P. Rosenthal

Critical current densities and transport in superconducting YBa2Cu3O7−δ films made by electron beam coevaporation

Thin films of the high Tc superconductor YBa2Cu3O7−δ were prepared and characterized. The films made on SrTiO3 showed epitaxial growth and high critical current densities in excess of 9×104 at 78 K and 2×106 at 4.2 K. Also, surface pinning in the parallel direction was found to be very high. The measurements put a lower limit on the depairing critical current density of 5×107 A/cm2.

Thin-film synthesis of the high- T c oxide superconductor YBa 2 Cu 3 O 7 by electron-beam codeposition

The successful synthesis of high- T c YBa 2 Cu 3 O 7 films by means of electron-beam codeposition are reported. Several important growth parameters have been surveyed in a preliminary way. The substrates investigated include Al 2 O 3 , ZrO 2 , MgO, and SrTiO 3 , The films were characterized by resistivity measurements, x-ray diffraction, microprobe, and Rutherford backscattering analysis. Some TEM and critical current density studies were also carried out. The best results to date have been obtained on SrTiO 3 substrates with which polycrystalline epitaxial growth has been achieved. Resistive superconducting transitions with zero resistance at 89.5 K and a 2 K width have been observed in these films.

Flux focusing effects in planar thin‐film grain‐boundary Josephson junctions

We have studied the magnetic interference of the critical currents of synthetic planar thin‐film grain‐boundary Josephson junctions. We find that the effective area of these junctions scales as the square of the width w in contrast to the usual w(2λ+d) dependence of sandwich‐type Josephson junctions. This behavior is a simple consequence of the magnetic response of thin‐film superconductors to perpendicular applied fields. A model based on the London theory yields the observed behavior. In addition, we find the correction to the interference pattern due to the effect of the corners.

Feasibility of the high Tc superconducting bolometer

A design analysis is given for a bolometric infrared detector that uses the resistive transition of a high‐temperature superconductor as the temperature sensing element, and liquid nitrogen (LN) as the coolant. It is shown that for highly oriented c‐axis films, the measured low‐frequency noise causes little or no degradation of the performance. With the incoming radiation chopped at 10 Hz, noise equivalent powers (NEP) in the range (1–20)×10−12 W Hz−1/2 should be achievable. These values compare favorably with the NEP of other detectors operating at or above LN temperatures for wavelengths greater than 20 μm.

Magnetic flux noise in thin-film rings of YBa2Cu3O7−δ

The low‐frequency magnetic flux noise in thin‐film rings of YBa2Cu3O7−δ (YBCO) is measured over the temperature range 1.3–125 K by means of a dc superconducting quantum interference device (SQUID) maintained at liquid‐helium temperatures. Below the transition temperature Tc of the YBCO, the spectral density of the noise scales as 1/f, where f is the frequency, and generally increases with increasing temperature. The magnitude of the noise depends strongly on the microstructure of the film, and is lowest for a sample which is predominantly oriented with its c axis perpendicular to the substrate. These results imply that SQUID’s and flux transformers of YBCO must be fabricated from highly oriented films to produce good resolution at low frequencies.

In situ growth of superconducting YBaCuO using reactive electron-beam coevaporation

Conditions required for in situ growth of YBaCuO thin films by reactive electron-beam evaporation have been explored. Three sources of activated oxygen (atomic oxygen from microwave discharge, plasma generated by electron beams, and an ion beam) were compared. The best results so far were obtained with atomic oxygen. Epitaxial films with high critical currents were grown on SrTiO/sub 3/ [100] and [110], Al/sub 2/O/sub 3/ (1102), and MgO [100] at 600 degrees C. Evaporation rates were controlled with a rate monitor using atomic absorption. >

Low frequency resistance fluctuations in films of high temperature superconductors

Low-frequency voltage fluctuations in thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ at and above the superconducting transition temperature have a spectral density proportional to the ratio of the average voltage across the film to the frequency. The ratio of the spectral density to the average voltage decreases markedly as the microstructure of the films is improved. In contrast to classic superconductors, the noise at the resistive transition does not arise from equilibrium temperature fluctuations. >

Microstructure of Cu‐ and Y‐rich YBa2Cu3O7−x thin films: Identification of the CuYO2 phase

The CuYO2 delafossite‐type structure has been identified as a second phase in thin films of Cu‐ and Y‐rich YBa2Cu3O7−x (123) made by electron beam co‐evaporation at conditions of low oxygen pressure; this phase occurs as extremely small, densely distributed, highly oriented precipitates within the 123 matrix. The smallest (sub‐100 A) [00.1] oriented precipitates of this hexagonal phase are strained to give a better lattice match to the surrounding 123 structure. The results suggest that, for thin film deposition conditions of sufficiently low temperature and pressure, there may be a tie line between YBa2Cu3O7−x and CuYO2 in the equilibrium phase diagram. The size and distribution of the precipitates is such as would be expected to influence structurally sensitive superconducting transport properties.

Anomalous compositional dependence in in situ growth of YBaCuO films at low oxygen pressure

The roles of composition and oxygen pressure in in situ growth of superconducting YBaCuO films have been studied. Anomalous results are observed at low oxygen pressure. Off‐stoichiometric, Ba‐deficient films have superior superconducting properties and more bulk‐like crystal structure compared to films made on the 1:2:3 stoichiometry. Films made at low pressure have an expanded c axis compared to the bulk value, although their Tc’s are only slightly depressed. Tc and c‐axis lattice parameters are reported as a function of the deposition conditions.

Flux noise and flux creep in YBCO thin films

The authors used a DC SQUID (superconducting quantum interference device) to measure the low-frequency magnetic flux noise produced by thin-film rings of YBa/sub 2/Cu/sub 3/O/sub 7- delta / (YBCO) with various microstructures. Below the transition temperature T/sub c/ of the YBCO, the spectral density of the noise scales as 1/f (f is the frequency) from 1 Hz to 1 kHz. This noise generally increases with temperature and vanishes abruptly at T/sub c/. Improvements in crystalline microstructure greatly reduce the magnitude of the noise, which was lowest for a highly orientated sample with its c-axis perpendicular to the substrate. Making a radial cut to interrupt current paths around the sample ring does not significantly affect the magnitude of the noise, demonstrating that the noise arises from a local mechanism such as the thermally activated hopping of flux bundles. Flux creep was observed in one sample cooled in a magnetic field of 1 mT, and the creep rate exhibited a sharp maximum near 80 K. It is concluded that SQUIDs and flux transformers of YBCO must be fabricated from highly orientated films to obtain low noise at low frequencies. >