Effect of the Vortices on the Nuclear Spin Relaxation Rate in the Unconventional Pairing States of the Organic Superconductor (TMTSF)2_2PF6_6

This Letter theoretically discusses quasiparticle states and nuclear spin relaxation rates $T_1^{-1}$ in a quasi-one-dimensional superconductor (TMTSF)$_2$PF$_6$ under a magnetic field applied parallel to the conduction chains. We study the effects of Josephson-type vortices on $T_1^{-1}$ by solving the Bogoliubov de Gennes equation for $p$-, $d$- or $f$-wave pairing interactions. In the presence of line nodes in pairing functions, $T_1^{-1}$ is proportional to $T$ in sufficiently low temperatures because quasiparticles induced by vortices at the Fermi energy relax spins. We also try to identify the pairing symmetry of (TMTSF)$_2$PF$_6$.Comment: 4+ pages, 4 figure

Possible high TcT_c superconductivity mediated by antiferromagnetic spin fluctuations in systems with Fermi surface pockets

We propose that if there are two small pocket-like Fermi surfaces, and the spin susceptibility is pronounced around a wave vector {\bf Q} that bridges the two pockets, the spin-singlet superconductivity mediated by spin fluctuations may have a high transition temperature. Using the fluctuation exchange approximation, this idea is confirmed for the Hubbard on a lattice with alternating hopping integrals, for which $T_c$ is estimated to be almost an order of magnitude larger than those for systems with a large connected Fermi surface.Comment: 5 pages, uses RevTe

Electronic structure and spontaneous internal field around non-magnetic impurities in spin-triplet chiral p-wave superconductors

The electronic structure around an impurity in spin triplet p-wave superconductors is studied by the Bogoliubov-de Gennes theory on a tight-binding model, where we have chosen $\sin{p_x}{+}{\rm i}\sin{p_y}$-wave or $\sin{(p_x+p_y)}{+}{\rm i}\sin{({-}p_x{+}p_y)}$-wave states which are considered to be candidates for the pairing state in Sr$_{2}$RuO$_{4}$. We calculate the spontaneous current and the local density of states around the impurity and discuss the difference between the two types of pairing. We propose that it is possible to discriminate the two pairing states by studying the spatial dependence of the magnetic field around a pair of impurities.Comment: 4 pages, 4 figure

Dynamical Generation of Non-Abelian Gauge Group via the Improved Perturbation Theory

It was suggested that the massive Yang-Mills-Chern-Simons matrix model has three phases and that in one of them a non-Abelian gauge symmetry is dynamically generated. The analysis was at the one-loop level around a classical solution of fuzzy sphere type. We obtain evidences that three phases are indeed realized as nonperturbative vacua by using the improved perturbation theory. It also gives a good example that even if we start from a trivial vacuum, the improved perturbation theory around it enables us to observe nontrivial vacua.Comment: 31 pages, published versio

Nuclear Magnetic Relaxation Rate in Iron-Pnictide Superconductors

Nuclear magnetic relaxation rate 1/T_1 in iron-pnictide superconductors is calculated using the gap function obtained in a microscopic calculation. Based on the obtained results, we discuss the issues such as the rapid decrease of 1/T_1 just below the transition temperature and the difference between nodeless and nodal s-wave gap functions. We also investigate the effect of Coulomb interaction on 1/T_1 in the random phase approximation and show its importance in interpreting the experimental results.Comment: Proceedings of 9th International Conference on Materials and Mechanisms of Superconductivity. To be published in Physica

Superconductivity and spin correlation in organic conductors: a quantum Monte Carlo study

The d-wave pairing correlations along with spin correlation are calculated with quantum Monte Carlo method for the two-dimensional Hubbard model on lattice structures representing organic superconductors $\kappa$-(BEDT-TTF)$_2$X and (TMTSF)$_2$X. In both cases the pairing correlations for superconducting order parameters with nodes are found to be enhanced. The symmetry and the enhancement of the pairing is systematically correlated with the spin structure factor, suggesting a spin-fluctuation mediated pairing. We have further found that, as we deform the Fermi surface to make the system approach the half-filled square lattice, the coherence of the pairing saturates while the local pairing amplitude continues to increase.Comment: 5 pages, RevTeX, uses epsf.sty and multicol.st

"Pudding mold" band drives large thermopower in Nax_xCoO2_2

In the present study, we pin down the origin of the coexistence of the large thermopower and the large conductivity in Na$_x$CoO$_2$. It is revealed that not just the density of states (DOS), the effective mass, nor the band width, but the peculiar {\it shape} of the $a_{1g}$ band referred to as the "pudding mold" type, which consists of a dispersive portion and a somewhat flat portion, is playing an important role in this phenomenon. The present study provides a new guiding principle for designing good thermoelectric materials.Comment: 5 page

Simple Real-Space Picture of Nodeless and Nodal s-wave Gap Functions in Iron Pnictide Superconductors

We propose a simple way to parameterize the gap function in iron pnictides. The key idea is to use orbital representation, not band representation, and to assume real-space short-range pairing. Our parameterization reproduces fairly well the structure of gap function obtained in microscopic calculation. At the same time the present parameterization is simple enough to obtain an intuitive picture and to develop a phenomenological theory. We also discuss simplification of the treatment of the superconducting state.Comment: 4 page

A unified origin for the 3D magnetism and superconductivity in Nax_xCoO2_2

We analyze the origin of the three dimensional (3D) magnetism observed in nonhydrated Na-rich Na$_x$CoO$_2$ within an itinerant spin picture using a 3D Hubbard model. The origin is identified as the 3D nesting between the inner and outer portions of the Fermi surface, which arise due to the local minimum structure of the $a_{1g}$ band at the $\Gamma$-A line. The calculated spin wave dispersion strikingly resembles the neutron scattering result. We argue that this 3D magnetism and the spin fluctuations responsible for superconductivity in the hydrated systems share essentially the same origin.Comment: 5pages, 6figure