J. Bindslev Hansen

Smooth YBa2Cu3O7−x thin films prepared by pulsed laser deposition in O2/Ar atmosphere

We report on pulsed laser deposition of YBa2Cu3O7−x in a diluted O2/Ar gas resulting in thin epitaxial films which are almost outgrowth‐free. Films were deposited on SrTiO3 or MgO substrates around 800 °C at a total chamber pressure of 1.0 mbar, varying the argon partial pressure from 0 to 0.6 mbar. The density of boulders and outgrowths usual for laser deposited films varies strongly with Ar pressure: the outgrowth density is reduced from 1.4×107 to 4.5×105 cm−2 with increasing Ar partial pressure, maintaining a critical temperature Tc,zero≊90 K and a transport critical current density Jc(77 K)≥106 A/cm2 by extended oxygenation time during cool down.

Andreev reflections at interfaces between δ‐doped GaAs and superconducting Al films

By placing several Si δ‐doped layers close to the surface of a GaAs molecular beam epitaxy–grown crystal, we achieve a compensation of the Schottky barrier and obtain a good Ohmic contact between an in situ deposited (without breaking the vacuum) Al metallization layer and a highly modulation doped (n++) conduction layer embedded below the δ‐doped layers in the GaAs crystal. When cooled to below the critical temperature (≊1.2 K) of Al, superconductivity is induced in the conductive layer of the semiconductor. We have studied the current voltage (I–V) characteristics in a planar geometry where the Al has been removed in a thin stripe. We find a manifestation of the superconducting energy gap and a rich fine structure at injection energies both below and above the gap.

Double-sided YBa2Cu3O7-x thin films for microwave applications

YBa2Cu3O7-x(YBCO) thin films with superconducting transition temperatures above 91 K were deposited on both sides of double-sided, polished LaAlO3(100) substrates. Using ex situ furnace annealing, the substrates could be kept at 45 degrees C during the codeposition of Y, Cu and BaF2. No O2 was necessary in the deposition chamber. The furnace annealing was carried out in a mixture of Ar, O2 and H2O vapour. The gas flow was 1000 sccm at an O2 partial pressure of 1.0 mbar. The maximum annealing temperature was 810 degrees C which was maintained for 1 h. Double-sided YBCO superconducting thin films may provide a good base material for microwave devices in which one superconducting side acts as a low-loss ground plane.

An automated 55 GHz cryogenic Josephson sampling oscilloscope

A computer‐automated superconductive 55 GHz sampling oscilloscope based on 4 kA/cm2, Nb/Nb2O5/Pb edge Josephson junctions is presented. The Josephson sampler chip was flip‐chip bonded to a carrier chip with a coplanar transmission line by use of a novel flip‐chip bonding machine. A 5.6 ps step pulse was successfully coupled in to the transmission line and 18.5 GHz multiple reflections plus a parasitic oscillation at 43 GHz were observed.

Tilted-axes YBCO thin films: from vicinal range to step bunching

Tilted-axes YBCO thin films were deposited on NdGaO3 (NGO) substrates with surface inclination from the standard (110) crystallographic plane. All deposited films were epitaxial with the (001) YBCO plane aligned with the {110} planes of the substrate. Structural and morphological studies revealed three angular ranges of the film formation: a small-angle “vicinal” range, where the films to not differ much from films deposited on (110) NGO; a high-angle range, characterized by “step bunching” during the thin film growth; and an intermediate range, providing very smooth films with the best crystal structure. Differences in film morphology and structure are probably due to an increasing density of seeding centers on the surface of tilted-axes substrate with an increase of inclination angle. The reason for the high-quality film formation in the intermediate range is probable the matching of the density of seeding centers and the diffusion length of the atoms on the surface. This model is corroborated by differences in the intermediate range angles for different deposition techniques. The intermediate range thin films showed also the best electrical properties (Tc, Jc(77 K), Rs(77K, 10 GHz).

Noise In Superconducting Josephson Junctions

This is a review of the noise properties of superconducting Josephson junctions, i.e. superconducting tunnel junctions and superconducting metallic weak links (point contacts, microbridges). For tunnel junctions theoretical models have been worked out and they agree fairly well with experimental results obtained on well-characterized tunnel barriers. For metallic weak links no general theory exists yet, and calculations of the noise properties based on the phenomenological Resistively-Shunted Junction (RSJ) model predicts white noise power levels which are 2–100 times smaller than the measured values.

The plasma resonance in microwave-driven small Josephson tunnel junctions

We report on measurements of the plasma resonance in small Josephson tunnel junctions both in the linear and the non-linear regime. The effect of a static magnetic field on the junctions dynamics was also investigated. All measurements were compared with calculations based on the resistor-capacitor shunted junction model.

Low-frequency flux noise in YBCO dc SQUIDs cooled in static magnetic fields

The low-frequency flux noise in bicrystal and step-edge YBa2Cu3Ox dc SQUIDs has been investigated. The width, w, of the superconducting strips forming the SQUID frame was varied from 4 to 42 µm. The SQUIDs were cooled in static magnetic fields up to 150 µT. Two types of low-frequency noise dominated, namely 1/f-like noise and random telegraph noise giving a Lorentzian frequency spectrum. The 1/f noise performance of the w = 4, 6 and 7 µm SQUIDs was almost identical, while the SQUIDs with w = 22 and 42 µm showed an order of magnitude higher noise level. Our analysis of the data suggests an exponential increase of the 1/f noise versus the cooling field, exhibiting a characteristic magnetic field around 40 µT.

Transient Response of Superconducting Tin Microbridges to Supercritical Current Pulses

We report on measurements of the transient response of long tin microbridges to current pulses exceeding the critical current. The voltage response was investigated by the use of superconducting sampling and pulse-generation circuits. Analysis of the delay time until the normal state voltage develops gives an estimate of 150 picoseconds for the inelastic electron-phonon scattering time in tin. We have also investigated fast voltage pulses which apparently are induced by the current-driven motion of flux-tubes across the bridges.

Observation of distinct, temperature dependent flux noise near bicrystal grain boundaries in YBa2Cu3O7−x films

Abstract The characteristics of the magnetic flux noise in high temperature superconducting thin-films of yttrium-barium-copper-oxide (YBa 2 Cu 3 O 7 ) in the vicinity of artificial grain boundaries have been studied by means of a low critical temperature superconducting quantum interference device (SQUID) magnetometer. The measured power spectrum of the flux noise typically shows 1/ f -behaviour at low frequencies. This is, in particular, evident near the transition temperature. When measuring the flux noise near an artificial grain boundary we observe in selected samples additional Lorentzian type spectra in narrow temperature ranges indicative for well-defined barriers for flux hopping. Such spectra are not observed when measuring away from the grain boundary. The distinct fluctuators are seen in the 60–80 K interval corresponding to barrier heights of order 0.1–0.4 eV. If present in Josephson junctions patterned across such artificial grain boundaries, they could lead to serious limitations in the junction performance in certain temperature ranges.