Nonequilibrium Peierls Transition

Nonequlibrium phase transition of an open Takayama-Lin Liu-Maki chain coupled with two reservoirs is investigated by combining a mean-field approximation and a formula characterizing nonequlibrium steady states, which is obtained from the algebraic field approach to nonequlibrium statistical mechanics. When the bias voltage is chosen to be a control parameter, the phase transition between ordered and normal phases is found to be first or second order. Then, the voltage-current characteristics is S-shaped in some parameter region. In contrast, when the current is chosen to be a control parameter, all the non-trivial solutions of the self-consistent equation are found to become stable. In this case, the phase transition between the ordered and normal phases is always second order and negative differential conductivity appears at low temperature.Comment: 30 pages, 21 frigure

In-plane Anisotropy on Resistivity and Thermopower in The Misfit Layered Oxide Bi2-xPbxSr2Co2Oy

We investigated the in-plane anisotropy on the resistivity and thermopower of Bi2-xPbxSr2Co2Oy single crystals, which have a misfit structure between the hexagonal CoO2 layer and the rock salt Bi2Sr2O4 layer. The resistivity and thermopower show significantly large anisotropy, which exceeds two at maximum. This anisotropy would come from the anisotropic pseudogap formation enhanced by the misfit structure. The thermopower changes with Pb doping to take a maximum at x=0.4. The misfit structure improves the thermoelectric properties through chemical pressure. The power factor is as large as 9 muW/cmK2 at 100 K for x=0.6, which is the highest value for thermoelectric oxides at 100 K.Comment: 4 pages, 3 figures, submitted to Jpn. J. Appl. Phy

Ga3+^{3+}-substitution effects in the weak ferromagnetic oxide LaCo0.8_{0.8}Rh0.2_{0.2}O3_{3}

Magnetization and x-ray diffraction have been measured on polycrystalline samples of LaCo$_{0.8-y}$Rh$_{0.2}$Ga$_{y}$O$_{3}$ for $0 \leq y \leq 0.15$ in order to understand the spin state of Co$^{3+}$ through the Ga$^{3+}$ substitution effect. The ferromagnetic order in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$ below 15 K is dramatically suppressed by the Ga$^{3+}$ substitution, where the ferromagnetic volume fraction is linearly decreased. The normal state susceptibility also systematically decreases with the Ga content, from which we find that one Ga$^{3+}$ ion reduces 4.6 $\mu_{\rm B}$ per formula unit. We have evaluated how the concentration of the high-spin state Co$^{3+}$ changes with temperature by using an extended Curie-Weiss law, and have found that the substituted Rh$^{3+}$ ion stabilizes the high-spin state Co$^{3+}$ ion down to low temperatures. We find that Ga$^{3+}$ preferentially replaces the high-spin state Co$^{3+}$, which makes a remarkable contrast to our previous finding that Rh$^{3+}$ preferentially replaces the low-spin state Co$^{3+}$. These results strongly suggest that the magnetically excited state of LaCoO$_{3}$ at room temperature is a mixed state of high-spin state Co$^{3+}$ and low-spin state Co$^{3+}$.Comment: 5 pages, 6 figure