Laser processing for deposition of high Tc superconducting films

Model considerations are applied to investigate the process of pulsed laser deposition of high T_c superconducting films. At low power densities, heat conduction and evaporation above a thermal threshold energy dominate. Increase of the laser flux results in the generation of a dense plasma with a mass ablation flow away from the target. According to different wave lengths the relation of the energy density to the pulse duration is estimated. Very short light pulses above a threshold energy density favour the ablation of a stoichiometric mass flow from a multicomponent target and suppress evaporation according to different vapor pressures.     

On the empirical correlation between the superconducting Tc and the Hall coefficient

It is shown that the occurence of superconductivity and the sign of the Hall coefficient are correlated in transition and non-transition elements and alloys.     

Tc enhancement!

The equilibrium critical temperature of a superconducting aluminum strip has been determined with three different methods. The results agree to within 1 mK. By applying microwave radiation superconductivity was maintained up to 57 mK above this critical temperature. These data clearly establish the existense of T_c enhancement.     

Mechanism of high Tc superconductivity and and anti-ferromagnetic order of high Tc oxides

We propose, here, new theory of high T_c superconductivity mechanism. The hole-Cooper pair formation and the Bose condensation of the Cooper pairs occur in the high T_c superconducting oxides such as Bi–Sr–Ca–Cu–O. The atractive interaction is due to the antiferromagnetic order and its fluctuation of CuO₂ plane. Dependence of T_c on the numbers of CuO₂ layers is quantitatively analized.     

Itinerant ferromagnets above Tc

We point out that itinerant ferromagnets differ from Heisenberg ones because the magnetic energy is mainly obtained in the paramagnetic phase. This makes any mean-field theory break down. In order to correct it, one must use the concept of reaction field first introduced by Onsager. We present a theory which takes this into account and permits us to describe in a unified way both the very weak and the pure 3d metallic ferromagnets. In particular we obtain a functional of the magnetization which is valid for the two limits. A consequence of this fact is that the 3d metals behave as very weak ferromagnets close to T_c.     

Analysis of anisotropy of high Tc superconducting oxides using multi-CuO2-layer structure model

The anisotropy of high Tc superconducting oxides such as Bi₂Sr₂Ca₁Cu₂O₈ is analized by use of the multi-CuO₂-layer structure model. It is found that the anisotropy decreases when Tc increases. Under high pressure, the anisotropy also decreases.     

一个新的Tc公式

从Eliashberg方程导出了一个适用于λ小的超导体的T_c公式。 关键词：     

Tc in two-zone superconductors with non-magnetic impurities

The two-zone model is used to investigate the influence of nonmagnetic scattering on the superconducting transition temperature. The transition temperature decreases with increasing impurity concentration, and either saturates or goes to zero exponentially at a critical concentration. The type of behaviour is determined by the interaction parameter η∞, which is calculated from the parameters of the model.     

Laser-modulated reflectance of SbSI near Tc

Reflectance modulation spectra of the semiconducting ferroelectric compound SbSI, at wavelengths near the fundamental edge and at temperatures around the phase-transition are reported. Modulation was obtained by the periodical injection of free carriers produced by a laser beam chopped at 10.18 Hz. In order to explain the mechanism of photoreflectance in SbSI, a comparison of the experimental data has been made both with electroreflectance spectra and eith the theoretical curve expected under the assumption of a periodical change of free carriers density.     

缺陷调控临界温度梯度超导膜的磁通整流反转效应

超导涡旋运动引起的棘齿效应可以广泛应用于磁通泵、整流器和超导开关等装置.金兹堡-朗道理论是研究超导磁通涡旋问题强有力的工具和手段.本文采用有限差分法数值求解时间相关的金兹堡-朗道方程,利用快速傅里叶变换方法求解耦合的热传导方程,数值模拟了临界温度梯度超导薄膜磁通涡旋动力学行为,提出了一种新的调节超导整流效应的方式,并研...     

