysteretic two-junction superconducting quantum interference device (SQUID) driven by one sinusoidal rf signal is presented. The operation is based on imbalanced evolution of the junction phases owing to the tilted interference diagrams. Dependence of ZCSS generation upon the frequency and amplitude of a rf signal was numerically obtained. A test circuit was designed and fabricated using a Nb∕AlOx∕Nb junction technology. When we irradiated a rf signal of 10GHz, a ZCSS at the voltage position of the first order was observed. The polarity of the step voltage was switched by a dc control flux to the SQUID loop. The experimental bias-current versus voltage and control-current versus voltage curves were quantitatively reproduced by numerical simulation.
Microbridge-type Josephson junctions made of granular bulk YBCO superconductors were experimentally studied at microwave frequencies. Although the bridge included many grain boundaries, clear Shapiro steps were induced at every hf/2e voltage for microwave frequencies, and step height dependence for microwave power similar to that of a single weak link bridge was observed. A heterodyne mixing experiment of microwaves at about 20 GHz was also performed using the detector. The IF signal showed peak amplitudes corresponding to the peak dynamic resistance due to nonlinearity resulting from the Josephson effect. It was confirmed that the detector operates stably for the Josephson-effect heterodyne mixer at 60 K or below. The detector has potential for high sensitivity and good impedance matching due to its large dynamic resistance and high normal state resistance, respectively.
A high-Tc dc superconducting quantum interference device (SQUID) with three Josephson junctions (JJs) in series in each of its arms has been fabricated. Its Shapiro steps were studied using microwave (rf) radiation of 10 GHz and weak magnetic fields. The appearance of giant Shapiro steps and of some of half-integer steps was observed. Separation between the adjacent Shapiro steps could be tuned by rf magnetic fields and small external dc magnetic fields. This phenomenon was analyzed by phase locking the JJs in the SQUID.