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Attempts to induce delocalisation of n and p-type carriers in iono-covalent systems represent a very important challenge in order to discover exotic metals. This idea is strongly supported by the concept of Cu2+-Cu3+ mixed valence that we developed in the 80′s for the realisation of numerous metallic cuprates. We show here that K.A. Muller's discovery of high Tc superconductivity in one of these exotic metals has boosted the research in a vast field of high Tc superconducting cuprates. Importantly, we demonstrate that the Cu+-Cu2+ mixed valence concept also allows numerous thermoelectric copper sulphides with exotic p-type metallic properties to be generated. We discuss the role of the Ni+-Ni2+ mixed valence in the metallic properties and thin film superconductivity of nickelates. [ABSTRACT FROM AUTHOR]

We show that doping of hole charge carriers leads to formation of electric dipolar clusters in cuprates. They are created by many-body interactions between the dopant ion outside and holes inside the CuO planes. Because of the two-fold degeneracy holes in the CuO plane cluster into four-particles resonance valence bond plaquettes bound with dopant ions. Such dipoles may order into charge-density waves (CDW) or stripes or form a disordered state depending on doping and temperature. The lowest energy of the ordered system corresponds to a local anti-ferroelectric ordering. The mobility of individual disordered dipoles is very low at low temperatures and they prefer first to bind into dipole-dipole pairs. Electromagnetic radiation interacts strongly with electric dipoles and when the sample is subjected to it the mobility changes significantly. This leads to a fractal growth of dipolar clusters. The existence of electric dipoles and CDW induce two phase transitions with increasing temperature, melting of the ordered state and disappearance of the dipolar state. Ferroelectricity at low doping is a natural consequence of such dipole moments. We develop a theory based on two-level systems and dipole-dipole interaction to explain the behavior of the polarization as a function of temperature and electric field. [ABSTRACT FROM AUTHOR]

Abstract: Self-assembled nanoscale channels may naturally arise in the growth process of grain boundaries (GBs) in high critical temperature superconductor (HTS) systems, and deeply influence the transport properties of the GB Josephson junctions (JJs). By isolating nano-channels in YBCO biepitaxial JJs and studying their properties, we sort out specific fingerprints of the mesoscopic nature of the contacts. The size of the channels combined to the characteristic properties of HTS favors a special regime of the proximity effect, where normal state coherence prevails on the superconducting coherence in the barrier region. Resistance oscillations from the current–voltage characteristic encode mesoscopic information on the junction and more specifically on the minigap induced in the barrier. Thouless energy emerges as a characteristic energy of these types of Josephson junctions. Possible implications on the understanding of coherent transport of quasiparticles in HTS and of the dissipation mechanisms are discussed, along with elements to take into account when designing HTS nanostructures. [Copyright &y& Elsevier]

Abstract: High temperature superconducting cables for transport currents well above 1kA, assembled from a number of coated conductor tapes, are required for application in transformers, generators and, with further increased currents, for future fusion reactor coils. At present the assembling of meander shaped tapes by the Roebel technique seems to be the most promising way of manufacturing high current cables. With short cable samples it was previously shown that the problem of the in the plane inflexibility of the tapes can be overcome by this technique. For some ac applications the losses are however still too high. Since the hysteresis losses in the tapes are dominant in the frequency range below 100Hz, the main emphasis should be directed to reduce this loss component. Several routes to achieve this goal are investigated: striations in the individual tapes reduce the effective width of the superconducting layer and hence the hysteresis losses, but the problem of twisting of these striations is not yet solved. A second route is the reduction of the meander shaped Roebel tape width to a value in the region of 1mm which allows the assembling of a correspondingly larger number of tapes, e.g. 30 or 40. This would lead to a cable approaching a square cross section with reduced losses and, as another important advantage, with a certain in-plane flexibility. The critical current degradation of tapes with 1 and 2mm width is investigated and ac-loss results of cables with 2mm meander tapes are presented. [Copyright &y& Elsevier]