Glitz

The crystal structure of the orthorhombic and tetragonal phases of La(Ba 2-xLax)Cu3-yO 6+x/2-y+ z are determined on twinned crystals. The orthorhombic structure, obtained for low x, is close to the regular Y-Ba-Cu-O type (twin a * b * c-b * a * c), but is highly copper deficient on the Cu(1) site (~ 30 %). The local correlations (ξ ~ 20 Å) between copper atoms and vacancies, as deduced from X-ray diffuse scattering, correspond to a short-range segregation of vacancies in chains. As a consequence of the large amount of defects, these crystals are non-typical semiconductors. The tetragonal structure, x ≃ 0.50, leads to tri-twinned crystals with 90° faulting, a * a * 3 a-a * 3 a * a -3 a * a * a (a, the perovskite lattice constant). In these materials the copper sites are found to be strongly anharmonic. This is due to the disorder introduced by the La-Ba substitution. These crystals are also semiconductors with a T-1/4 activation law for the conductivity which indicates that variable range hopping is expected to set in, a consequence of localization by the disorder

The magnetic interactions in spin-glasslike Ge/1-x-y/Sn/x/Mn/y/Te diluted magnetic semiconductor

We investigated the nature of the magnetic phase transition in the Ge/1-x-y/Sn/x/Mn/y/Te mixed crystals with chemical composition changing in the range of 0.083 < x < 0.142 and 0.012 < y < 0.119. The DC magnetization measurements performed in the magnetic field up to 90 kOe and temperature range 2-200 K showed that the magnetic ordering at temperatures below T = 50 K exhibits features characteristic for both spin-glass and ferromagnetic phases. The modified Sherrington - Southern model was applied to explain the observed transition temperatures. The calculations showed that the spin-glass state is preferred in the range of the experimental carrier concentrations and Mn content. The value of the Mn hole exchange integral was estimated to be J/pd/ = 0.45+/-0.05 eV. The experimental magnetization vs temperature curves were reproduced satisfactory using the non-interacting spin-wave theory with the exchange constant J/pd/ values consistent with those calculated using modified Sherrington - Southern model. The magnetization vs magnetic field curves showed nonsaturating behavior at magnetic fields B < 90 kOe indicating the presence of strong magnetic frustration in the system. The experimental results were reproduced theoretically with good accuracy using the molecular field approximation-based model of a disordered ferromagnet with long-range RKKY interaction.Comment: 9 pages, 6 figure

Flux flow noise in high Tc superconductors thin films

We have studied the fraction pinned of vortex in a type II superconductors. The sample used is the YBa2Cu3O7-d thin films. This fraction pinned is determined from the comparison of the experimental results and the theoretical results previously definite. In continuation one will study the variations of this fraction according to the temperature of the applied magnetic field and the electric current crossing the sample.We have studied the fraction pinned of vortex in a type II superconductors. The sample used is the YBa2Cu3O7-d thin films. This fraction pinned is determined from the comparison of the experimental results and the theoretical results previously definite. In continuation one will study the variations of this fraction according to the temperature of the applied magnetic field and the electric current crossing the sample

Pinning of vortices in superconductors at high critical temperature

Due to problem identification and enumeration of defects in HTC superconductors, the correlation between current density and the transition of vortex pinning volume is not completely mastered. However, many theoretical and experimental studies have been developed to better understand the problem of dissipation due to vortices in type II superconductors

Interplay between the glassy transition and granular superconductivity in organic materials

It is known that some (BEDT-TTF)2X layered organic superconductors undergo a glassy transition near 80 K. Our purpose is to exploit quenched disorder to get new insights on both the superconducting state (T < 12 K) and the glassy transition by studying the superconducting properties as functions of annealing time (ta) and temperature (Ta) around 80 K. The main results on the fully deuterated kappa-(BEDT-TTF)2Cu[N(CN)2]Br compound are: 1) The data can be described by a percolation cluster model. 2) At short time scales, the clusters grow with ta following a power law. 3) At large time scales the clusters grow toward a thermodynamic state following a stretched exponential law in (1 - exp(-(t/tau)beta)with beta varying from about 0.5 to 1 in our Ta range (65 - 110 K). 4) The relaxation time follows an Arrhenius law tau(T)=tau0exp(U/T) with U around 2660 K and 1/tau0 around 2x1013 s-1. 5) The asymptotic magnetization fits with a scaling law with Tg around 55K and n around 3.2. The results are consistent with a Ising spin-glass-like model.Comment: 17 pages; 4 figure

