David A. Rudman

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.

Scanning tunneling microscopy of the surface morphology of YBa2Cu3Ox thin films between 300 and 76 K

Scanning tunneling microscopy (STM) images of YBa2Cu3Ox (YBCO) thin films show different growth mechanisms depending on the deposition method and substrate material. We present images of YBCO films sputter deposited onto MgO and SrTiO3, and laser ablated onto LaAlO3 showing screw dislocation and ledge growth mechanisms. At room temperature we observed an anomalous tunneling conductance near the edge of growth steps which causes a large apparent step‐edge height in the STM image. This effect decreases with decreasing temperature, so that the step height approaches the expected value for one unit cell of 1.2 nm at 76 K. This phenomenon reflects changes in either the surface tunneling barrier or tunneling density of states upon cooling.

Pulsed Laser Deposition of Superconducting Nb-Doped Strontium Titanate Thin Films

We report on the growth of superconducting Nb-doped SrTiO3 thin films on LaAlO3 substrates by pulsed laser deposition. We find optimum Nb doping and high mobility for growth near 870 °C and chamber pressures below 3×10−4 Pa (2×10−6 Torr). The transport properties were measured from 300 K to 150 mK. The most highly doped samples display metallic behavior, and are superconducting below 350 mK, with transition temperatures similar to those of bulk single crystals.

Thin film reaction kinetics of niobium/aluminum multilayers

Phase formation kinetics in Nb/Al multilayered thin films having overall compositions of 25, 33, 50, and 75 at. % Al have been investigated using scanning calorimetry, x‐ray diffraction, and cross‐sectional transmission electron microscopy. The first phase to form upon annealing the Nb/Al layered structure of all samples is the NbAl3 intermetallic. Calorimetry clearly identifies the NbAl3 formation to be a two‐stage process. The first stage is the formation of a planar layer by nucleation and growth to coalescence while the second stage is the thickening of the planar layer. The large amount of heat released (and hence large volume fraction of NbAl3 formed) during the first reaction stage is consistent with heterogeneous nucleation at well‐isolated sites in the Nb/Al interface. This is surprising in light of the large thermodynamic driving force expected for nucleation and suggests that the local nonequilibrium nature of the Nb/Al interface greatly reduces the driving force for nucleation. The next phase ...

Geometry dependence of nonlinear effects in high temperature superconducting transmission lines at microwave frequencies

We investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films. The measured power in the third-harmonic signal changes systematically with film thickness, center conductor linewidth, and line length. We analyze these results using a simple model for a transmission line with a nonlinear inductance arising from a current-dependent superconducting penetration depth. This analysis describes quantitatively the observed differences in harmonic generation for transmission lines of different dimensions, and yields a single geometry-independent parameter (the nonlinear scaling current density J0) to quantify the observed nonlinear behavior. For the thin film samples studied here J0=3.0×107 A/cm2 at 76 K for all geometries investigated. These results provide the means to establish a lower limit for the expected nonline...

Antenna‐coupled high‐Tc air‐bridge microbolometer on silicon

An antenna‐coupled high‐Tc superconducting microbolometer on a silicon substrate, operating at infrared wavelengths, is described. This detector incorporates a silicon‐micromachined yttria‐stabilized zirconia air bridge at the feed of a planar lithographic antenna to simultaneously minimize the thermal conductance and the heat capacity of the bolometer. At an operating temperature of 87.4 K, the optical responsivity measured using a 300‐K blackbody source over a 0.2–2.9 THz bandwidth is 2900 V/W, the optical noise‐equivalent power (NEP) is 9×10−12 W/Hz1/2, and the time constant is <10 μs. This NEP is nearly a factor of 2 lower than the previous record for a liquid‐nitrogen‐cooled thermal detector, and the time constant is several orders of magnitude shorter.

Microwave properties of voltage-tunable YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///SrTiO/sub 3/ coplanar waveguide transmission lines

To explore the electrical characteristics of monolithic microwave circuits with integrated high-temperature superconductor and ferroelectric materials, we fabricated a series of coplanar waveguide transmission lines in laser-deposited YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// and SrTiO/sub 3/ thin films. We characterized the voltage-tunable two-port microwave response of the transmission lines at cryogenic temperatures using a calibrated network analyzer system. Total phase shifts and phase tuning in these devices increased for increasing ferroelectric film thickness with only moderate increases in transmission loss.<<ETX>>

Time-Division SQUID Multiplexers

SQUID multiplexers make it possible to build arrays of thousands of low-temperature bolometers and microcalorimeters based on superconducting transition-edge sensors with a manageable number of readout channels. We discuss the technical tradeoffs between proposed time-division multiplexer and frequency-division multiplexer schemes and motivate our choice of time division. Our first-generation SQUID multiplexer is now in use in an astronomical instrument. We describe our second-generation SQUID multiplexer, which is based on a new architecture that significantly reduces the dissipation of power at the first stage, allowing thousands of SQUIDs to be operated at the base temperature of a cryostat.

Vortex images in thin films of YBa2Cu3O7−x and Bi2Sr2Ca1Cu2O8+x obtained by low‐temperature magnetic force microscopy

We have imaged vortices in superconducting thin films with a low‐temperature magnetic force microscope that utilizes microfabricated piezoresistive cantilevers with built‐in tips. The films of YBa2Cu3O7−x and Bi2Sr2Ca1Cu2O8+x, are made by laser ablation and molecular beam epitaxy, respectively. The vortices usually appear as round features in the noncontact image with a diameter of about 1 μm or slightly less. In some cases the position of the vortices is correlated to defects observed in the topographic image of the same area. The vortices move sometimes, especially after taking a topographic (contact mode) scan.

Microcalorimeter energy-dispersive spectrometry using a low voltage scanning electron microscope

We describe the current performance of the prototype microcalorimeter energy‐dispersive spectrometer (µcal EDS) developed at NIST for X‐ray microanalysis. We show that the low‐energy µcal EDS, designed for operation in the energy range 0.2–2 keV, offers significant advantages for low‐beam‐energy microanalysis. We present several examples in which the prototype µcal EDS has been used to solve problems in low‐voltage microanalysis, including the analysis of tungsten silicide (WSi2), titanium nitride (TiN) and barium titanate (BaTiO3) and the measurement of chemical shifts in Fe and C compounds.