摘要:

EXAFS measurements were done on Bi2Sr2CaCu2O8glass and glass‐ceramic to study the local environment of copper and strontium. X‐ray absorption measurements at the Cu K‐edge indicate that copper atoms are surrounded by four oxygen atoms in the glass and six oxygens in the glass‐ceramic with Cu‐O bond distances of 1.83 A˚ and 1.91 A˚, respectively. Measurements at the Sr K‐edge indicate that the Sr‐O distance in the glass‐ceramic is 2.70 A˚ with a coordination of nine.

ISSN:0094-243X    

DOI:10.1063/1.40276

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Glancing angle ion beam milling has been used to successively thinc‐axis oriented Ba2YCu3O7films fabricated using anin situpulsed laser deposition process. The systematics of the thinning process are characterized by scanning electron microscopy, ion channeling by Rutherford back‐scattering spectrometry, and measurements of the resistance transitions. Comparisons are made with the results of previously published work in whichex situannealed films, fabricated in a three‐source evaporator and having a significantly different surface morphology, were subjected to a similar thinning treatment.

ISSN:0094-243X    

DOI:10.1063/1.40277

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Ceramic films relating to high‐Tc superconductors were grown in a UHV deposition‐analysis system. Against our intuition that large oxygen deficiency is inevitable in the growth of oxide films in UHV, required oxidation state was achieved either by using such a highly oxidative gas as NO2or by choosing thermodynamically stable oxides. Two‐dimensional growth of ceramics were verified in MBE growth of Bi‐Sr‐Cu‐O as well as in laser MBE growth of various ceramic thin films. The growth mode and the oxidation state of the films were investigated byin situRHEED and XPS analyses. Discussion is extended to the possibility of our method for constructing new ceramics layers which may exhibit higher Tc.

ISSN:0094-243X    

DOI:10.1063/1.40278

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

For YBCO epitaxial thin films the basal plane transport critical current densityJc, flowing perpendicular to an applied magnetic fieldH, depends sensitively on the orientation of the crystal with respect toH. In particular,Jcis sharply peaked and greatly enhanced whenHis precisely parallel to the copper‐oxygen planes. Experiments on a series of epitaxial monolithic and superconductor‐insulator multilayer thin films provide clear evidence that the enhancement is a bulk, rather than surface or thin sample, phenomenon. Measurements of the orientation dependence are presented and compared with a model of ‘‘intrinsic flux pinning’’ by the layered crystal structure.

ISSN:0094-243X    

DOI:10.1063/1.40279

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

High Tcsuperconductor films were prepared by laser deposition using a SIEMENS XP 2020 excimer laser. Epitaxial YBaCuO films of high quality are obtained for a uniform laser beam energy density of about 4 J/cm2, a substrate temperature above 730 °C and an oxygen pressure of about 0.4 mbar. These films show high DC critical current densities as 5×106A/cm2at 77 K or 6×107A/cm2at 4.2 K and zero field and excellent microwave properties. The performance of first passive microwave devices made from these films is promising. Also epitaxial Bi2Sr2CaCu2Oxfilms were prepared on SrTiO3substrates. The inductively measured Tc, onsetreached 83 K. Critical current densities up to 107A/cm2at 4.2 K and 106A/cm2at 50 K were obtained. In magnetic field, these films show a dramatic dependence on the field direction, with jcbeing almost field‐independent (up to 7 T) at low temperatures, when the field lies within the film plane.

ISSN:0094-243X    

DOI:10.1063/1.40227

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Y‐base and Bi‐base high‐Tcoxide superconductors have been successfully synthesized by the citric acid salt decomposition process.Tc’s of 92 K and 109 K have been obtained after a relatively short sintering in the Y‐system and Bi‐system, respectively. The fine dispersion of particles Ag in the Y‐base oxide appreciably decreases the sintering temperature and enhances the transportJc. The intermediate press increases bothTcandJcin bi‐base oxide. AJc(77 K) of 1500 A/cm2has been obtained in the sintered Bi‐base specimen.

ISSN:0094-243X    

DOI:10.1063/1.40228

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Stable bulk superconductors in the Tl‐Ca‐Ba‐Cu‐O system with zero resistance up to 125 K are discussed. Structural, magnetic, and electronic transport properties, including thermoelectric power data, are presented on a variety of phases in this system. The anisotropic resistivity of Tl2Ba2CaCu2O8single crystals is discussed. The in‐plane and c‐axis resistivities are ∼7×10−4&OHgr;‐cm and 2×10−1&OHgr;‐cm, respectively, at room temperature. The c‐axis resistivity decreases with decreasing temperature in the temperature range from room temperature down to the superconducting transition temperature, and the second derivative of &rgr;cis negative, unlike that of YBa2Cu3O7and Bi2Sr2.2Ca0.8Cu2O8single crystals. The anisotropic thermopower of Tl2Ba2CaCu2O8single crystals is also described. The room temperature value ofSabis about 14 &mgr;V/K.Sabincreases linearly with decreasing temperature at a rate of 0.06–0.065 &mgr;V/K2down to about 160 K, below whichSabflattens off and drops to zero atTc.Scat room temperature is around 30 &mgr;V/K and depends only weakly on temperature. Conventional scattering mechanisms cannot explain these normal state properties self‐consistently.

ISSN:0094-243X    

DOI:10.1063/1.40229

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Magnetically aligned polycrystalline samples of high‐Tcsuperconductors have been shown to display improved critical currents, but are still limited by the presence of intergranular weak links. We discuss a new technique, involving biaxial granular alignment, which can in principle overcome the weak link problem. We discuss recent experimental progress in demonstrating biaxial alignment.

ISSN:0094-243X    

DOI:10.1063/1.40230

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Two kinds of superconducting Pb‐based layered copper oxides, (Pb, Cu)Sr2(Y,Ca)Cu2O7(the ‘‘1222’’ phase) and (Pb,Cu) (Ln,Ce)2(Sr,Ln)2Cu2O9(Ln: rare‐earth element; the ‘‘1212’’ phase), are synthesized and characterized. Although most of previously‐known Pb‐based cuprate superconductors form in a midly reducing atmosphere, these compounds form in an oxidizing atmosphere. Single‐phase ceramics of the ‘‘1212’’ compounds are obtained in the compositional range of (Pb1+x)/2Cu(1−x)/2)Sr2(Y1−xCax)Cu2Oz(x=0∼0.35 and z=∼7). The Pb‐based ‘‘1222’’ compounds are the first ‘‘1222’’ superconductor as well as the first superconducting lead cuprate containing ‘‘fluorite‐type’’ block layers. The maximum values of the observed superconducting onset temperatures are 65 K and 28 K for the ‘‘1222’’ phase and for the ‘‘1222’’ phase, respectively.

ISSN:0094-243X    

DOI:10.1063/1.40231

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Transport properties of YBa2(Cu0.98M0.02)3Oy(M=Fe, Co, Ni) superconducting thin films are studied and compared with the results measured in doped bulk materials. Critical current density as a function of magnetic field shows that Ni‐doping has a larger effect on intragranular current while Fe‐doping seems to have more influence on the intergranular coupling. All three doped samples show the same field‐dependence on their volume pinning forceFpexcept in low field region.Fpcan be scaled to a universal functionFp/Fp,max=b(1−b)2indicating that similar pinning mechanism is involved in all these samples. Finally, Hall effect measurements show that the effective carrier concentration is decreased in these doped films.

ISSN:0094-243X    

DOI:10.1063/1.40232

出版商:AIP    

年代:1991

数据来源: AIP