A. Deville

Gadolinium substitution for yttrium and chemical preparation effects in YBa2Cu3O7: an epr study

Summary Compounds with yttrium totally or partially substituted by gadolinium in High-T c superconductor YBa 2 Cu 3 O 7 were prepared. The superconductivity is not affected by this substitution. An orthorhombic structure has been observed in GdBa 2 Cu 3 O 7 sintered and single crystals. The EPR measurements show linewidth dependence on chemical preparation in GdBa 2 Cu 3 O 7 . In Gd-low concentration compounds (x ≤ 0.02), a narrow parasite signal, due to the Y 2 BaCuO 5 green phase, appears superimposed to the principal Gd 3+ ions EPR line.

10 GHz Rs measurements of YBa2Cu3O7. Influence of a magnetic field

Abstract We examine the 10 GHz surface resistance of YBa 2 Cu 3 O 7 single crystals, without and with a continuous magnetic field up to 2T, in both cases without modulation of the magnetic field. We compare the results in a continuous field with those we obtain using a modulation. The results are briefly discussed.

Some experimental and theoretical aspects of the surface impedance in bulk high-temperature superconductors at 10 GHz

Abstract After a presentation of the theoretical framework, and a short review of existing r.f. results in high-temperature superconductors, we present our own 10 GHz measurements on YBa2Cu3O7 single crystals and sintered samples. We use an electron spin resonance (ESR) heterodyne spectrometer and measurements of the reflection coefficient and resonant frequency of a cavity to get information not only on the surface resistance, R S , as generally done, but also on the surface reactance, X S . The theoretical analysis of the frequency shift is made by adapting Slaters perturbation method to the present problem. In order to explain both the R S and X S results, and previous ESR observations, while keeping theoretical simplicity, we are led to suggest that in the normal state these materials show an unconventional skin effect, where the phenomenon is governed not by the d.c. conductivity, but by an effective (lower) conductivity, the other characteristics being unchanged. We briefly discuss the superconductor results and the validity of the two-fluid model.

Temperature variation of the electron resonance line for the insulating spin-glass Eu0.4 Sr0.6 S

Abstract The electron resonance of Eu2+ in a single crystal of the insulating spin glass system Eu0.4 Sr0.6 S between 1.4 and 300 K is presented. It is possible to determine directly Tg from linewidths measurements. We first discuss the high temperature results, then the behaviour around the spin freezing temperature Tg. Our experimental results disagree with Salamons predictions. At 1.4 K the frequency dependence of the resonance field around 9.3 GHz is well described by assuming the existence of an internal field Hi ≅ 1 kG.

ESR in the Y1−xGdxBa2Cu3O7 system below Tc

Abstract The ESR spectra from GdBa2Cu3O7 between 300 and 10 K are presented and shortly discussed. Below Tc, the signal from sintered samples is lost (X band) or weak (Q band). The ESR line from single crystals, while Dysonian and isotropic at 300 K, is symmetrical and significantly anisotropic below 40 K, and presents then an additional narrowing when H // ab. In the dilute compounds (x ≲ .02) an experimental answer is given to a question raised LT19, to be solved before low T studies.