Possible spin-triplet ff-wave pairing due to disconnected Fermi surfaces in Nax_xCoO2⋅y_2\cdot yH2_2O

We propose that spin-triplet pairing mechanism due to disconnected Fermi surfaces proposed in our previous study [Phys. Rev. B {\bf 63} 174507 (2001)] may be at work in a recently discovered superconductor Na$_x$CoO$_2$ $\cdot y$ H$_2$O. We introduce a single band effective model that takes into account the pocket-like Fermi surfaces along with the van Hove singularity near the K point found in the band calculation results. Applying fluctuation exchange method and solving the linearized {\'E}liashberg equation, the most dominant pairing is found to have spin-triplet $f$-wave symmetry, where the nodes of the gap function do not intersect the pocket Fermi surfaces. Presence of finite $T_c$ is suggested in sharp contrast with cases when the gap nodes intersect the Fermi surface.Comment: A revised version of the paper, which has been accepted for publication in Physical Reiview Letters. Some sentences and paragraphs added in the discussion part (page 4

Strong magnetic field enhancement of spin triplet pairing arising from coexisting 2kF2k_F spin and 2kF2k_F charge fluctuations

We study the effect of the magnetic field (Zeeman splitting) on the triplet pairing. We show generally that the enhancement of spin triplet pairing mediated by coexisting $2k_F$ spin and $2k_F$ charge fluctuations can be much larger than in the case of triplet pairing mediated by ferromagnetic spin fluctuations. We propose that this may be related to the recent experiment for (TMTSF)$_2$ClO$_4$, in which a possibility of singlet to triplet pairing transition has been suggested.Comment: 5 page

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

Realization of Berezinskii's superconductivity in quasi-one-dimensional systems

We revisit the pairing symmetry competition in quasi-one-dimensional systems. We show that spin-triplet s-wave pairing, where the pair is formed by electrons with different times and has an odd-frequency symmetry, can be realized in systems with strong one-dimensionality when the strength of charge fluctuation dominates over spin fluctuation. The present study provides a novel microscopic mechanism for this exotic pairing originally proposed by Berezinskii in 1974.Comment: 4 pages, 4 figure

Strong parity mixing in the FFLO superconductivity in systems with coexisting spin and charge fluctuations

We study the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state of spin fluctuation mediated pairing, and focus on the effect of coexisting charge fluctuations. We find that (i) consecutive transitions from singlet pairing to FFLO and further to $S_z=1$ triplet pairing can generally take place upon increasing the magnetic field when strong charge fluctuations coexist with spin fluctuations, and (ii) the enhancement of the charge fluctuations lead to a significant increase of the parity mixing in the FFLO state, where the triplet/singlet component ratio in the gap function can be close to unity. We propose that such consecutive pairing state transition and strong parity mixing in the FFLO state may take place in a quasi-one-dimensional organic superconductor (TMTSF)$_2X$.Comment: 5 pages, 5 figures. To be published in Phys. Rev. Let

Competition between singlet and triplet pairings in Na_xCoO_2 yH_2O

We discuss the pairing symmetry of a cobaltate superconductor Na$_x$CoO$_2\cdot y$ H$_2$O by adopting an effective single band model that takes into account the $e_g'$ hole pockets, as discussed in our previous paper [to appear in Phys. Rev. Lett.] Here we consider the off-site repulsions in addition to the on-site repulsion considered in our previous study. We show that the spin-triplet f-wave pairing proposed in our previous study is robust to some extent even in the presence of off-site repulsions. However, f-wave pairing gives way to singlet pairings for sufficiently large values of off-site repulsions. Among the singlet pairings, i-wave and extended s-wave pairings are good candidates which do not break time reversal symmetry below $T_c$ in agreement with the experiments.Comment: 12 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

Predominant Magnetic States in Hubbard Model on Anisotropic Triangular Lattices

Using an optimization variational Monte Carlo method, we study the half-filled-band Hubbard model on anisotropic triangular lattices, as a continuation of the preceding study [J. Phys. Soc. Jpn 75, 074707 (2006)]. We introduce two new trial states: (i) A coexisting state of (\pi,\pi)-antiferromagnetic (AF) and a d-wave singlet gaps, in which we allow for a band renormalization effect, and (ii) a state with an AF order of 120^\circ spin structure. In both states, a first-order metal-to-insulator transition occurs at smaller U/t than that of the pure d-wave state. In insulating regimes, magnetic orders always exist; an ordinary (\pi,\pi)-AF order survives up to t'/t\sim 0.9 (U/t=12), and a 120^\circ-AF order becomes dominant for t'/t \gsim 0.9. The regimes of the robust superconductor and of the nonmagnetic insulator the preceding study proposed give way to these magnetic domains.Comment: 11 pages, 14 figure

Unconventional pairing originating from disconnected Fermi surfaces in superconducting LaFeAsO1−x_{1-x}Fx_x}

For a newly discovered iron-based high $T_c$ superconductor LaFeAsO$_{1-x}$F$_x$, we have constructed a minimal model, where inclusion of all the five Fe $d$ bands is found to be necessary. Random-phase approximation is applied to the model to investigate the origin of superconductivity. We conclude that the multiple spin fluctuation modes arising from the nesting across the disconnected Fermi surfaces realize an extended s-wave pairing, while d-wave pairing can also be another candidate.Comment: 5 pages, 2 figures, to be published in Phys. Rev. Let