S. Proyer

Particulates on pulsed-laser deposited YBaCuO films

Abstract Different types of particulates observed on high temperature superconducting Yue5f8Baue5f8Cuue5f8O films prepared by pulsed-laser deposition on MgO substrates are investigated as a function of deposition parameters and classified according to their size, shape, and chemical composition. Among the parameters studied in detail are the oxygen pressure, the substrate temperature, the laser energy, fluence, spot size, and the number of pulses. The interrelations between the different particulates, the ablation and deposition rates, the shape of the visible plasma plume, and the film properties have been investigated.

Patterning of YBCO films by excimer-laser ablation

Thin films of YBa2Cu3O7 coated with a photoresist have been patterned by means of XeCl- and KrF-excimer-laser light projection. Lines with widths down to 3.5 μm have been fabricated without degradation in Tc and jc. The technique is compared with wet-chemical etching and with ns and fs excimer-laser patterning of films without a protective layer.

STM observation of dislocation chains suitable for flux pinning in YBa2Cu3O7−x films

Abstract Systematic variations of the film-growth conditions during pulsed-laser deposition (PLD) with regard to substrate temperature, oxygen partial pressure and substrate quality, allow us to establish correlations between the as-grown defect structure and the current transport properties of c -axis oriented YBa 2 Cu 3 O 7− x films. By investigating the film surface with scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) the nature and the density of various defects as well as the general film-growth mode were determined. SEM pictures show the overall surface roughness and the presence of other phases (outgrowths), while higher-resolution STM pictures reveal small-scale defects down to the nm range. Transport current measurements on patterned parts of the same films show differences in the magnitude of J c as well as in its field and angle dependence. Influences of the defect structure on intrinsic pinning, weak-link properties and special features in the angular dependence of J c are discussed. Dislocation chains extending over several layers along the c -axis within the terraces of growth spirals are of special interest with regard to flux pinning. The existence of these dislocation chains can be directly implied from the present data. A higher substrate roughness and a faster growth speed of the terraces along the ab plane seem to favor the appearance of the dislocation chains.

Time-integrated photography of laser-induced plasma plumes

The shape of the plasma plume induced by KrF-laser irradiation of Y−Ba−Cu−O, Cu−O, and Cu targets in O2 and N2 atmosphere was investigated by time-integrated photography. The dependence on laser fluence, spot size, and pressure of the ambient gas was studied. Special emphasis was put on the ablation of YBCO in O2 atmosphere under experimental conditions that are typical for thin-film deposition.

Synthesis of YBa2-xSrxCu3O7-δ by pulsed-laser deposition

Abstract This superconducting films of single-phase YBa2-xSrxCu3O7-δ with values of x up to 1.8 have been synthesized by means of pulsed-laser deposition (PLD). When increasing x from zero to 1.8, the zero resistance temperature decreases from 91 K to 57 K; simultaneously the length of the c-axis decreases from 11.68 A to 11.45 A. The temperature dependence of the critical current density has been measured for materials with various values of x.

Femtosecond-excimer-laser patterning of YBa2Cu3O7 films

Projection patterning of Y-Ba-Cu-O films on (100) SrTiO3 and (100) MgO substrates using femtosecond KrF-excimer-laser pulses is reported

The morphology of YBa2Cu3O7−α thin films investigated by STM

Abstract Scanning tunnelling microscopy (STM) was employed to study the surface of c -axis oriented YBCO thin films. The films with a thickness of typically about 300 nm were grown onto single crystal (100) MgO by pulsed laser deposition. In addition to the well known growth spirals, “frozen in” clusters were observed. The latter result in a high density of holes with sizes ranging from a few to about 50 nm. These holes may serve as pinning sites. Sub-unit cell steps were found in regions between neighboring growth spirals. In samples exposed to air for several weeks, a change in surface morphology was found. Samples that were patterned by wet-chemical etching did not show the characteristic growth steps. Irradiation with fast neutrons slightly increased the density of nm sized holes.

Anisotropic pinning and dimensionality effects in YBa2Cu3O7-x thin films☆

Abstract Transport measurements were made on c -axis orientated thin films of YBa 2 Cu 3 O 7 − x in order to obtain the magnetic field and the temperature dependence of the in-plane critical current density J c ab . By rotating the samples in the external field, the anisotropy of flux pinning was assessed for this material. Sets of J c ( θ , T , B ) curves, obtained for a variety of differently produced samples, allow us to distinguish between intrinsic pinning properties due to the crystal structure and the nature of the superconducting state, which influences the shape of the flux lines, and pinning by defects. A gradual cross-over from a quasi two-dimensional superconducting state at low temperatures to a three-dimensional state at high temperatures is suggested. We analyse the influence of this effect on the temperature, field and angular dependence of J c ab .

The preparation and electrical properties of Bi2Sr2CaCu2Oy thick films with high Tc on (100) MgO substrate

Thick c-oriented films, of single-phase Bi2Sr2CaCu2Oy (Bi-2212) were prepared on (100) MgO substrates by employing a partial melting method. The effect of quenching on Tc0 was studied. The highest Tc0 obtained by resistance measurement was 92 K, whereas 93.2 K was achieved by AC susceptibility measurements. The best results for the critical current density Jc, measured in zero magnetic field, were 1.3*103 A cm-2 at 77 K and 7*103 A cm-2 at 53 K. Near Tc the temperature dependence of Jc can be described by Jc approximately (1-T/Tc)2. At temperatures Tc<<Tc the dependence of Jc on temperature can be described by a homogeneous superconductor flux creep model.

Novel approach for the analysis of the fluctuation magnetoconductivity in YBa2Cu3O7 thin films

Abstract We report measurements of the paraconductivity and of the fluctuation magnetoconductivity as a function of both the magnetic field up to 13 T and the temperature in thin epitaxial films of YBa 2 Cu 3 O 7 . An analysis within the model for dirty-limit superconductors yields consistent values for the in-plane, the out-of-plane coherence lengths, and the quasiparticle phase coherence time. Although an estimation of the mean free carrier path indicates that YBa 2 Cu 3 O 7 is a clean superconductor, the appropriate clean-limit theory produces an unrealisticly short phase coherence time, around half the value of the transport scattering time, and fails to fit the paraconductivity data. We show that if the magnetoconductivity due to the orbital motion of the normal-state quasiparticles is taken into account, the Maki-Thompson fluctuation contribution at high temperatures is significantly smaller than predicted by both clean- and dirty-limit theories. This result suggests non-s-wave pairing in YBa 2 Cu 3 O 7 .