Correctors for some asymptotic problems

In the theory of anisotropic singular perturbation boundary value problems, the solution u ɛ does not converge, in the H 1-norm on the whole domain, towards some u 0. In this paper we construct correctors to have good approximations of u ɛ in the H 1-norm on the whole domain. Since the anisotropic singular perturbation problems can be connected to the study of the asymptotic behaviour of problems defined in cylindrical domains becoming unbounded in some directions, we transpose our results for such problems

Absence of large nanoscale electronic inhomogeneities in the Ba(Fe1-xCox)2As2 pnictide

75As NMR and susceptiblity were measured in a Ba(Fe1-xCox)2As2 single crystal for x=6% for various field H values and orientations. The sharpness of the superconducting and magnetic transitions demonstrates a homogeneity of the Co doping x better than +-0.25%. On the nanometer scale, the paramagnetic part of the NMR spectra is found very anisotropic and very narrow for H//ab which allows to rule out the interpretation of Ref.[6] in terms of strong Co induced electronic inhomogeneities. We propose that a distribution of hyperfine couplings and chemical shifts due to the Co effect on its nearest As explains the observed linewidths and relaxations. All these measurements show that Co substitution induces a very homogeneous electronic doping in BaFe2As2, from nano to micrometer lengthscales, on the contrary to the K doping.Comment: 6 pages, 4 figure

The critical current density and the vortex pinning in high quality YBa2Cu3O7-d thin films

We have measured the critical current density Jc of high quality c-axis oriented YBa2Cu3O7-d thin films. Measurements were performed for various magnetic field and temperature values, and as a function of the angle q between the c-axis and the applied magnetic field direction. A maximum of the critical current density was obtained when the flux lines are aligned along the CuO planes (q = 90°); another maximum in Jc was also observed when the magnetic field is adjusted parallel to the c-axis. We attribute these effects to different, intrinsic and extrinsic, pinning mechanisms of vortices in the sample.We have measured the critical current density Jc of high quality c-axis oriented YBa2Cu3O7-d thin films. Measurements were performed for various magnetic field and temperature values, and as a function of the angle q between the c-axis and the applied magnetic field direction. A maximum of the critical current density was obtained when the flux lines are aligned along the CuO planes (q = 90°); another maximum in Jc was also observed when the magnetic field is adjusted parallel to the c-axis. We attribute these effects to different, intrinsic and extrinsic, pinning mechanisms of vortices in the sample

Magnetic relaxation phenomena and cluster glass properties of La{0.7-x}Y{x}Ca{0.3}MnO{3} manganites

The dynamic magnetic properties of the distorted perovskite system La{0.7-x}Y{x}Ca{0.3}MnO{3} (0 <= x <= 0.15) have been investigated by ac-susceptibility and dc magnetization measurements, with emphasis on relaxation and aging studies. They evidence for x >= 0.10 the appearance of a metallic cluster glass phase, that develops just below the ferromagnetic transition temperature. The clusters grow with decreasing temperature down to a temperature T(f0) at which they freeze due to severe intercluster frustration. The formation of these clusters is explained by the presence of yttrium induced local structural distortions that create localized spin disorder in a magnetic lattice where double-exchange ferromagnetism is dominant.Comment: Accepted for publication in Phys. Rev.

The vortex motion and the magnetization study in YBa2Cu3O7-d single crystal

We have investigated the vortex motion and the magnetization of high critical temperature superconductors YBa2Cu3O7-d Measurements were made using a vibrating sample magnetometer technique. Several magnetization hysteresis cycles have been obtained for different angles, θ, between the applied magnetic field, H, and the crystallographic c-axis. For T = 30 K we observed a central peak and for T = 80 K we observed a second peak or "fishtail".We have investigated the vortex motion and the magnetization of high critical temperature superconductors YBa2Cu3O7-d Measurements were made using a vibrating sample magnetometer technique. Several magnetization hysteresis cycles have been obtained for different angles, θ, between the applied magnetic field, H, and the crystallographic c-axis. For T = 30 K we observed a central peak and for T = 80 K we observed a second peak or "fishtail"