Quantum Impurities and the Neutron Resonance Peak in YBa2Cu3O7{\bf YBa_2 Cu_3 O_7}: Ni versus Zn

The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin dynamics of an optimally doped high temperature superconductor is compared in two samples with almost identical superconducting transition temperatures: YBa$_2$(Cu$_{0.97}$Ni$_{0.03}$)$_3$O$_7$ (T$_c$=80 K) and YBa$_2$(Cu$_{0.99}$Zn$_{0.01}$)$_3$O$_7$ (T$_c$=78 K). In the Ni-substituted system, the magnetic resonance peak (which is observed at E$_r \simeq$40 meV in the pure system) shifts to lower energy with a preserved E$_r$/T$_c$ ratio while the shift is much smaller upon Zn substitution. By contrast Zn, but not Ni, restores significant spin fluctuations around 40 meV in the normal state. These observations are discussed in the light of models proposed for the magnetic resonance peak.Comment: 3 figures, submitted to PR

Spin Susceptibility in Underdoped YBa2Cu3O6+x\bf YBa_2Cu_3O_{6+x}

We report a comprehensive polarized and unpolarized neutron scattering study of the evolution of the dynamical spin susceptibility with temperature and doping in three underdoped single crystals of the \YBCO{6+x} high temperature superconductor: \YBCO{6.5} (Tc = 52 K), \YBCO{6.7} (Tc = 67 K), and \YBCO{6.85} (T_c = 87 K). Theoretical implications of these data are discussed, and a critique of recent attempts to relate the spin excitations to the thermodynamics of high temperature superconductors is given.Comment: minor revisions, to appear in PR

In-situ YBa2Cu3Ox growth rate analysis

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Élaboration et performances de matériaux supraconducteurs YBa

Industrial applications of the bulk superconducting YBa2Cu3O7 material imply to control the growth of large oriented monodomains in samples of big size (several centimeters). The laboratory EPM-Matformag is committed to produce such materials according to three different methods (zone melting, solidification controlled by a magnetic field, crystal growth from a seed). The results obtained show that it is possible by such methods to elaborate a material with high performances at the centimeter scale and to produce it in series. The availability of such materials allows the measure of physical properties on a large scale and the testing of prototypes for cryo-electrotechnical applications (magnetic bearing, flywheel, coupling device, current lead...)

Twin plane influence on critical transport current along the

The magnetic-field angular dependence of the c-axis transport critical current density $J_{\rm c} (\theta,B)$ in bulk-textured single-domain YBCO samples is studied with the field in the ab-plane. Above a matching field B_{\mit\Phi} associated with the twin boundary network, $J_{\rm c} (\theta)$ shows a fourfold peak structure related to vortex pinning by alternatively the (110) and $(1\bar 10)$ twin planes directions. The peak value is independent of the field intensity up to 8 T, due to long-range pinning by twin planes. Below B_{\mit\Phi}, the twin plane network gives rise to a lock-in of the vortices characterised by an angular and field-independent Jc

Influence des macles sur le courant critique de transport selon c et sur la ligne d'irréversibilité dans les plans ab étudiée dans des échantillons monodomaines texturés d'YBaCuO

La densité de courant critique selon l'axe c (Jc) est mesurée à 77 K en transport et par des mesures magnétiques sur les mêmes échantillons texturés monodomaines d'YBaCuO. L'influence des plans de macles (TP) sur Jc et sur la ligne d'irréversibilité $B(T^*$) est étudiée en faisant tourner le champ dans les plans ab, perpendiculairement à l'axe c. Les résultats montrent que dans ces composés, Jc et $B(T^*$) sont fortement influencés par le piégeage par les TP. Divers comportements des lignes de flux résultant de l'influence de ces défauts corrélés sont proposés selon les gammes de température et de champ étudiées

Thermoelectric Properties of Ternary and Quaternary Mo6 and Mo9 Cluster Selenides

International audienceMo-based cluster compounds containing Mo6 and Mo9 cluster units have long been known for their rich chemistry and the diversity and complexity of their crystal structures. While most studies have mainly focused on their crystallographic properties, recent investigations have pointed out their potential for thermoelectric applications in power generation. These compounds derive their good properties from the three-dimensional arrangement of the clusters between which cations reside. This inherent disorder strongly limits the ability of these materials to transport heat that often leads to a temperature dependence of the lattice thermal conductivity that mirrors that observed in glassy systems. In addition, most of these compounds can be driven from a metallic toward a semiconducting state through insertion of additional cations. Here, we review the recent progress made on determining the transport properties of these compounds, discussing in particular the key ingredients that lead to their peculiar thermal properties, and examine possible future directions to further enhance their thermoelectric properties