G.K.G. Hohenwarter

Confocal resonators for measuring the surface resistance of high‐temperature superconducting films

A quasioptical technique of measuring superconductor surface resistance using a confocal resonator has been developed. The method has advantages of nondestructive analysis, high sensitivity, easy extension to higher frequencies, convenient experimental setup, and flexibility in sample size. Tl‐Ca‐Ba‐Cu‐O high‐temperature superconducting films have been measured with this technique and the measured surface resistances were less than 0.01 Ω at 36.135 GHz and 77 K. The measurements have been performed from 29 to 39 GHz, and all films showed roughly a quadratic dependence of surface resistance with frequency.

S parameter measurements on single superconducting thin‐film three‐terminal devices made of high‐Tc and low‐Tc materials

We have investigated three‐terminal single‐layer thin‐film superconducting devices made of YBaCuO and Nb. The devices incorporate regions of weak superconductivity in multiple parallel links that are influenced by current in a separate control line. These experiments were designed to study the possible application of this device as an rf amplifier. With the device biased, rf power was applied to the control line and the transmission coefficient, S21, was measured. The reverse transmission coefficient, S12, was also measured for comparison. Upon biasing into a flux flow state, the S21 of the device at rf frequencies was found to increase 10–15 dB over the zero bias value and over the reverse feed (S12 ) value. The device behaved linearly up to power inputs of +5 dBm (1‐dB compression point). The bandwidth was limited only by the impedance transformer.

JOSEPHSON VORTEX FLOW IN SUPERCONDUCTING SINGLE-CRYSTAL BI2SR2CACU2OX

Using various size rectangular mesas formed by photolithographically patterning and etching on single‐crystal Bi2Sr2Ca1Cu2Ox superconductors, we have obtained c‐axis volt‐ampere characteristics as a function of magnetic field applied parallel to the a‐b planes. Enhanced sensitivity with field perpendicular to the long side was observed even in mesas with dimensions smaller than the magnetic penetration depth λc. This can be explained in terms of viscous flow of Josephson vortices. The measurements are in good quantitative agreement with theoretical models for Josephson vortex motion in layered superconductors. Vortex flow coexists with the multiple hysteretic structure previously presented as evidence that this material behaves as a stack of underdamped intrinsic Josephson junctions.

YBa2Cu3O7 nanobridges fabricated by direct‐write electron beam lithography

A direct method for nondamaging, nanometer‐scale patterning of high Tc superconductor thin films is presented. We have fabricated superconducting nanobridges in high‐quality, epitaxial thin‐film YBa2Cu3O7 (YBCO) by combining direct‐write electron beam lithography and an improved aqueous etchant. Weak links with both length and width dimensions less than 20 nm have exhibited critical currents at 77 K of 4–20 μA and IcRn products of 10–100 μV which compare favorably with results for other YBCO junction technologies. We have used this technique in the fabrication of a shock‐wave pulse former as an initial demonstration of its applicability to monolithic superconductive electronics.

Single superconducting thin film devices for applications in high T/sub c/ materials circuits

The authors investigated several different devices based on regions of weak superconductivity and multiple parallel links in thin films. Devices were fabricated with Nb and YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Hysteretic symmetric and asymmetric I-V (current-voltage) curves have been observed. Flux flow was indicated. Device switching properties and the dependence of the flux-flow signature in the I-V curve on applied magnetic field were explored. Contrary to vortex flow devices based on Josephson junctions, the devices described here do not possess a tunnelling barrier and are made of only a single superconducting layer. Hence they should be applicable to electronic circuits based on high-T/sub c/ superconducting materials without the need for tunnel junctions. >

Characteristics of superconducting flux-flow transistors

The operational characteristics and physics of three superconducting thin-film-based transistor structures are compared. The devices are based on the motion of quantized vortices, either Josephson fluxons in a long tunnel junction of Abrikosov fluxons in a superconducting film. The transistor amplification mechanism is accomplished by controlling magnetic field at the boundaries of the structures. An overview of the present understanding of device mechanisms and measured characteristics, including voltampere relations and small and large signal circuit parameters is provided. Demonstrated applications and anticipated limitations are discussed.

Fabrication of TlCaBaCuO step‐edge Josephson junctions with hysteretic behavior

One way to create hysteretic Josephson junctions from the currently available nonhysteretic high temperature superconducting junctions is to artificially add capacitance. We have adapted a multilayer technique for artificial capacitance addition/grain boundary modification to produce TlCaBaCuO step‐edge junctions exhibiting large amounts of hysteresis at 77 K. A gaplike structure is present at 32–36 mV that has a temperature dependence that does not contradict that predicted by the Bardeen‐Cooper‐Schrieffer theory. In addition, the junctions show fast switching times, less than a fixture limited 20 ps, and Fraunhofer‐like dependence of critical current on magnetic field.

A Josephson junction to FET high speed line driver made of TlCaBaCuO

A Tl-Ca-Ba-Cu-O superconducting flux flow transistor (SFFT) was used as an active impedance converter between Josephson and FET circuitry. The input of the flux flow device is a control line of low impedance that can be driven by a tunnel junction. The output is the signal across the SFFT which is made of a parallel array of weak links. The output impedance is typically greater than 5 Omega , with a maximum voltage swing of over 100 mV into a 50- Omega system. The switching of an all-Nb junction induced a 90-mV voltage swing at the FET input and over 200 mV at the FET output. The line driver can operate anywhere between 4.2 K and 85 K with minor changes in speed (+or-5 ps) and output level (+or-10 mV). The switching time measured was about 100 ps and was fixture limited.

A film transmission line resonator to measure the microwave surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7-x/

The authors constructed microstrip transmission line resonators with YBa/sub 2/Cu/sub 3/O/sub 6.9/ ground planes, SiO dielectric, and Nb top conductors. The film resonator is a low-impedance section n lambda /2 long, and the wave is launched from a coaxial line. Loose coupling is provided by the discontinuity between 50- Omega feed lines and the 0.05- Omega resonator section. After de-embedding the transmittance data of the structure, the surface resistance of the ceramic superconductor can be computed. The surface resistance of YBa/sub 2/Cu/sub 3/O/sub 6.9/ at 4.2 K was about a factor of 60 higher than the theoretical value for Nb at 4.2 K over the range 2.9-6 GHz. The power law behavior versus frequency of the surface resistance is roughly quadratic, which is consistent with two-fluid analysis. >

Magnetic field sensitivity of variable thickness microbridges in TBCCO, BSCCO, and YBCO

We describe results of a study comparing the magnetic field sensitivities of variable thickness bridge (VTB) arrays fabricated in TBCCO, BSCCO, and YBCO thin films. Identical structures were patterned in a variety of films, and the bridges were thinned by four different methods. Analysis of the data yields experimental evidence as to the suitability of these types of films for devices such as the superconducting flux flow transistor (SFFT) which is based on this geometry. The volt-ampere characteristics of the arrays were measured in low uniform magnetic fields (/spl les/130 G) and in nonuniform fields (/spl les/5 G) produced by a nearby control line. For these films in this geometry, no measurable effect of the control line magnetic field was observed. Large values of transresistance and current gain could only be attained through a thermal mechanism when the control line was driven normal. Upper bounds for (magnetically generated) transresistance (/spl les/5 m/spl Omega/) and current gains (/spl les/0.005) have been inferred from the uniform field data assuming a standard best-case device geometry. All volt-ampere curves followed closely a power law relationship (V/spl sim/I/sup n/), with exponent n /spl sim/1.2-10. We suggest materials considerations that may yield improved device performance.<<ETX>>