International Journal of Modern Physics B

The symmetry properties and the general overview of the superconductivity theory in the itinerant ferromagnets and in materials without space parity are presented. The basic notions of unconventional superconductivity are introduced in broad context of multiband superconductivity which is an inherent property of ferromagnetic metals or metals without centre of inversion.

The free polaron energy levels in finite GaAs/Al_xGa_1-xAs parabolic quantum wells have been investigated by a modified variational method. The effect of the electric field, the electron-phonon interaction including the longitudinal optical phonons and the four branches of interface optical phonons, and the effect of spatial dependent effective mass have been considered in the calculation. The dependence of the energies of free polarons on the alloy composition x is given. The numerical results for finite GaAs/Al_xGa_1-xAs parabolic quantum wells are obtained and discussed. The results show that the effect of the electric field and the interface optical phonons as well as the longitudinal optical phonons on the energy levels is obvious. One can find that the effect of the spatially dependent effective masses on the energy levels in finite parabolic quantum wells is considerable except for large well width. Thus, the electron-phonon interaction and the effect of the spatially dependent effective mass should not be neglected for the study of the electron state problem in finite parabolic quantum wells.

The Pr-substituted YBa₂Cu₃O_7-δ(Y123) superconducting systems with the content of 0.0–1.0 have been systematically studied by positron lifetime and infrared absorption experiments. The results show that the short lifetime τ₁ decreases as a function of Pr-substitution x below x=0.6, but when above x=0.6, it increases inversely. The long lifetime τ₂ decreases as a function of Pr-substituted x. Based on the present results, we discuss that Pr⁴⁺ substitutes on Ba²⁺ ion and forms Pr_Ba defects. The variation of τ₁ is caused by the oxygen vacancies and τ₂ is probably affected by the absence of Cu due to Pr_Ba defects. The infrared absorption of Pr-substituted Y123 systems gives three variational modes, located at 560 cm^-1(A₁), 1435 cm^-1(A₂) and 1631 cm^-1(A₃) respectively. Here the A₃ mode shows that with Pr content increasing from x=0.1 to 1.0, the infrared absorption decreases and the peak tends to broaden. So, combining with the results of the positron lifetime spectra, we can argue that there always exists portion of Pr_Ba defects in the Pr-substituted systems, and the Pr⁴⁺ substituted into Ba²⁺ is also increasing with the Pr content increasing. When the Pr content is 0.6, the systems have a metal-insulator transition and Pr_Ba defects begin to domain the samples' microstructure.

Based on the results of existing studies of the minority game, we further explored the periodic properties of the time series of population and discovered its exhibition of a separate three-bump distribution, in which an unusual periodic pattern is existent. By defining "the variance of scores" of all the strategies, we found a method to identify the distinctive features between periodic and non-periodic distributions. We also found there exists a critical point at which periodic behavior disappears when N goes from large to small.

The problem of extrapolating the series in powers of small variables to the region of large variables is addressed. Such a problem is typical of quantum theory and statistical physics. A method of extrapolation is developed based on self-similar factor and root approximants, suggested earlier by the authors. It is shown that these approximants and their combinations can effectively extrapolate power series to the region of large variables, even up to infinity. Several examples from quantum and statistical mechanics are analyzed, illustrating the approach.

We establish that the response to static perturbations of two-dimensional electron systems (2DESs), exposed to magnetic field B, in states with the Landau-level filling fractions ν_e close to one such as 1/2, is singularly different from that of 2DESs at small ‖B‖. Thus in addition to demonstrating the severe inadequacy of the treatment of composite fermions as non-interacting particles, we show, in contradiction to a recent theoretical finding, that the physical origin of the observed behavior of the dc longitudinal magneto-resistivity for ν_e≈1/2 in periodically modulated 2DESs remains to be determined.

Polycrystalline samples of Ru_1-xSb_xSr₂GdCuO₈ doped with Sb, in which magnetic order and superconductivity coexist with T_magnetic≫T_c, were prepared by a solid state reaction technique. Lattice parameters, electrical resistivity, Hall and thermopower coefficients measurements on the same sintered ceramic samples are presented.

Both Hall effect and thermopower show anomalous decreases below T_magnetic which might be explained with a simple two-band model in the RuO₂ layer at T_magnetic. It was also observed that the Sb doping reduce the conductivity of the system and the transition temperature decreases with increasing Sb content. This may be due to a distortion of RuO₆ octahedra which is responsible for the increase of hole localization.

In this article, a low cost chemical bath deposition (CBD) technique has been used for the preparation of Cu_2-xSe thin films on to glass substrate. Different thin films (0.2–0.6 μm) were prepared by adjusting the bath parameter like concentration of ammonia, deposition time, temperature of the solution, and the ratios of the mixing composition between copper and selenium in the reaction bath. From these studies, it reveals that at low concentration of ammonia or TEA, the terminal thicknesses of the films are less, which gradually increases with the increase of concentrations and then drop down at still higher concentrations. It has been found that complexing the Cu²⁺ ions with TEA first, and then addition of ammonia yields better results than the reverse process. The film thickness increases with the decrease of value x of Cu_2-xSe.