摘要:

A microscopic theory based on the idea of hybridization-induced wide-band bipolarons is proposed to explain the occurrence of high-temperature superconductors such as La2−x-BaxCuO4, and La2−xSrxCuO4. In our view, these are such that bipolarons are formed whose singlet ground state possesses a large binding energy (Δ) with bandwidth W (>Δ) as a result of two distinct electron-phonon mechanisms and two types of hybridization between Cu2+d electrons and other electrons. This wide-band bipolaron gas has features similar to Cooper pairs and leads to a superconducting state with high Tcin which all occupied electron states participate. The large electron-phonon coupling and a largeWboth cause the transition temperature Tc, to rise spectacularly.

ISSN:0950-0839    

DOI:10.1080/09500838708214693

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Several biphasic compositions of the type Y3−xBa3+xCu6O14show an onset of superconductivity in the 90-115K range, attaining zero resistance in the 70-85K range. The phase responsible for superconductivity in these compositions is a perovskite oxide of composition YBa2Cu3O7. This oxide annealed in oxygen shows the onset of superconductivity at 120K and zero resistance at 87K. YBa2,Cu3O7shows the highest Meissner effect of all oxide superconductors. The superconducting behaviour of the two perovskite oxides, Y0.95Ba1.95,Cu3O7and Y1.05Ba1.95Cu3O7show interesting features; a marked decrease in resistivity is observed from room temperature itself in the former oxide with zero resistance at 89K. Electron microscopy and infrared spectra of these oxides are briefly discussed.

ISSN:0950-0839    

DOI:10.1080/09500838708214694

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

A quasicrystalline Al-Fe phase has been formed by room-temperature ion mixing of AI-Fe multiple layers with no additional postannealing. The quasicrystalline phase was dispersed as grain in an amorphous matrix. The size of the grains was about 10-30nm, and the composition was determined to be Al80Fe20. The structure of the quasicrystalline phase was identified by calculating some 20 sharp diffraction rings in the selected-area diffraction pattern and found to be similar to that of the rapidly quenched AI-Mn quasicrystalline phase.

ISSN:0950-0839    

DOI:10.1080/09500838708214695

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The paper is concerned with the interpretation of steady-state photoconductivity results on undoped a-Si at temperatures of 50K and below which lead to an essentially constant value of the (photogeneration efficiency x mobility x lifetime) product νμτ≃10−11cm2V−1. Measurements on p+-i-n+junctions and Cr-i-n+barriers were carried out to determine the above parameters separately: (i) steadystate reverse saturation currents gave a generation efficiency of ν≃ 5×10-2below 50K, suggesting that geminate recombination limits the generation process. (ii) the electron drift mobility μethrough the tail states and the charge extracted from the absorption region of the incident light were investigated by transient experiments, these showed that μeτais limited to about 3×10−10cm2V−1at low T. The independent results account for the observed νμτ values and suggest that, contrary to the interpretation of Hoheisel, Carius and Fuhs (1984), the main contribution to the low-temperature photoconductivity arises from transport in tail states.

ISSN:0950-0839    

DOI:10.1080/09500838708214696

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Time-of-flight measurements on molecularly doped polymers have revealed that, the transient current shapes change from being virtually non-dispersive to highly dispersive (featureless) and back to non-dispersive as the shallow trap concentration is increased from 0 to ∼ 1 mol%. The shallow traps were introduced into the molecularly doped system by mixing the charge-transporting molecule with small amounts of another transporting molecule having a significantly lower ionization potential. The results support the model of Schmidlin for single-trap-controlled transport. Based on this model, the attempt-to-escape frequency and the activation energy for escape from the trap are found to be ∼4 × 1012s−1and 0·56eV, respectively.

ISSN:0950-0839    

DOI:10.1080/09500838708214697

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

In certain ferroelectric liquid crystals that contain the biphenylyl moiety, the spontaneous polarization undergoes a change in direction relative to the optical axis of the system as a function of temperature. Consequently, at a characteristic temperature for a particular derivative, the polarization vanishes. A model based on the presence of at least two species, interconvertible via an activation barrier is proposed to explain this extraordinary behaviour of the polarization. It is suggested that these species are most likely identified with the rotational conformers of these materials while they are in their liquid-crystalline states.

ISSN:0950-0839    

DOI:10.1080/09500838708214698

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Experimental results for two-dimensional grain growth are compared with the results obtained from computer-generated random threefold vertex networks with a fixed one-particle distribution function. A considerable discrepancy in the two-particle distributions implies some kind of short-range order. New results in the computer simulation of normal grain growth are reported, demonstrating a similar kind of ordering. A simple relation is suggested for the quantitymn, introduced by Aboav and the second moment of the one-particle distribution μ2

ISSN:0950-0839    

DOI:10.1080/09500838708214699

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The infrared spectra of several type Ia diamond crystals expected to contain voidites have been examined in the wavenumber range 4000–2700 cm−1. The absence of any absorption which could possibly be attributable to the stretching of N─H bonds makes unlikely recent suggestions that the voidites may contain a solid phase of NH3.

ISSN:0950-0839    

DOI:10.1080/09500838708214700

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

An empirical scaling law for the real part of the frequency-dependent electrical conductivitσ′(ω) of brine-saturated porous materials in relation to its d.c. limit has been identified. This scaling law relates the conductivity as a function of frequency, porosity and salinity to a function of only one variable that is the ratio of the frequency to the d.c. conductivity in appropriate units. This result is very similar to what has been reported in hopping semiconducting systems.

ISSN:0950-0839    

DOI:10.1080/09500838708214701

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The process of anodic oxide growth has been investigated. It is shown that the obtained experimental results confirm the Fehlner-Mott oxidation model in the case of InSb, a typical compound of the 111-V group. We also present a kinetic theory of the anion migration in the formed oxide. The theory is based on the following assumptions.(a)The only species which moves through the formed oxide is the OH−anion.(b)The chemical reactions of OH−with In3+and Sb3+cations take place simultaneously and independently, in a thin layer of the oxide/InSb interface. The resulting solution of the kinetic equations describes the anion distribution in the formed oxide. A simple expression for the rate of oxidation is obtained. Another possible application of the presented theory is suggested.

ISSN:0950-0839    

DOI:10.1080/09500838708214702

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor