Relaxation Effects in the Transition Temperature of Superconducting HgBa2CuO4+delta

In previous studies on a number of under- and overdoped high temperature superconductors, including YBa_{2}Cu_{3}O_{7-y} and Tl_{2}Ba_{2}CuO_{6+\delta}, the transition temperature T_c has been found to change with time in a manner which depends on the sample's detailed temperature and pressure history. This relaxation behavior in T_c is believed to originate from rearrangements within the oxygen sublattice. In the present high-pressure studies on HgBa_{2}CuO_{4+\delta} to 0.8 GPa we find clear evidence for weak relaxation effects in strongly under- and overdoped samples ($T_c\simeq 40 - 50 K$) with an activation energy $E_{A}(1 bar) \simeq 0.8 - 0.9 eV$. For overdoped HgBa_{2}CuO_{4+\delta} E_{A} increases under pressure more rapidly than previously observed for YBa_{2}Cu_{3}O_{6.41}, yielding an activation volume of +11 \pm 5 cm^{3}; the dependence of T_c on pressure is markedly nonlinear, an anomalous result for high-T_c superconductors in the present pressure range, giving evidence for a change in the electronic and/or structural properties near 0.4 GPa

Interplay of structural and electronic phase separation in single crystalline La(2)CuO(4.05) studied by neutron and Raman scattering

We report a neutron and Raman scattering study of a single-crystal of La(2)CuO(4.05) prepared by high temperature electrochemical oxidation. Elastic neutron scattering measurements show the presence of two phases, corresponding to the two edges of the first miscibility gap, all the way up to 300 K. An additional oxygen redistribution, driven by electronic energies, is identified at 250 K in Raman scattering (RS) experiments by the simultaneous onset of two-phonon and two-magnon scattering, which are fingerprints of the insulating phase. Elastic neutron scattering measurements show directly an antiferromagnetic ordering below a N\'eel temperature of T_N =210K. The opening of the superconducting gap manifests itself as a redistribution of electronic Raman scattering below the superconducting transition temperature, T_c = 24K. A pronounced temperature-dependent suppression of the intensity of the (100) magnetic Bragg peak has been detected below T_c. We ascribe this phenomenon to a change of relative volume fraction of superconducting and antiferromagnetic phases with decreasing temperature caused by a form of a superconducting proximity effect.Comment: 9 pages, including 9 eps figures, submitted to PR

Effect of pressure on the crystal field splitting in rare earth pnictides and chalcogenides

On fait le point sur la situation expérimentale concernant la variation avec la pression du champ cristallin des pnictures et des chalcogénures de praséodyme. Cette situation expérimentale est comparée aux prédictions du modèle des charges ponctuelles. On discute de la séparation des contributions de l'échange et du champ cristallin à partir des décalages de fréquence RMN ou des mesures de susceptibilité ; on considéré les problèmes associés a l'explication de ces effets par des modèles mettant en jeu les électrons de conduction.The experimental situation for the pressure dependence of the crystal field of praseodymium pnictides and chalcogenides is reviewed and compared with the predictions of the point charge model. The problem of separating exchange and crystal field contributions from the measured NMR frequency shift or susceptibility measurements is discussed as well as problems with explaining these effects with conduction electron related models

Fermi surface measurements in actinide metals and compounds

On discute d'abord les différentes techniques de mesure des paramètres de la surface de Fermi et leur application aux systèmes d'actinides. On étudie spécialement l'effet dHvA et on présente des résultats généraux sur l'effet dHvA dans les composés d'actinides. L'effet dHvA a été mesuré dans l'U-α et il est présenté ici en détail. Aucune des fréquences observées ne correspond à celles des surfaces fermées. Les résultats sont en accord qualitatif avec les calculs de Freeman, Koelling et Watson-Yang.The various techniques of measuring Fermi surface parameters are briefly discussed in terms of application to actinide systems. Particular emphasis is given to the dHvA effect. Some general results found in the dHvA studies of actinide compounds are given. The dHvA effect has been measured in α-U and is presented in detail. None of the observed frequencies corresponds to closed surfaces. Results are compared to the calculations of Freeman, Koelling and Watson-Yang where qualitative agreement is observed

VOLUME DEPENDENCE OF THE ELECTRONIC STRUCTURE OF Cu2Mg

Nous avons mesuré plusieurs sections de la surface de Fermi de Cu2Mg en fonction de la pression et nous comparons les résultats à des calculs de structure de bandes où l'on fait varier le volume.We have made measurements of the pressure dependence of several cross sections of the Fermi surface of Cu2Mg and compare them with a volume dépendent band structure calculation