摘要:

As‐deposited epitaxial YBa2Cu3Oxthin films have been prepared by ArF excimer laser deposition followed by a rapid cooling with controlled oxygen pressure. The best film after the rapid cooling has aTcwith a zero resistance of 88 K and aJc(77 K) of 5.5×105A/cm2. It has been observed that thec‐lattice parameter decreases, that is, the oxygen contentxincreases from ∼6 to 7, with increasing oxygen pressure during the rapid cooling. The relation betweenxand oxygen pressure is almost consistent with that for a powder sample being in a thermal equilibrium. We conclude that the oxygen content in the as‐deposited films is determined by the thermal equilibrium oxygen content during the cooling process.

ISSN:0003-6951    

DOI:10.1063/1.102449

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Perfect epitaxial growth of superconducting YBa2Cu3O7−xthin films have been achieved on (100) SrTiO3, (110) SrTiO3, and (100) ZrO2substrates using a modified planar dc magnetron sputtering system. The films exhibit zero resistances at 87–90 K with transition widths of about 2 K. The critical current density of 1.4×106A/cm2at 77 K so far has been measured on the (100) SrTiO3substrate. The epitaxial orientation of the thin films was influenced by the substrate orientation, the substrate temperature, and the oxygen partial pressure. The quality of growth and the epitaxial orientation of the films were examined by electron channeling, x‐ray diffraction, and reflection high‐energy electron diffraction techniques. The preferential epitaxial orientation is discussed.

ISSN:0003-6951    

DOI:10.1063/1.102450

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Critical‐current criteria based on electric field or resistivity can present a number of problems in defining critical current, especially for highTcsuperconductors in the vicinity of the critical temperature or upper critical field. The resulting critical‐current densityJccan be quite arbitrary, since it depends strongly on criterion level at high fields and temperatures. TheseJcdefinitions also create problems in distinguishing between superconductors and high‐conductivity normal metals such as copper. They can also biasJcdata when superconductors are compared that have different values of normal‐state resistivity. To minimize these problems, an intrinsicJccriterion is proposed, which effectively separates superconducting and normal‐state properties. Based on the long‐standing concept of a flux‐flow resistivity,Jcis defined as the current where the tangent to theE‐Jcurve at a given electric field level extrapolates to zero electric field. This determines an offsetJcthat minimizes the above problems. The criterion is particularly useful nearTcor near the effective upper critical field where theE‐Jcharacteristic starts to approach ohmic behavior.

ISSN:0003-6951    

DOI:10.1063/1.102259

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Semiquantitative tests are considered of models, proposed earlier by this author, in which highTcsuperconductivity follows from the exchange of unspecified intermediate bosons (IB) with &ohgr;≫&ohgr;phononbetween fermionic pairs in different CuO2layers. Earlier predictions of the dependence ofTc(n) on the numbernof layers per cluster are extended to include the possibility thatTc(n=1)<∼10 K in the Tl 1201 and 2201 compounds. In common with other authors rapid saturation ofTc(n) with increasingnis found here. Nonetheless, the experimental value ofTc(n=1) in the Tl and Bi systems can play an important role in discriminating between the various models empirically.

ISSN:0003-6951    

DOI:10.1063/1.101619

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

We have investigated the transport current‐induced resistive transitions in predominantlyc‐axis oriented crystalline Y1Ba2Cu3O7−xsuperconducting thin films with scanning electron microscopic (SEM) imaging and its correlation with dc current‐voltage (I‐V) measurements. We find that there is a nonlinear gradual transition region in theI‐Vcurves that is caused by macroscopic effects (current ‘‘crowding’’, substrate defects, film thickness variation, etc.) and by microscopic dissipation effects. At a high current there is an abrupt transition to the normal state in theI‐Vcurve due to Joule heating. The SEM mapping provides a direct image of the above effects and is useful for characterization and study of fundamental transport properties of highTcthin films.

ISSN:0003-6951    

DOI:10.1063/1.101620

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Surface resistance measurements of films of YBa2Cu3O7deposited onto single‐crystal substrates of LaGaO3and SrTiO3have been made at a frequency of 22 GHz. The measurements were made in either a copper or niobium cavity by replacing the end wall with the superconducting film. Typical surface resistance at 20 K are 1–2 m&OHgr; for films on LaGaO3and 6–8 m&OHgr; for films on SrTiO3, as measured in the copper cavity. The LaGaO3values lie within the sensitivity range of the Cu cavity (∼2 m&OHgr;) and can only be considered upper limits. Similar measurements in a Nb superconducting cavity resulted in a surface resistance value of 0.2±0.1 m&OHgr; at 4 K for the best LaGaO3‐based film. This value is more than an order of magnitude lower than Cu, and suggests that LaGaO3‐based films may offer immediate advantages in a number of applications.

ISSN:0003-6951    

DOI:10.1063/1.101621

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Ag‐sheathed powder‐in‐tube wires on the basis of the highTcsuperconductor YBa2Cu3Oxare produced by a drawing process. After cold working, the wires are not superconducting. By heat treatment at temperatures >900 °C, lattice defects are healed and the contact between the grains is improved. After this heat treatment the wires are slowly cooled in an O2atmosphere in order to adjust the optimum O2stoichiometry. The annealing temperature has a strong influence on the attainable critical current density. By annealing a wire with 420 ppm carbon content at 950 °C,jcvalues of 5020 A/cm2at 4.2 K and 1130 A/cm2at 77 K were achieved. Drastically reduced critical current densities in wires with high carbon content demonstrate the importance of a low carbon content for achieving high critical current densities in YBa2Cu3Ox.

ISSN:0003-6951    

DOI:10.1063/1.102451

出版商:AIP    

年代:1989

数据来源: AIP

摘要:

Observations of thin films of amorphous TbFeCo by electron microscopy with a resolution limit of better than 0.2 nm invariably reveal the presence of very fine microcrystals (sizes ∼2–5 nm). Bright field imaging, in both plan view and in cross section, indicates that these nanocrystals have the crystal structure of &agr;‐Fe or Co. In contrast, crystals of Tb metal (or Tb oxides) have not been observed. Similar observations of FeCo thin films, prepared under identical experimental conditions, reveal much larger crystals (typically 10–50 nm). It is concluded that the presence of the rare earth element Tb is crucial to the microstructure displayed by this kind of material.

ISSN:0003-6951    

DOI:10.1063/1.101622

出版商:AIP    

年代:1989

数据来源: AIP