T-duality of ZZ-brane

We examine how nonperturbative effects in string theory are transformed under the T-duality in its nonperturbative framework by analyzing the c=1/2 noncritical string theory as a simplest example. We show that in the T-dual theory they also take the form of exp(-S_0/g_s) in the leading order and that the instanton actions S_0 of the dual ZZ-branes are exactly the same as those in the original c=1/2 string theory. Furthermore we present formulas for coefficients of exp(-S_0/g_s) in the dual theory.Comment: 37 pages, no figure, LaTeX; (v2) version published in Physical Review

Thermoelectric figure of merit of tau-type conductors of several donors

Dimensionless thermoelectric figure of merit $ZT$ is investigated for two-dimensional organic conductors $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, $\tau$-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$and$\tau$-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ ($y \le 0.875$), respectively. The $ZT$ values were estimated by measuring electrical resistivity, thermopower and thermal conductivity simultaneously. The largest $ZT$ is 2.7 $\times$ 10$^{-2}$ at 155 K for $\tau-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, 1.5 $\times$ 10$^{-2}$ at 180 K for $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and 5.4 $\times$ 10$^{-3}$ at 78 K for $\tau-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, respectively. Substitution of the donor molecules fixing the counter anion revealed EDT-S,S-DMEDT-TTF is the best of the three donors to obtain larger $ZT$.Comment: proceedings of ISCOM 2009 (to be published in Physica B

A quantum Monte Carlo study on the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a triangular lattice

We study the superconducting Kosterlitz-Thouless transition of the attractive Hubbard model on a two-dimensional triangular lattice using auxiliary field quantum Monte Carlo method for system sizes up to $12\times 12$ sites. Combining three methods to analyze the numerical data, we find, for the attractive interaction of $U=-4t$, that the transition temperature stays almost constant within the band filling range of $1.0 < n < 1.4$, while it is found to be much lower in the $n<1$ region.Comment: RevTeX 6 page

High temperature superconductivity in dimer array systems

Superconductivity in the Hubbard model is studied on a series of lattices in which dimers are coupled in various types of arrays. Using fluctuation exchange method and solving the linearized Eliashberg equation, the transition temperature $T_c$ of these systems is estimated to be much higher than that of the Hubbard model on a simple square lattice, which is a model for the high $T_c$ cuprates. We conclude that these `dimer array' systems can generally exhibit superconductivity with very high $T_c$. Not only $d$-electron systems, but also $p$-electron systems may provide various stages for realizing the present mechanism.Comment: 4 pages, 9 figure

Three-orbital study on the orbital distillation effect in the high Tc cuprates

Our recent study has revealed that the mixture of the dz2 orbital component into the Fermi surface suppresses Tc in the cuprates such as La2CuO4. We have also shown that applying hydrostatic pressure enhances Tc due to smaller mixing of the Cu4s component. We call these the "orbital distillation" effect. In our previous study, the 4s orbital was taken into account through the hoppings in the dx2-y2 sector, but here we consider a model in which of the dx2-y2, dz2 and 4s orbitals are all considered explicitly. The present study reinforces our conclusion that smaller 4s hybridization further enhances Tc.Comment: 4 pages, 2 figures, submitted as a proceeding of ISS2012(Tokyo

Two-band Fluctuation Exchange Study on the Superconductivity of β′\beta'-(BEDT-TTF)2_2ICl2_2 under High Pressure

We study the pressure dependence of the superconducting transition temperature of an organic superconductor $\beta'$-(BEDT-TTF)$_2$ICl$_2$ by applying the fluctuation exchange method to the Hubbard model on the original two-band lattice at 3/4-filling rather than the single band model in the strong dimerization limit. Our study is motivated by the fact that hopping parameters evaluated from a first-principles study suggest that the dimerization of the BEDT-TTF molecules is not so strong especially at high pressure. Solving the linearized Eliashberg's equation, a d$_{xy}$-wave-like superconducting state with realistic values of $T_c$ is obtained in a pressure regime somewhat higher than the actual experimental result. These results are similar to those obtained within the single band model in the previous study by Kino {\it et al}. We conclude that the resemblance to the dimer limit is due to a combination of a good Fermi surface nesting, a large density of states near the Fermi level, and a moderate dimerization, which cooperatively enhance electron correlation effects and also the superconducting $T_c$.Comment: 6 pages, 8 figure