高Tc超导体的表面微波吸收

测量不同粒度及低真空条件下高T_c超导样品Bi-Pb-Sb-Sr-Ca-Cu-O的非共振微波吸收信号,实验表明,这个对温度和磁场十分敏感的信号还与样品颗粒的大小及氧气氛有关;并经计算证明,信号强度与近表面趋肤深度壳层Meissner态的体积成正比。 关键词：     

关于高Tc问题的一个建议

我们建议:把超导体分成A型和B型两种。前者,λ<Λ,后者,λ>Λ。Λ的物理意义在正文中给出。本文分析表明了,有效声子谱对这两种超导体T_c的影响是不同的,因而提高它们T_c方法也是不一样的。提高A型超导体T_c的最有效方法是增大λ。提高B型超导体T_c的最有效方法是增大又λ〈ω²〉。 关键词：     

Collective modes in charged superconductors near Tc

The dynamics of charged superconductors is described within a phenomenological model. It exhibits near T_c a screened propagating mode, which represents high frequency second sound. The damping is due to friction, to the conversion of normal to superfluid, and diffusion.     

Gap anisotropy in superconducting alloys with localized states within the energy gap

The effect of paramagnetic impurities on the gap function of an anisotropic superconducting alloy is studied within the framework of BCS and Shiba Rusinov theory. It is found that the anisotropy of the gap function increases with increasing impurity concentration but the increase is additionally affected by the parameter ?₀ which describes the relative position of the localized state within the gap.     

Effect of scattering processes on the dynamics of electrons in narrow energy bands

We have investigated the effect of a magnetic field and the degree of filling and width of the band on the scattering processes on the basis of the Hubbard method. The electrical conductivity tensor was calculated with account taken of damping processes. It is shown that the relaxation times depend essentially on the electron spin orientations. Theordering of the electron spins under the influence of the external magnetic field leads to an increase of the electrical conductivity tensor, that in turn leads to a negative magnetoresistance.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 65–69, June, 1976.     

Correlation of Tc with structure in the density of states in the new high-Tc superconductors

The variation of T_c with hole concentration in the new copper oxide superconductors can readily be understood in terms of a peak in the density of states associated with the CuO₂ planes. The data are consistent with either simple phonon-mediated pairing or an indirect excitonic pairing. Effects due to interlayer coupling are considered. The sharp decrease of T_c near half-filling is probable due to strong spin fluctuations.     

Anomalous phonon thermal resistivity in superconducting Ti55Nb45 alloys

The thermal conductivity of cold-worked and heat-treated Ti₅₅Nb₄₅ alloys has been measured in the temperature range between 0.6 and 1.5 K. Anomalously strong scattering of phonons for every sample and the remarkable enhancement in the thermal resistivity caused by the annealing at 500°C on cold-worked alloys have been found. These behaviors indicate the existence of an anomalous phonon scattering mechanism besides dislocation scattering in Ti₅₅Nb₄₅ alloys. Electron diffraction patterns show the circular diffuse streaks characteristic of the precursory lattice distortion for the ω atomic configuration. Two kinds of models which correlate the anomalies in the phonon scattering with the lattice instability of the β phase of Ti-Nb matrix are described.     

紧密堆积结构化合物的超导临界温度和导带的异质杂化

用LMTO能带论方法分析了密堆结构复式格子的导带中角动量杂化情况变化的规律。指出异质配位(在最近邻位置上出现电子结构差异大的原子)是显著强化导带的角动量杂化程度的必要条件。结合前人对电子-声子耦合超导原理的研究结果,可以对密堆结构化合物得出如下关系:高T_c高η强的角动量杂化强异质配位。这个原理与A15结构,B1结构及其它多种密堆结构化合物的多方面的实验结果普遍符合。 关键词：     

Magnetic correlations in 3d ferromagnets above Tc

A review is presented of our current neutron scattering experiments on Fe and Ni above the Curie temperature. Our experimental results show that the picture of propagating spin waves above T_c, widely accepted since 1973, is incorrect. In addition, we will demonstrate that over wide ranges of ω, q and temperature, both Fe and Ni, as well as Pd₂MnSn, follow a simple paramagnetic scattering function of the spin diffusion type.     

Occupation of energy bands by electrons in transition metals and alloys

A method is proposed for calculating the degree of occupation of energy bands by electrons with variously directed spins in transition metals and their alloys in a ferromagnetic state. The method is used to calculate the degree of occupation of bands in metals of the first transition period and in several binary and ternary alloys based on these metals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 64–71, September, 1977.