摘要:

The essential features and new developments of the s‐channel theory of superconductivity are presented. Applications to the &mgr;sRand Hall number experiments are analyzed. The relations between small coherence length, Bose‐Einstein condensation and highTcare emphasized.

ISSN:0094-243X    

DOI:10.1063/1.40234

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Several structural, normal‐state, and superconductive properties of the high‐Tccopper oxides are discussed as a prelude to the proposal of a correlation‐bag model of Cooper‐pair formation. All copper oxides have intergrowth structures, which introduce not only anisotropy—both electronic and mechanical—but also internal electric fields normal to the intergrowths in addition to the local electric fields at copper due to the oxygen coordination. The significance of these internal fields for the oxidation state of the superconductive CuO2layers is illustrated, and the need for all Cu of a CuO2layer to have the same oxygen coordination is emphasized. The role of interlayer bond‐length mismatch is illustrated by the A2CuO4compounds. Evidence for a first‐order phase change on going from the antiferromagnetic‐semiconductor to the superconductor phase is presented, and XPS data indicative of a change from more ionic to more covalent Cu‐3d, O‐2p&sgr;bonding across this phase change is cited. Evidence for intermediate correlation energies (U≳W) in the superconductive phase and an equilibrium reaction Cu3++O2−&rlarr2;Cu2++O−near cross‐over are argued to give rise to instabilities that cannot be expressed as a negative‐U charge‐density wave; they therefore give rise to dynamic charge fluctuations that are manifest as spin fluctuations in the hole‐poor regions and as paired holes in the hole‐rich regions. Where the pair density is large enough for pair‐pair interactions, a cooperative condensation into the superconductive state occurs. Pair formation is stabilized not only by the electron‐phonon interactions, but also by modulation of the bandwidth—and hence also the correlation energy—within a charge fluctuation.

ISSN:0094-243X    

DOI:10.1063/1.40273

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

In this paper the results of electron tunneling measurements in two classes of high Tcsuperconductors are reported. In the cubic BiO3materials (notably BaPb1−xBixO3and Ba1−xKxBiO3) the results show these materials to be BCS superconductors. Indications point to electron‐phonon coupling as the pair binding mechanism but quantitative verification still eludes experiment. In the cuprates (most of the data is on YBa2Cu3O7‘‘gap‐like’’ structure is observed but it is not BCS‐like. Potential reasons for this deviation are discussed and some indications in the tunneling data for the pairing mechanism are revealed.

ISSN:0094-243X    

DOI:10.1063/1.40244

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A major driving force in the field of high Tcsuperconductivity during the first few years has been the search for new materials. ‘‘Why copper and bismuth oxides?’’ is a question that is often raised. This paper summarizes recent results on nonsuperconducting BaSn1−xSbxO3and La1−xSrxNiO4, which were studied in an attempt to address that question. Recent results on the new 60 K superconductor La1.6Sr0.4CaCu2O6are also described.

ISSN:0094-243X    

DOI:10.1063/1.40209

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The theory of high‐temperature superconductivity is discussed in terms of layered structures. (AIP)

ISSN:0094-243X    

DOI:10.1063/1.40220

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The in‐plane resistivity data, &rgr;∥(T), as a function of temperature for various Cu‐oxide superconductors have provided strong evidence for a Fermi‐liquid normal state and also important clues for understanding the mechanism responsible for high‐temperature superconductivity. A dominant quadratic temperature dependence of &rgr;∥(T) above Tcis observed in the electron‐ doped NdCeCuO system and several relatively low‐Tchole‐ doped cuprates. On the other hand, high TcCu oxides (Tc≳80 K) are always characterized by a linear temperature dependence of &rgr;∥(T). Within the framework of the BCS phonon‐mediated pairing, the closeness of a 2D van Hove singularity in the density of states to the Fermi level provides a basis for understanding the origin of high‐temperature superconductivity, the correlation between Tcand &rgr;∥(T), anomalous isotope effects and several other normal‐state and superconducting properties.

ISSN:0094-243X    

DOI:10.1063/1.40224

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Four different temperature regimes can be distinguished in flux‐creep relaxation‐rate data on many high‐temperature superconductors: 1) the ultralow temperature region where initial results show a temperature‐independent relaxation suggesting vortex tunneling, 2) a low‐temperature region where the relaxation increases linearly with temperature in agreement with conventional flux creep theory, 3) an intermediate region where the normalized relaxation rate becomes constant, as suggested by a vortex glass or collective pinning model, and finally 4) a high‐temperature region near the irreversibility line, where experiments are still inconclusive.

ISSN:0094-243X    

DOI:10.1063/1.40225

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Nonlinear magnetic behavior in high‐Tcsuperconductors as reflected by the appearance of harmonics in ac magnetic response, yields valuable information about transition temperature, critical fields and the dynamics of fluxons. In this paper we discuss the specific applications of the third harmonic measurements in developing (i) a high resolution spectrometer for determining Tc’s and (ii) a microscope which reveals local variation of Tc and the other superconducting properties.

ISSN:0094-243X    

DOI:10.1063/1.40226

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

A systematic study of the YBa2Cu3Oy(YBCO)/Ag2O composite shows that the presence of Ag at high temperature is to stabilize the superconducting YBCO phase. Ag is believed to serve as catalyst which causes partial melting of YBa2Cu3Oyphase at 985 °C and aided grain growth. Some of Ag particles were trapped in the grain or at grain boundaries during rapidly grain growth process and resulted as pinning centers. For the strong pinning samples, two distinguished resistive transitions attributed to the intra‐grain and the inter‐grain region are observed. Thermally activated flux creep is insignificant in these samples.

ISSN:0094-243X    

DOI:10.1063/1.40260

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We present direct observation of twin boundary pinning and Lorentz force induced flux motion in twinned, untwinned, and oxygen deficient crystals of YBa2Cu3O7−&dgr;. We find that twin boundaries act as strong pinning centers only when the vortex lines are pinned within the twin boundary planes. In addition, we find anomalous broadening in the superconducting resistive transition in magnetic fields in an oxygen dificient crystal where Tc<30 K.

ISSN:0094-243X    

DOI:10.1063/1.40261

出版商:AIP    

年代:1991

数据来源: AIP