Manifestation of hopping conductivity and granularity within phase diagrams of LaO1-x F x BiS2, Sr1-x La x FBiS2 and related BiS2-based compounds.

Layered BiS [Formula: see text] -based series, such as LaO [Formula: see text] F [Formula: see text] BiS [Formula: see text] and Sr [Formula: see text] La [Formula: see text] FBiS [Formula: see text] , offer ideal examples for studying normal and superconducting phase diagram of a solid solution that evolves from a nonmagnetic band-insulator parent. We constructed typical [Formula: see text] phase diagrams of these systems based on events occurring in thermal evolution of their electrical resistivity, [Formula: see text]. Overall evolution of these diagrams can be rationalized in terms of (i) Mott-Efros-Shklovskii scenario which, within the semiconducting [Formula: see text] regime ([Formula: see text] metal-insulator transition), describes the doping influence on the thermally activated hopping conductivity. (ii) A granular metal (superconductor) scenario which, within [Formula: see text], describes the evolution of normal and superconducting properties in terms of conductance g, Coulomb charging energy E c and Josephson coupling J; their joint influence is usually captured within a [Formula: see text] phase diagram. Based on analysis of the granular character of [Formula: see text], we converted the [Formula: see text] diagrams into projected g - T diagrams which, being fundamental, allow a better understanding of evolution of various granular-related properties (in particular the hallmarks of normal-state [Formula: see text] feature and superconductor-insulator transition) and how such properties are influenced by x, pressure or heat treatment.

Large enhancement of the thermopower in Na(x)CoO2 at high Na doping.

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate Na(x)CoO2 exhibits several interesting electronic phases as the Na content x is varied, including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, Na(x)CoO2 has moderately large thermopower S and conductivity sigma. However, the prospects for thermoelectric cooling applications faded when the figure of merit Z was found to be small at this composition (0.60.75, S undergoes an even steeper enhancement. At the critical doping x(p) approximately 0.85, Z (at 80 K) reaches values approximately 40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.

Anomalous hall heat current and nernst effect in the CuCr2Se4-xBrx ferromagnet.

In a ferromagnet, an anomalous Hall heat current, given by the off-diagonal Peltier term alpha(xy), accompanies the anomalous Hall current. By combining Nernst, thermopower, and Hall experiments, we have measured how alpha(xy) varies with hole density and lifetime tau in CuCr2Se4-xBrx. At low temperatures T, we find that alpha(xy) is independent of tau, consistent with anomalous-velocity theories. Its magnitude is fixed by a microscopic geometric area A approximately 34 A(2). Our results are incompatible with some models of the Nernst effect in ferromagnets.