摘要:

The use of epitaxial growth for the deposition of superconducting structures (layers, heterostructures, superlattices) is briefly discussed. The technique enables to obtain enhanced superconducting properties makes possible the formation of new phases and metastable compositions, etc. Excellent quality films can be grown using buffer layers. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.42093

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The discovery of a critical current density persisting out to high fields violates early expectations of weak‐link behavior in polycrystalline YBa2Cu3O7and Bi‐based superconductors. Explanations in terms of non‐weak‐linked portions of grain boundaries or the brick‐wall model are assessed.

ISSN:0094-243X    

DOI:10.1063/1.42052

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

This note is a summary of the talk presented at the meeting. It describes our current understanding of the weak link nature of grain boundaries in the high temperature cuprate superconductors. A hypothesis is advanced which suggests how large critical current in polycrystalline wires can be attainable.

ISSN:0094-243X    

DOI:10.1063/1.42063

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Structure and superconductivity of ultrathin films as thin as a unit‐cell size have been investigated. Oscillations in the reflection high energy electron diffraction (RHEED) specular intensity during growth of the superconducting oxides on SrTiO3indicate that the growth occurs in monomolecular layer by monomolecular layer mode. The film thickness can be exactly controlled by monitoring the RHEED intensity. The isolated ultrathin YBa2Cu3O7−x(YBCO) films sandwiched in between PrBa2Cu3O7−x(PrBCO) layers were prepared using the growth interruption technique by oscillations monitoring. A one unit‐cell YBCO layer has a superconducting transition with the onset temperature at 70K and the zero resistance at 30K. The overlayer of PrBCO was required to provide the underlying one unit‐cell YBCO layer with hole carriers sufficient to induce superconductivity. The reduction of the onset temperature is found to be mainly due to the decrease of the hole carriers but not due to the absence of the interlayer coupling. The resistive transitions for the YBCO layers with the thickness below 5 unit‐cells and also for the YBCO (1 unit‐cell)/PrBCO (1 unit‐cell) superlattice are magnetic field independent up to 10T when the field is applied parallel to the CuO2layers. In perpendicular magnetic fields, the activation energy of flux flow for the isolated ultrathin layers is nearly proportional to the number of unit‐cells. The activation energy for the YBCO (1 unit‐cell)/PrBCO (1 unit‐cell) superlattice is comparable to the value of the isolated layers, suggesting that the flux lines alongc‐axis are limited to each one unit‐cell layer of YBCO.

ISSN:0094-243X    

DOI:10.1063/1.42068

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Epitaxial YBa2Cu3O7−&dgr;‐based superconducting superlattices have been fabricated using pulsed laser deposition in whichc‐axis oriented YBa2Cu3O7−&dgr;layers as thin as one unit cell thick are separated by relatively thick PrBa2Cu3O7−&dgr;‐based barrier layers. The superlatticeTc(R=0) decreases rapidly with increasing barrier layer thickness, but then saturates at a finiteTcfor YBa2Cu3O7−&dgr;layers as thin as a singlec‐axis unit cell The superconducting properties of YBa2Cu3O7−&dgr;‐based superlattices are shown to depend strongly on the electronic properties of the barrier layers. The resistive transition width decreases significantly as the hole carrier density in the barrier layers is increased. However,Tc(onset) does not change, contrary to predictions of hole filling models. Theoretical analyses suggest that the broadening of the resistive transition for the thinnest YBa2Cu3O7−&dgr;layers is most likely due to a crossover to 2D resistive behavior involving thermally‐generated vortices. Scanning tunneling microscopy reveals that epitaxial YBa2Cu3O7−&dgr;thin films grow unit cell‐by unit cell, by a terraced‐island growth mode. This terraced microstructure explains the steps found in ultrathin YBa2Cu3O7−&dgr;layers in these superlattices. These steps may act as superconducting weak links, providing support for 2D Josephson‐coupled‐array models of superconducting superlattices.

ISSN:0094-243X    

DOI:10.1063/1.42087

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We review our results on the optimization of the production parameters which has led to the fabrication of very high quality thin films of superconducting Ba2YCu3O7−&dgr;by co‐evaporation of Y, Cu, and BaF2followed by a two stage anneal. The significance of critical annealing and deposition parameters is discussed. We also discuss our work correlating some of the superconducting properties of these films with their structural features.

ISSN:0094-243X    

DOI:10.1063/1.42100

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We report the preparation of (Pb0.75Cu0.25)Sr2(Y1−yCay)Cu2O7thin films by laser ablation. The films are growninsituon (100) LaAlO3at the relatively low substrate temperature of 620 °C. The target does not have the same stoichiometry as the thin films in that it is lead enriched. The films are highly oriented with thec‐axis perpendicular to the substrate and exhibit a surface morphology which is unique to the oxide superconductors. By varying the Y/Ca ratio it is possible to systematically vary the onset of the superconducting transition temperature from 10 K to a maximum of 86 K. We also report the successful preparation of highly oriented Nd1.85Ce0.15CuO4+&dgr;thin films grown on (100) SrTiO3substrates. By carefully controlling the thin film’s oxygen content through vacuum anneals it is possible to obtain superconducting transitions which onset at temperatures as high as 23 K.

ISSN:0094-243X    

DOI:10.1063/1.42110

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Post‐deposition annealing of thin films of YBa2Cu3O7(YBCO) at temperatures of 850–900 °C and in atmospheric pressure oxygen has yielded films with values of critical current density and microwave surface resistance over large areas comparable to those obtained byinsitumethods. A recent improvement in the properties of post‐annealed films has come about by using low oxygen partial pressures, which allow annealing temperatures to be lowered as has been done byinsitumethods. Surface smoothness, critical current density, and ac losses are all improved by using the new post‐annealing conditions. These results mean that high quality thin films of YBCO can be made over large areas and with double‐sided coating by ambient temperature film deposition and post‐annealing in a tube furnace.

ISSN:0094-243X    

DOI:10.1063/1.42045

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The structural and electrical properties ofin‐situlaser deposited thin films of YBa2Cu3O7−xon SrTiO3(110) substrates were studied as a function of deposition temperature. It was found that at low temperatures, the films with (110) orientation dominated. At high temperatures, the (103) orientation was obtained. The lattice mismatch model can qualitatively explain the orientation of the films at different deposition temperatures, but the transition point for the orientation of the film was found to shift as much as 60 °C to higher temperature when compared to the predictions of the lattice mismatch model. TheJcwas found to be extremely anisotropic. For (103) oriented films at 70 K, theJc’s were found to be 0.85×106A/cm2and 5×102A/cm2along the substrate ⟨100⟩ and ⟨011⟩ directions respectively.

ISSN:0094-243X    

DOI:10.1063/1.42050

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Tl‐Ba‐Ca‐Cu‐O superconducting thin films have been prepared using both laser ablation and ‘‘fog’’ spray techniques. The zero resistance temperature varied between 92 K and 119 K.

ISSN:0094-243X    

DOI:10.1063/1.42058

出版商:AIP    

年代:1992

数据来源: AIP