E. Seidl

Magnetization of low-κ superconductors I the phase transition at Hc1

Abstract Experimental and theoretical investigations of the magnetization in low-κ superconductors are reported. From experiments on a series of Ta, Nb, V and LaAl 2 superconductors doped with varying amounts of impurity atoms, the phase boundaries between type-I superconductivity, type-II superconductivity with a first order phase transition at the lower critical field H c1 and “normal” type-II superconductivity could be established experimentally and compared with theory. The analysis in terms of weak-coupling isotropic, strong-coupling isotropic and strong-coupling anisotropic theories demonstrates clearly that the location of the phase boundaries is determined primarily by anisotropy effects with only contributions of coupling corrections. The magnitude of the discontinuity at H c1 , on the other hand, is affected by both coupling and anisotropy effects in nearly the same way. In all cases, the validity of the Ginzburg-Landau theory for temperatures near T c is confirmed with high accuracy.

Anisotropy effects in the upper critical field of niobium: Theory and experiment

Abstract A series of polycristalline niobium samples of varying impurity content was prepared and characterized thoroughly in order to study anisotropy effects in terms of mean square anisotropy parameters. From this analysis of experimental results on the temperature and impurity dependence of H c2 in terms of strong coupling theory we obtain two mean square anisotropy parameters, namely 〈a 2 〉 for the electron-phonon interaction anisotropy and 〈b 2 〉 for the anisotropy of the Fermi velocity. The Fermi velocity is found to be 0.57·10 6 m/s. With these parameters the theory describes the experimental data within experimental accuracy.

Structure and superconductivity of YBa2Cu3O7-x-BaTiO3 composites

Abstract Bulk and film samples have been prepared of composite YBa2Cu3O7-x-BaTiO3. The X-ray diffraction, SHG, resistivity and magnetic susceptibility investigations have been performed. The superconductive phase transition is broadened towards lower temperatures in the presence of BaTiO3, the resistivity of the normal phase being increased. Changes in behaviour of the composite films can be partly explained by weak contacts between particles of YBa2Cu3CV7-x during the preparation.

Magnetization of low-κ superconductors II. The lower critical field Hc1

Abstract In this second part of a series of three publications on the magnetization of low-κ superconductors, we present experimental and theoretical results on the lower critical field H c1 . We concentrate on highly pure niobium and vanadium samples. Both materials are type-II superconductors with a first order transition at H c1 . After a brief description of experimental techniques, the theoretical methods used in the present study are outlined. We show that a successful theoretical analysis of experimental data requires the inclusion of both strong-coupling and anisotropy effects. Because of the first order transition at H c1 , the thermodynamics of the interacting vortex lattice has to be solved in order to obtain H c1 . In this way, we achieve excellent agreement between experiment and theory, consistent with our results on other characteristic features of the magnetization in these superconductors. Near T c , the validity of Ginzburg-Landau theory is confirmed with high accuracy.

Superconducting alloys with paramagnetic impurities (part II) magnetization and upper critical fields of LaAl2 with dilute Gd impurities

Abstract Magnetization and AC susceptibility were made on single- and polycrystalline samples of LaAl 2 doped with Gd in the concentration range 0⩽ n I ⩽0.7 at%. The depression of T c with Gd content is found to be in excellent agreement with previous experimental work. Concerning the temperature dependence of H c2 , peaks at low temperatures (⋍400mK) and downward curvatures towards T =0 have been observed. The upper critical field of the clean material can be explained quantitatively in terms of anisotropic Eliashberg theory using 〈 a 2 〉 =0.01 for the electron-phonon coupling anisotropy parameter and 〈 b 2 〉 =0.16 for the anisotropy of the Fermi velocity. With these values and the generalized phonon density of states reported by Yeh et al. excellent agreement between theory and experiment was achieved. Furthermore, results on the Gd doped materials can be explained quantitatively on the basis of strong coupling theory, if spin orientation effects of the Gd atoms are included in the theoretical treatment.

Magnetization of low-κ superconductors III the generalized Ginzburg-Landau parameters κ1 and κ2

Abstract In this third part of a series of three publications on the magnetization of low-κ superconductors, we present experimental and theoretical results on the generalized Ginzburg-Landau parameters κ 1 and κ 2 in Nb and V. In addition to the mixed state theories used throughout this work (weak- and strong-coupling isotropic theory and strong-coupling theory including a model anisotropy for the electronic density of states), the anisotropic Eliashberg theory of upper critical fields was employed to evaluate κ 1 . In this case, excellent agreement between theory and experiment is achieved. The application of the mixed state theories also emphasizes the role of anisotropy, whereas strong-coupling corrections are found to be rather moderate, especially for κ 2 . Both theory and experiment show a distinctly different temperature dependence of κ 2 in clean and dirty superconductors.

Investigation of the phase transition at Hc1 in low-K superconductors

Abstract Several low- K superconductors (Nb, Ta, V, LaAl 2 ) were doped with small amounts of impurities in order to vary the Ginzburg-Landau parameter near its critical value K = 1 2 . The corresponding magnetization curves show type-I, type-II/1 and type-II/2 behavior depending on the magnitude of K at T c . The experimental results demonstrate that the phase boundaries separating the different types of phase transitions in the t- K plane are material independent, in remarkable contrast to theoretical results based on weak-coupling calculations. The anisotropic strong-coupling theory on the other hand agrees well with the bulk of the data.

Symmetry considerations forHc2 anisotropy experiments in cubic superconductors

We report on an evaluation ofHc2 anisotropy effects in arbitrary planes and directions of cubic superconductors. Using Mercator projections, information on the three-dimensional variation ofHc2 is obtained. The results are discussed for the three main crystallographic planes of the cubic lattice and evaluated in terms of their usefulness for an accurate experimental determination of the anisotropy coefficientsA1. Although some planes show interesting features because of special symmetry conditions, we conclude that careful measurements in the (110) plane represent the optimum choice for an accurate determination of the anisotropy coefficients.