摘要:

Low Jc values are typical for polycrystalline high temperature superconductors, especially in magnetic fields. On the other hand, recent results on some polycrystalline samples are showing encouraging Jc values. Grain boundary defects which limit current will be considered. The intrinsic properties of grain boundaries in the high Tc materials will be emphasized. These properties such as the atomic arrangements at grain boundaries are inherent to a given material. It will be shown that a plausible explanation for the low Jc values in c‐axis aligned YBa2Cu3O7compared to Bi2Sr2Ca1−xCuxOyis the ability to transfer supercurrents across basal plane boundaries in the latter material.

ISSN:0094-243X    

DOI:10.1063/1.40262

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Changes in various physical properties due to both addition of Ag and substitution of Ag for Cu in the N‐type superconductor system Nd‐Ce‐Cu‐O were investigated by measurements of electrical resistivity, magnetization, X‐ray powder diffraction, X‐ray photoelectron spectroscopy (XPS) and UV photoelectron spectroscopy (UPS). There is only a small decrease in the superconducting transition temperature with Ag dopants up to 40% (compared with Cu). The normal‐state resistivity decreases with the addition of Ag; however, it increases with the substitution of Ag for Cu. The M‐H hysteresis loops at 5 K were enhanced for the 20% Ag‐doped samples. The superconductivity in the Ag‐doped Nd‐Ce‐Cu‐O samples depends greatly on the reduction processing conditions.

ISSN:0094-243X    

DOI:10.1063/1.40263

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We present measurments of thea‐banisotropy of the normal state resistivity &rgr;a/&rgr;band measurements of the normal state magnetization along the three crystal axes. &rgr;a/&rgr;bincreases with increasing temperature and reaches a (sample dependent) value of 1.2 to 1.85 near room temperature. The results show that the CuO chains are metallic and that they can have a conductivity comparable to that one of the CuO2planes. A model of a parallel circuit of planes and chains is used to analyse the data. Superconducting fluctuations are found to be much more pronounced in the planes than in the chains. The normal state susceptibility shows the onset of strong diamagnetic fluctuations at temperatures below 200 K. At higher temperatures the anisotropy &khgr;c≳&sgr;a≳&sgr;bis observed and compared to predictions based on a model of local Cu‐moments in the planes and chains.

ISSN:0094-243X    

DOI:10.1063/1.40264

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

In order to interpret the ‘irreversibility line’ found in most found in most high‐Tcsuperconductors, it is necessary to understand how pinning modifies the flux lattice melting transition. Here, the effect of pinning is incorporated in a quasi‐two‐dimensional formulation, which should be appropriate for these highly anisotropic materials. Comparing the results to experiment, it is found that the flux lattice appears to be very different in YBa2Cu3O7−&dgr;than in the Bi‐ and Tl‐layered materials.

ISSN:0094-243X    

DOI:10.1063/1.40265

出版商:AIP    

年代:1991

数据来源: AIP

ISSN:0094-243X    

DOI:10.1063/1.40254

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Recent high resolution angle resolved photoemission spectra for the high temperature superconductors are analyzed on the basis of a fermi‐liquid theory of highly correlated metals. Due to the presence of large many‐body effects, the region over which typical Landau fermi‐liquid properties holds is small. The spectrum is dominated by the self‐energy variation over an intermediate range of energies, and in this region the spectrum bears a close resemblance to that found in the phenomenological marginal fermi‐liquid theory of Varmaet al.

ISSN:0094-243X    

DOI:10.1063/1.40255

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

Some high temperature superconductors require atmospheres of oxygen for the formation of the superconducting phase, and others do not. The former corrode more easily and includes the doped compound (La1.5Ba0.5)CuO4. In the latter class, Cu ions inhabit sites with large ionic potential, which apparently allows the formation of high charge states such as Cu (d8) or CuO+. An elementary code was used to examine potentials on a number of defect sites. Thus a high charge state on polyvalent Cu is a feature common to both types of superconductor. The use of Madelung potential calculations is reviewed for the construction of band energy diagrams, and for determining cohesion energies and stability of defects.

ISSN:0094-243X    

DOI:10.1063/1.40256

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

The magnetic properties of crystalline superconductors in an externally applied field can be modeled by a system of interacting weak link supercurrent loops. The quantum state of the interacting system is described by a set of integersnj. In each quantum statenthe magnetic flux through each independent loop is quantized. The magnetic field dependence of the energy in each quantum state, E(n, Bext), is computed in terms of classical parameters (mutual inductance matrix), quantum numbersnand the external magnetic fieldBext. Transitions between states are driven by absorption and emission of microwave quanta. Back reaction of the network on the field eliminates ‘frustration.’ The model is applied to bulk and single crystal high‐Tcsuperconductors and the results are in qualitative agreement with microwave absorption experiments. The theory predicts the possibility of energy bands in superconductors.

ISSN:0094-243X    

DOI:10.1063/1.40257

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

With a view to monitor the formal valence of copper in YBaCuO system, substitutions of divalent (Ca) and tetravalent (Pr) impurities at Y‐site have been carried out. In addition to this the cooperative effect of these substitutions at Y‐site on the structural and superconducting behavior of YBaCuO system has been investigated using X‐ray and susceptibility measurements. The changes in the superconducting transition temperature have been discussed in the context of oxygen content and changes in the formal copper valence in the YBaCuO system.

ISSN:0094-243X    

DOI:10.1063/1.40258

出版商:AIP    

年代:1991

数据来源: AIP

摘要:

We show that the structure inab‐plane infrared spectra of the oxide superconductors can be understood in terms of the electron‐phonon interaction. The minima in the optical conductivity seen in the oxygen breathing mode region translate to peaks in the frequency dependent scattering rate. The position and the width of the peaks are in reasonable agreement with structure seen in time‐of‐flight neutron spectroscopy. We interpret the appearance of phonon modes in the highly metallicab‐plane in terms of a charged phonon model borrowed from the theory of organic conductors.

ISSN:0094-243X    

DOI:10.1063/1.40259

出版商:AIP    

年代:1991

数据来源: AIP