M. W. Cromar

High‐TC superconductor‐normal metal‐superconductor Josephson microbridges with high‐resistance normal metal links

We have developed an in situ process for fabricating high transition temperature superconductor‐normal metal‐superconductor microbridges using a step edge to define the normal metal length. Critical current‐normal resistance products over 1 mV have been measured at low temperature in devices with high‐resistivity Ag‐Au alloy bridges. Results on samples with Ag bridges are compared with the alloy data as an initial test of recent theories of SNS Josephson junctions. Josephson effects have been demonstrated in these devices at temperatures higher than 80 K. Clearly defined rf steps have been observed, with power dependence qualitatively similar to theoretical predictions.

Operation of a Y‐Ba‐Cu‐O rf SQUID at 81 K

An rf superconducting quantum interference device (SQUID) has been made from bulk Y‐Ba‐Cu‐O. The device displays quantum interference effects and operates with useful signal levels up to 81 K. The SQUID is formed from a ring of Y‐Ba‐Cu‐O which is broken in the cryogenic environment and then recontacted. Estimates of the SQUID noise performance are given.

Very low noise, tightly coupled, dc SQUID amplifiers

We have fabricated and tested thin film, niobium edge junction, double transformer, dc superconducting quantum interference devices (SQUID’s) that were stable under room‐temperature storage and thermal cycling and that had very good noise performance. The input inductance, approximately 1.7 μH, was large enough to facilitate good matching to many experiments. When the SQUID was operated as a small‐signal amplifier, the minimum detectable energy per unit bandwidth (Se) was 5×10−33 J/Hz at 100 kHz, referred to the SQUID loop (uncoupled). The minimum detectable energy per unit bandwidth was 1.8×10−31 J/Hz at 100 kHz, referred to the input coil. The SQUID’s had good characteristics for flux‐locked operation since the small signal Se was low over a substantial range of bias current and magnetic flux. For operation in a flux‐locked feedback circuit, Se was 6×10−32 J/Hz at 1 kHz.

Well coupled, low noise, dc SQUIDs

We have designed, fabricated, and tested a Double Transformer (DT) coupled dc SQUID (Superconducting Quantum Interference Device) with low noise, an input inductance of 1μH and a smooth input-output characteristic. A transmission line model is presented to explain a resonance in the input-output characteristic of early versions of this device. Guided by the results of numerical simulations a new version of this device has been built and tested. Experimental results are presented that show that the resonance can be moved to a higher voltage by reducing the area of the SQUID loop. The voltage-external flux characteristic of some of these new devices agrees to within 10% with computer simulations. The minimum detectable energy per unit bandwidth (MDE) referred to the SQUID loop, is 10h, where h is Plancks constant. Computer simulations indicate an MDE of 6h.

YBa/sub 2/Cu/sub 3/O/sub 7- delta //insulator multi-layers for crossover fabrication

The development of thin-film dielectrics compatible with the epitaxial growth of YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) is crucial to the fabrication of multilayer device and circuit structures. The authors investigated the YBCO/SrTiO/sub 3/ (STO) system by fabricating YBCO/STO bilayers and simple YBCO/STO/YBCO crossover structures. The thin films were deposited in situ by pulsed-laser deposition and analyzed using X-ray diffraction and scanning electron microscopy. The film interfaces were characterized by secondary ion mass spectrometry (SIMS) depth profiling. The authors have developed photolithographic and wet-etching processes for patterning the crossovers which are compatible with these materials. The crossover structures were characterized by resistance and insulator pinhole density as well as by the superconducting properties of the patterned top and bottom YBCO electrodes (critical temperature, T/sub c/, and critical current density, J/sub c/). Using SrTiO/sub 3/ as the insulating layer, crossovers were made with good isolation between layers (>100 M Omega ) and high J/sub c/ even in the top electrode (J/sub c/(76 K)>10/sup 5/ A/cm/sup 2/).

Tightly coupled dc SQUIDs with resonance damping

We have reduced the effect of resonances on washer style dc SQUIDs coupled to input flux transformers and analyzed our damping structures using a distributed circuit model. A resistance of 1/spl Omega/ is placed across each turn of a 137-turn coil coupled to a planar washer dc SQUID reducing the structure in the voltage-flux curve thus extending the range of current biases over which the device operates. The energy sensitivity of the SQUID is predicted to not be degraded by the intracoil resistors.

Current-voltage characteristics of nanoampere Josephson junctions

We have studied the current-voltage characteristics of small area tunnel junctions at temperatures below 1 K. The junctions were made in an edge geometry with a Nb base electrode and had areas less than .05 μm2and critical currents in the nA range. Although the measured I-V characteristics resemble those of ordinary hysteretic junctions, the supposed zero-voltage portion of the curve proved to have a finite slope and to deviate from zero voltage. For these junctions it is apparently possible for occasional 2π phase slips to occur without switching to the usual voltage state. This behavior can be explained either by macroscopic quantum tunneling or by a model in which the effective shunt conductance of the junction is frequency dependent.

Excess low-frequency flux noise in dc SQUIDs

We have fabricated dc superconducting quantum interference devices (SQUIDs) incorporating Nb/Al-oxide/Nb Josephson junctions in both stripline and washer geometries. Low-frequency noise in excess of that predicted by the resistively-shunted junction model is present in both geometries and is demonstrated to be flux noise. This flux noise is not environmental. Improvements in fabrication processing over the past four years have reduced the level of this flux noise. SQUIDs are now fabricated with PdAu resistors, Nb wiring layers, and SiO/sub 2/ interlayer dielectric. In our best well-coupled SQUIDs, the white-noise energy sensitivity is 5/spl times/10/sup -31/ J/spl middot/s, with a 1/f knee below 0.1 Hz. We believe further reduction in the flux noise might be obtained with the use of on-chip flux shielding and/or trapping structures.

Switching noise in YBa/sub 2/Cu/sub 3/O/sub x/ 'macrobridges'

Intermittent switching was observed in the voltage current characteristics of thin-film bridges of YBa/sub 2/Cu/sub 3/O/sub x/. At a fixed bias point there are multiple metastable voltage states with lifetimes which depend on the bias current and applied magnetic field. The microbridges are made of thin ( >

Equivalent Flux Noise in a YBa2Cu3Ox rf SQUID

We have measured the noise in flux-locked rf SQUIDs made of bulk YBa2Cu3Ox both in a He cooled cryostat and in liquid nitrogen (LN2). Our best results at 75 K show a spectral density of the equivalent flux noise equal to 4.5×10-40/√Hz. There is considerable variation in the performance of SQUIDs made from nominally similar material.