Robustness of s-wave superconductivity against Coulomb interactions in Nax_xCoO2_2

We study the depairing effect due to Coulomb interactions in Na$_x$CoO$_2$. We consider the electron-phonon coupling and the Coulomb interactions, and determine $T_c$ for s-wave superconductivity by solving the linearized Eliashberg equation. When we consider shear phonons as well as breathing phonons, $T_c$ is enhanced by Suhl-Kondo (SK) mechanism. Since SK mechanism is strong against Coulomb interactions, $T_c$ remains finite even if strong Coulomb interactions cancel out the attractive force due to breathing phonons. The orbital degree of freedom is important to understand the mechanism of superconductivity in Na$_x$CoO$_2$.Comment: 2pages, 2figures, Proceedings of M2S-HTSC conference in Dresden, 200

Spin fluctuations and weak pseudogap behaviors in Na_{0.35}CoO_2:renomarization of band structure

We analyze the normal electronic states of Na$_{0.35}$CoO$_2$ based on the multi-orbital Hubbard model using the FLEX approximation. The fundamental electronic property of this system is drastically changed by the presence or absence of the small hole pockets associated with the $e_g'$ orbital. This change of the Fermi surface topology may be caused by the crystalline electric splitting due to the trigonal distortion. When small hole pockets are absent, the weak pseudogap bahaviors appear in the density of states and the uniform spin susceptibility, which are observed by recent experiments. We estimate the mass enhancement factor of quasiparticle $m^*/m\simeq 1.5\sim 1.8$. This result suppports recent ARPES measurements.Comment: 2 pages, 2 figures, submitted to proceedings of LT24, Orlando, FL, August 2005; added referenc

Tachyon condensation in unbalanced D-anti D system

The tachyon condensation is studied in asymmetric D-anti D systems. Taking a system of two pairs of D5-anti D5 in type IIB superstring theory in the background of large N D5-branes, we show that one BPS D1-brane comes out after the condensation. It is also seen that the BPS D1-brane feels no force from the background D5-branes. We also show that the inclusion of the fluctuation fields gives an expected Dirac-Born-Infeld (DBI) action of the resultant D1-brane. On the other hand, in the case of one pair of D5-anti D5 in the same background, we show that the resultant BPS D3-brane experiences attractive force from the background D5-branes.Comment: 18 pages, minor corrections, to appear in Prog. Theor. Phy

JAPANESE FIRMS’ DEBT POLICY AND TAX POLICY

Understanding the effects of marginal tax rate on debt policy is crucial not only for　considering various capital structure theories of firms but also for evaluating corporate tax reform proposals. In this empirical study, we have found a positive relation in most cases between the firm-specific marginal tax rates (simulated using the method of Shevlin (1990) and Graham (1996)) and the debt ratio increase of Japanese firms. This result shows that the marginal tax rates significantly affect the debt policies of Japanese firms. Corporate tax reform to produce equal treatment of equity and debt is desirable in Japan.debt, capital structure, marginal tax rate, corporate tax

Tunneling Conductance and Surface States Transition in Superconducting Topological Insulators

We develop a theory of the tunneling spectroscopy for superconducting topological insulators (STIs), where the surface Andreev bound states (SABSs) appear as helical Majorana fermions. Based on the symmetry and topological nature of parent topological insulators, we find that the SABSs in the STIs have a profound structural transition in the energy dispersions. The transition results in a variety of Majorana fermions, by tuning the chemical potential and the effective mass of the energy band. We clarify that Majorana fermions in the vicinity of the transitions give rise to robust zero bias peaks in the tunneling conductance between normal metal/STI junctions.Comment: 6 pages, 4 figure