Thermopower analysis of the electronic structure around metal-insulator transition in V1-xWxO2

Electronic structure across the metal-insulator (MI) transition of electron-doped V1-xWxO2 epitaxial films (x = 0-0.06) grown on alfa-Al2O3 substrates was studied by means of thermopower (S) measurements. Significant increase of |S|-values accompanied by MI transition was observed, and the transition temperatures of S (TS) decreased with x in good linear relation with MI transition temperatures. |S| values of V1-xWxO2 films at T > TS were constant at low values of 23 microV K-1 independently of x, which reflects a metallic electronic structure, whereas, those at T < TS almost linearly decreased with logarithmic W-concentrations. The gradient of -213 microV K-1 agrees well with -kB/e*ln10 (-198 microV K-1), suggesting that V1-xWxO2 films have insulating electronic structures with a parabolic density of state around the conduction band bottom.Comment: Accepted for publication as a Rapid Commun. in Phys. Rev.

Thermopower analysis of metal-insulator transition temperature modulations in vanadium dioxide thin films with lattice distortion

Insulator-to-metal (MI) phase transition in vanadium dioxide (VO2) thin films with controlled lattice distortion was investigated by thermopower measurements. VO2 epitaxial films with different crystallographic orientations, grown on (0001) alpha-Al2O3, (11-20) alpha-Al2O3, and (001) TiO2 substrates, showed significant decrease of absolute value of Seebeck coefficient (S) from ~200 to 23 microV K-1, along with a sharp drop in electrical resistivity (rho), due to the transition from an insulator to a metal. The MI transition temperatures observed both in rho (Trho) and S (TS) for the VO2 films systematically decrease with lattice shrinkage in the pseudo-rutile structure along c-axis, accompanying a broadening of the MI transition temperature width. Moreover, the onset TS, where the insulating phase starts to become metallic, is much lower than onset Trho. This difference is attributed to the sensitivity of S for the detection of hidden metallic domains in the majority insulating phase, which cannot be detected in rho-measurements. Consequently, S-measurements provide a straightforward and excellent approach for a deeper understanding of the MI transition process in VO2.Comment: To be published in Physical Review

Transport and magnetic properties of Co-doped BaFe_{2}As_{2} epitaxial thin films

We report resistivity, Hall coefficient, current-voltage characteristics, and magneto-optical imaging measurements of epitaxial Co-doped BaFe_{2}As_{2} thin films deposited on MgO(001) substrate. The Hall resistivity of the films has a substantial contribution arising from anomalous Hall effect of ferromagnetic components. The critical current density (J_{c}) of the films is ~2 MA/cm^{2} at low temperatures. Differential magneto-optical images of the remanent state give similar J_{c} values and also exhibit presence of extended defects in the film.Comment: 9 pages, 4 figure

Superconductivity in La1-xCexOBiSSe: carrier doping by mixed valence of Ce ions

We report the effects of Ce substitution on structural, electronic, and magnetic properties of layered bismuth-chalcogenide La1-xCexOBiSSe (x = 0-0.9), which are newly obtained in this study. Metallic conductivity was observed for x > 0.1 because of electron carriers induced by mixed valence of Ce ions, as revealed by bond valence sum calculation and magnetization measurements. Zero resistivity and clear diamagnetic susceptibility were obtained for x = 0.2-0.6, indicating the emergence of bulk superconductivity in these compounds. Dome-shaped superconductivity phase diagram with the highest transition temperature (Tc) of 3.1 K, which is slightly lower than that of F-doped LaOBiSSe (Tc = 3.7 K), was established. The present study clearly shows that the mixed valence of Ce ions can be utilized as an alternative approach for electron-doping in layered bismuth-chalcogenides to induce superconductivity