W. J. Skocpol

Coexistence of grains with differing orthorhombicity in high quality YBa2Cu3O7−δ thin films

High quality films of YBa2Cu3O7−δ on LaAlO3 have been grown by pulsed‐laser deposition at oxygen pressures of 3.4–54 Pa (25–400 mTorr). X‐ray diffraction reveals the coexistence of grains that align with the substrate axes (axial grains) and grains that are rotated by 0.4° from the substrate axes (diagonal grains). The axial grains are tetragonal while the diagonal grains achieve lattice parameters close to bulk YBa2Cu3O7. The relative proportion of axial grains accounts for the measured variations of normal‐state conductance and superconducting critical current density from film to film, based on a simple two‐dimensional model of randomly positioned, insulating axial grains.

High‐temperature dc superconducting quantum interference device with deep‐submicron YBa2Cu3O7 weak links

We have fabricated a YBa2Cu3O7 (YBCO) thin‐film dc superconducting quantum interference device (SQUID) with lithographically defined deep‐submicron weak links. At 77 K the voltage response to dc flux is sinusoidal and nonhysteretic, with maximum peak‐to‐peak amplitude of 5 μV. The maximum response is 8 μV at 70 K, where 2LI0/Φ0≊1. At lower temperatures, the maximum response oscillates in the range 4–6 μV. Random telegraph noise was observed near 30–40 K. Well‐behaved Shapiro steps were observable at all measured temperatures below Tc.

Megagauss exploration of Hc2 and vortex dynamics in YBa2Cu3O7−x thin films

Abstract New measurements of the dc resistivity ρ(T,H) of YBa2Cu3O7−x (YBCO) below 20K and in magnetic fields > 100T (> 1 Megagauss) show Hc2 ‖ c-axis is near 140T at low temperature. The onset of hydrodynamic vortex flow is also clearly seen and is in agreement with existing data. We have employed a linear fit of sample voltage vs. dB/dt at low fields to systematically remove inductive coupling from the signal voltage and extract ρ(T,H).

Etching and annealing of substrates for superconducting multilayers and devices

The structural and electrical quality of YBa/sub 2/Cu/sub 3/O/sub 7- delta / grown on ion-etched substrates by a variety of techniques was investigated. Co-evaporated YBa/sub 2/Cu/sub 3/O/sub 7- delta / films deposited and postannealed on etched LaAlO/sub 3/ substrates have substantially higher room-temperature resistivities and wider c-axis rocking curve widths than films on unetched substrates. Annealing of the etched substrate prior to evaporation narrows the c-axis rocking curve width and restores resistivity and critical current densities to values comparable to those of the unetched controls. Laser-ablated films on etched substrates show a smaller absolute magnitude of rocking curve broadening and no change in DC electrical properties. An increase of the inhomogeneous strain was also observed in coevaporated postannealed and laser-ablated films on etched substrates.<<ETX>>

Low-temperature critical field of YBCO

We have measured the upper critical fieldHC2(T) of YBa2Cu3O7 (YBCO) thin films in magnetic fields up to 140 T forH applied parallel to thec-axis of the film. The critical field in the zero-temperature limit is 138 T, and the temperature dependence does not fit any simple model.

Increased transition temperature in in situ coevaporated YBa2Cu3O7−δ thin films by low temperature post‐annealing

In situ coevaporated YBa2Cu3O7−δ thin films have a slightly depressed transition temperature Tc, though they have excellent radio‐frequency surface resistance characteristics. These films consistently have less orthorhombic strain than laser ablated or post‐annealed films. Low temperature (320–420 °C) post‐annealing of in situ coevaporated films in 100 kPa of O2 raised Tc to values as high as 91.5 K with some increase in the orthorhombic strain. All measured thin films show less variation of Tc with orthorhombic strain than does bulk material.

Two-GHz microstrip thin-film resonators of Nb and YBa/sub 2/Cu/sub 3/O/sub 7/

Dissipation and phase noise have been measured for 2-GHz linear microstrip resonators on 0.5-mm LaAlO/sub 3/. Fifty-ohm YBCO resonators with Nb ground planes (YBCO/Nb) showed quality factors of approximately 100,000 at 2 GHz and 4.2 K, somewhat less than measured in all-Nb resonators of the same geometry. The corresponding surface resistance of 10 microhms was maintained to average current densities of 1 MA/cm/sup 2/. All-YBCO resonators were measured to higher temperatures, and showed significant excess dissipation above about 60 K. The normalized phase noise of S/sub y/=-20/spl plusmn/2 was maintained to at least 30 K. This resonator was also operated in a resonator-stabilized oscillator configuration. The effect of package interactions on microstrip devices was also investigated.<<ETX>>