Magnetothemopower study of quasi two-dimensional organic conductor α\alpha-(BEDT-TTF)2_2KHg(SCN)4_4

We have used a low-frequency magneto-thermopower (MTEP) method to probe the high magnetic field ground state behavior of $\alpha$-(BEDT-TTF)$_2$KHg(SCN)$_4$ along all three principal crystallographic axes at low temperatures. The thermopower tensor coefficients ($S_{xx}, S_{yx}$ and $S_{zz}$) have been measured to 30 T, beyond the anomalous low temperature, field-induced transition at 22.5 T. We find a significant anisotropy in the MTEP signal, and also observe large quantum oscillations associated with the de Haas - van Alphen effect. The anisotropy indicates that the ground state properties are clearly driven by mechanisms that occur along specific directions for the in-plane electronic structure. Both transverse and longitudinal magnetothermopower show asymptotic behavior in field, which can be explained in terms of magnetic breakdown of compensated closed orbits.Comment: 9 pages, 10 figure

Electronic charge and orbital reconstruction at cuprate-titanate interfaces

In complex transition metal oxide heterostructures of physically dissimilar perovskite compounds, interface phenomena can lead to novel physical properties not observed in either of their constituents. This remarkable feature opens new prospects for technological applications in oxide electronic devices based on nm-thin oxide films. Here we report on a significant electronic charge and orbital reconstruction at interfaces between YBa2Cu3O6 and SrTiO3 studied using local spin density approximation (LSDA) with intra-atomic Coulomb repulsion (LSDA+U). We show that the interface polarity results in the metallicity of cuprate-titanate superlattices with the hole carriers concentrated predominantly in the CuO2 and BaO layers and in the first interface TiO2 and SrO planes. We also find that the interface structural relaxation causes a strong change of orbital occupation of Cu 3d orbitals in the CuO2 layers. The concomitant change of Cu valency from +2 to +3 is related to the partial occupation of the Cu $3d_{3z^2-r^2}$ orbitals at the interface with SrO planes terminating SrTiO3. Interface-induced predoping and orbital reconstruction in CuO2 layers are key mechanisms which control the superconducting properties of field-effect devices developed on the basis of cuprate-titanate heterostructures.Comment: 11 pages, 8 figures, to appear in the "Proceedings of Third Joint HLRB and KONWIHR Result and Reviewing Workshop", Springer 200

Anomalous Behavior of the Thermoelectric Power in the Vicinity of the Superconducting Transition in the Organic Superconductors κ\mathsf{\kappa}-(BEDT-TTF)2\mathsf{_2}Cu(NCS)2\mathsf{_2} and κ\mathsf{\kappa}-(BEDT-TTF)2\mathsf{_2}Cu[ N(CN)2\mathsf{_2}] Br

The in-plane thermoelectric power (TEP) of the layered organic superconductors $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ and $\kappa$-(BEDT-TTF)$_2$Cu[ N(CN)$_2$] Br has been studied in the vicinity of the superconducting transition under magnetic field. Below the transition temperature, an anomalous non-monotonic behavior of the TEP is found in the $\kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$ salt: instead of a gradual monotonic decrease, the TEP shows a prominent sign-change before vanishing. Similar, although less pronounced features have been found in the other compound. The change in sign of the TEP is explained in terms of the two-fluid counterflow model, assuming a predominant role of one of the two anisotropic bands (namely, the electron-like one) in the superconducting pairing.