M. Cyrot

Electrical transport properties and superconducting critical temperature of Ln1.85Ce0.15CuO4−y (Ln Nd, Sm, Eu) under quasihydrostatic pressure up to 100 kbar

Abstract We have measured the resistivity of electron-doped superconductors Ln1.85Ce0.15CuO4−y (Ln = Nd, Sm, Eu) under quasihydrostatic pressure up to 100 kbar. All the samples remain superconducting for this pressure. The ratio d log Tc/d log V was found to be of the same order of magnitude as the one measured under hydrostatic pressure up to 20 kbar, weak and positive, in contrast with the hole doped high Tc superconductors. The effect of applied pressure on Tc does not compare with that of chemical pressure.

Theoretical studies of Sr2VO4, a charge conjugate analog of La2CuO4

Abstract We have used local (spin) density theory to study the atomic geometry and electronic structure of K2NiF4-structure Sr2VO4. The calculations correctly predict the Cu Oz distance, reflecting a negative Jahn-Teller distortion (relative to La2CuO4). The oxygen p bands are filled, leaving one d electron per V, but in the paramagnetic phase three bands cross EF rather than having a single half-filled band. We find a ferromagnetic instability within local spin density theory, rather than the antiferromagnetic insulator seen experimentally.

Superconducting properties of Ln2−xMxCuO4−y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th) under pressure up TO 100 kbar

Abstract We have measured the pressure effects on the superconducting transition temperature (Tc) of Sm2−xCexCuO4−y for various x and y which show an asymmetrical behavior contrasting with what is observed in La2−xSrxCuO4 compounds. Previous data under hydrostatic pressure up to 20 kbar on the Ln1.85M0.15CuO4−y compounds are surveyed and the results of new resistivity measurements under quasihydrostatic pressure up to 100 kbar are given. Unlike the hole-doped high Tc superconductors, the quantity dLogT c dLogV was found to be weakly positive in the whole range of pressure. A simple chemical pressure analysis based on the dependence of Tc on the cell volume (derived from X-ray data) in the series of rare-earths, is not consistent with the pressure effects. The data are discussed in terms of interaction between magnetism and superconductivity.

BCS-type superconductivity due to correlations in high-Tc materials

The authors describe high-Tc superconductors as spin-1/2 doped Mott insulators. They hypothesise that the doped Mott insulator is not a Fermi liquid. They show that this leads to important consequences as far as superconductivity is concerned. In particular, they find s-wave superconductivity which behaves as the usual BCS superconductivity.

Granular Superconductivity in the Electron-Doped Compounds Ln2-xMxCuO4-y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th)

We study the superconducting properties of the Electron.-doped compounds Ln2-xMxCuO4-y (Ln = Pr, Nd, Sm, Eu; M = Ce, Th). Three characteristic temperatures of the superconducting transition are defined, corresponding to: the macroscopic resistive transition midpoint (Tc mid), the diamagnetic signal onset (Tcχ), and the intragrain transition temperature (Tcg). Tcχ and Tcg are found to vary weakly in the series of the studied compounds, in contrast with Tc mid, which is strongly reduced in Eu1.85Ce0.15CuO4-y and Sm1.85Th0.15CuO4-y samples.

Electronic properties and resonant states in AlMn(−Si) alloys☆☆☆

Experimental studies of quasi-crystalline Al100-xMnx(Si) alloys have shown that the magnitude of electrical resistivity as well as the excess specific heat term Δγ are proportional to the percentage of manganese in the samples as is the case in dilute Al-Mn alloys. The authors argue that the excess specific heat term and the high resistivity (about 150-200 μΩ cm for the quasi-crystalline phase) can be accounted for within the extended Friedel-Anderson s-d model in which the dominant scattering is due to the hybridization of d electrons with the sp conduction band. Therefore, the high resistivity in Al-Mn quasi-crystals is mainly due to the contributions from two effects: the existence of narrow resonant states of manganese at the Fermi level and the existence of correlation lengths of the order of 100-300 which cause the incoherence of transport electrons.

An alternative approach to the R.V.B. model, the spin one half doped mott insulator

Abstract In the limit of strong correlation, we present a model for high T c superconductivity which starts from a Mott insulator. By doping it, a B.C.S. condensation becomes possible due to the antiferromagnetic superexchange. We obtain physical results very similar to the usual B.C.S. superconductor.

The normal state of superconducting cuprates viewed as an excitonic metal

Abstract In Motts original idea, a Mott insulator is driven by the ionic potential which is not well screened. Following this idea, we have shown that, in the metallic state obtained upon doping, this potential creates new low energy excitations where a hole sits on an ion and the electron is in a virtual bound state around it. These low energy excitations which are excitonic like, so the name excitonic metal, leads to properties different from a conventional one. We describe the normal state of the superconducting cuprates as an excitonic metal.

Magnetic field induced transitions in Pr2NiO4 single crystals

Abstract Magnetization measurements in Pr2NiO4 single crystals were made up to 70kOe, for different orientations of the applied magnetic field with respect to the crystallographic axes, in the range from 1.5 to 300K. The Ni sublattice becomes antiferromagnetically ordered at TN=325K, as reported from previous neutron diffraction experiments1. There is also a structural transition at TC1=117K1, which allows a ferromagnetic component in the low temperature phase. Below TC1 different magnetic anomalies are observed, which have been interpreted as spin reorientations. Finally at low temperature the Pr sublattice is also ordered. Four different transitions induced by the external applied magnetic field parallel to the c-axis are visible in the range from 0 to 70 kOe, for T

On the Fermi liquid behaviour in doped antiferromagnetic correlated systems

A reformulation of the Gutzwiller-type approximation for so-called correlated ferrimagnetic states is developed to obtain an analytical expression for the ground-state energies. The physical properties of two-dimensional antiferromagnetic states in the doped regime, such as the longitudinal spin susceptibility and the charge compressibility, are calculated from this formalism. The authors have estimated the Landau parameters in Fermi liquid theory near the critical doping concentration for antiferromagnetism and the authors show the fundamental difference of these correlated antiferromagnetic systems with liquid 3He. In particular, the authors suggest the possibility of singlet superconductivity in the intermediate regime for the repulsive electron interaction (U approximately W) due to antiferromagnetic correlation.