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Journal of Applied Physics

Heterodyne microwave mixing in a superconducting YBa2Cu3O7−x coplanar waveguide circuit containing a single engineered grain boundary junction

R. Seed, C. Vittoria, A. Widom

Published Nov 15, 1994• 2 citations
Publisher

Abstract

The purpose of this work was to utilize the nonlinear current-voltage properties of induced grain boundaries in high temperature superconducting YBa2Cu3O7−x thin films to fabricate a planar microwave mixer. The experiment involved constructing a coplanar waveguide microwave circuit, the center conductor of which had a constriction patterned in it containing a single high angle grain boundary, thus forming a weak link junction. Analysis was provided by use of the resistively shunted junction model with excess current.

DOI
10.1063/1.358088

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The purpose of this work was to utilize the nonlinear current-voltage properties of induced grain boundaries in high temperature superconducting YBa2Cu3O7−x thin films to fabricate a planar microwave mixer. The experiment involved constructing a coplanar waveguide microwave circuit, the center conductor of which had a constriction patterned in it containing a single high angle grain boundary, thus forming a weak link junction. Analysis was provided by use of the resistively shunted junction model with excess current.
A microstrip transmission line was patterned from a superconducting YBa/sub 2/Cu/sub 3/O/sub 7-x/ film which was laser deposited on a SrTiO/sub 3/ bicrystal substrate. A microbridge in the microstrip was then patterned at the bicrystal interface to fabricate a superconducting grain boundary junction. Microwave mixing was observed with an input local oscillator frequency of 9.000 GHz, and an input signal frequency of 9.941 GHz. The output signal at 941 MHz was detected as a transmitted signal. The junction was current biased slightly above the critical current I/sub c/. The measured conversion loss was compared to the calculated value based upon a numerical solution of the equation for a superconducting weak link junction.>
A microwave mixer was patterned on a microstrip transmission line of superconducting YBCO (YBa/sub 2/Cu/sub 3/O/sub 7-x/). The YBCO film was epitaxially laser deposited on a SrTiO/sub 3/ bicrystal substrate. A weak link was constructed by patterning a microbridge in the microstrip at the bicrystal boundary. Microwave signals at 9.000 GHz and 9.941 GHz were applied at the input of the microstrip line. An output intermediate frequency signal was observed at 941 MHz and was detected as the transmitted signal. The microbridge junction, which behaved as a resistively shunted Josephson junction (RSJ), was current biased slightly above the critical current I/sub c/. The mixer conversion loss was measured at the input and output ports of the device package. The mixing efficiency was determined at the device junction. This efficiency was compared with the calculated efficiency determined by numerical solution of the Josephson equation for the weak link junction.>
Deliberate grain boundaries have been fabricated in epitaxial films of Tl2CaBa2Cu2O8 by growing the films on substrates with localized surface damage and on substrates with surface steps produced with an anisotropic etching process. Bowtie antennas were fabricated to couple millimeter wave radiation into the weak link junctions, which were used as the nonlinear elements. Heterodyne mixing of signals at 55 GHz and 94 GHz was observed in the integrated antenna/detector structures at temperatures below 101 K in the Tl-based films. In addition, observations were made at 55 GHz, using these structures as subharmonic mixers. We have designed an integrated microwave package to measure the noise and conversion efficiency of the grain-boundary junctions at frequencies between 9 GHz and 20 GHz.
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.
We have investigated the temperature dependence of microwave losses in the low-power limit and millikelvin temperature range of a thin-film YBa2Cu3O7-x(YBCO) λ/2 coplanar waveguide resonator patterned on a (La0.3Sr0.7) (Al0.65Ta0.35)O3(LSAT) substrate covered by a CeO2seed layer. The unloaded quality factor is mainly governed by the surface resistance of the YBCO film and the dielectric losses caused by resonant absorption due to a bath of two-level fluctuators in the dielectrics. The value of the unloaded quality factor at 20 mK and low power, i.e., Q0≃2000, allows for the realization of YBCO microwave quantum circuits implementing biepitaxial grain-boundary Josephson junctions.
Current-voltage characteristics and microwave mixing properties of step-edge microbridge Josephson junctions (SEJJs) made of YBaCuO thin films were systematically investigated. The step-edge was formed on the surface of a MgO(100) substrate employing the common photolithograph and wet-etching techniques. YBaCuO thin film was deposited by the off-axis sputtering method, and the microbridge junctions were fabricated so as to cross the step-edge. The SEJJs showed typical DC and AC characteristics which were explained by the resistively shunted junction (RSJ) model. Mixing experiments were carried out in the K-band microwave region. An intermediate frequency (IF) output with the frequency range of 20 MHz∼2.0 GHz was successfully obtained.