Superconductivity Driven by the Interband Coulomb Interaction and Implications for the Superconducting Mechanism of MgB2

Superconducting mechanism mediated by interband exchange Coulomb repulsion is examined in an extended two-band Hubbard models with a wide band crossing the Fermi level and coexisting with a narrower band located at moderately lower energy. We apply newly developed path-integral renormalization group method to reliably calculate pairing correlations. The correlation shows marked enhancement at moderate amplitudes of the exchange Coulomb repulsion taken smaller than the on-site repulsion for the narrower band. The pairing symmetry is s-wave while it has unconventional phases with the opposite sign between the order parameters on the two bands, in agreement with the mean-field prediction. Since the band structure of recently discovered superconductor MgB$_2$ shares basic similarities with our model, we propose that the present results provide a relevant clue for the understanding of the superconducting mechanism in MgB$_2$ as well as in this class of multi-band materials with good metallic conduction in the normal state.Comment: 4pages, 2figure

Transition from Band insulator to Bose-Einstein Condensate superfluid and Mott State of Cold Fermi Gases with Multiband Effects in Optical Lattices

We study two models realized by two-component Fermi gases loaded in optical lattices. We clarify that multi-band effects inevitably caused by the optical lattices generate a rich structure, when the systems crossover from the region of weakly bound molecular bosons to the region of strongly bound atomic bosons. Here the crossover can be controlled by attractive fermion interaction. One of the present models is a case with attractive fermion interaction, where an insulator-superfluid transition takes place. The transition is characterized as the transition between a band insulator and a Bose-Einstein condensate (BEC) superfluid state. Differing from the conventional BCS superfluid transition, this transition shows unconventional properties. In contrast to the one particle excitation gap scaled by the superfluid order parameter in the conventional BCS transition, because of the multi-band effects, a large gap of one-particle density of states is retained all through the transition although the superfluid order grows continuously from zero. A reentrant transition with lowering temperature is another unconventionality. The other model is the case with coexisting attractive and repulsive interactions. Within a mean field treatment, we find a new insulating state, an orbital ordered insulator. This insulator is one candidate for the Mott insulator of molecular bosons and is the first example that the orbital internal degrees of freedom of molecular bosons appears explicitly. Besides the emergence of a new phase, a coexisting phase also appears where superfluidity and an orbital order coexist just by doping holes or particles. The insulating and superfluid particles show differentiation in momentum space as in the high-Tc cuprate superconductors.Comment: 13 pages, 10 figure

Spin-gap phase in nearly-half-filled one-dimensional conductors coupled with phonons

Asymptotic properties of nearly-half-filled one-dimensional conductors coupled with phonons are studied through a renormalization group method. Due to spin-charge coupling via electron-phonon interaction, the spin correlation varies with filling as well as the charge correlation. Depending on the relation between cut-off energy scales of the Umklapp process and of the electron-phonon interaction, various phases appear. We found a metallic phase with a spin gap and a dominant charge- density-wave correlation near half filling between a gapless density-wave phase (like in the doped repulsive Hubbard model) and a superconductor phase with a spin gap. The spin gap is produced by phonon-assisted backward scatterings which are interfered with the Umklapp process constructively or destructively depending on the character of electron-phonon coupling.Comment: 14 pages, revtex, replaced 5 ps figures, published in PR

Superconductivity from Flat Dispersion Designed in Doped Mott Insulators

Routes to enhance superconducting instability are explored for doped Mott insulators. With the help of insights for criticalities of metal-insulator transitions, geometrical design of lattice structure is proposed to control the instability. A guideline is to explicitly make flat band dispersions near the Fermi level without suppressing two-particle channels. In a one-dimensional model, numerical studies show that our prescription with finite-ranged hoppings realizes large enhancement of spin-gap and pairing dominant regions. We also propose several multi-band systems, where the pairing is driven by intersite Coulomb repulsion.Comment: 4 pages, to be published in Phys. Rev. Let

Gapped spin liquid states in a one-dimensional Hubbard model with antiferromagnetic exchange interaction

We study the phase diagram of a one-dimensional extended Hubbard model with antiferromagnetic exchange interaction analytically and numerically. The bosonization and transfer-matrix renormalization group methods are used in the corresponding coupling regimes. At half-filling, the system is a Mott insulator with a finite spin excitation gap if the on-site Coulomb repulsion is fairly smaller than the antiferromagnetic exchange J. This Mott-insulator is characterized by the bond-charge-density-wave order or spontaneously dimerization. In the weak-coupling regime where the spin-charge separation holds approximately, the critical point separating the gapless and gapped spin liquid phases is U_c\sim J/2. However, as J increases, the spin-charge couplings become important and the critical point U_c is significantly suppressed and eventually tends to zero as J\to \infty. Away from half-filling, the charge gap completely collapses but the spin gap persists.Comment: 5 pages, 2 figures, to appear in PR

A revised checklist of Hawaiian mosses

A revised and updated literature-based checklist of Hawaiian mosses is presented. Geographic coverage includes the eight main Hawaiian Islands; the Northwestern Hawaiian Islands are excluded. The checklist is alphabetically ordered by scientific names; the family is noted for each genus. Synonyms and misapplied names are cross-referenced to the accepted names. A bibliography of supporting references is included

Fate of Quasiparticle at Mott Transition and Interplay with Lifshitz Transition Studied by Correlator Projection Method

Filling-control metal-insulator transition on the two-dimensional Hubbard model is investigated by using the correlator projection method, which takes into account momentum dependence of the free energy beyond the dynamical mean-field theory. The phase diagram of metals and Mott insulators is analyzed. Lifshitz transitions occur simultaneously with metal-insulator transitions at large Coulomb repulsion. On the other hand, they are separated each other for lower Coulomb repulsion, where the phase sandwiched by the Lifshitz and metal-insulator transitions appears to show violation of the Luttinger sum rule. Through the metal-insulator transition, quasiparticles retain nonzero renormalization factor and finite quasi-particle weight in the both sides of the transition. This supports that the metal-insulator transition is caused not by the vanishing renormalization factor but by the relative shift of the Fermi level into the Mott gap away from the quasiparticle band, in sharp contrast with the original dynamical mean-field theory. Charge compressibility diverges at the critical end point of the first-order Lifshitz transition at finite temperatures. The origin of the divergence is ascribed to singular momentum dependence of the quasiparticle dispersion.Comment: 24 pages including 10 figure

Optical Conductivity of the Two-Dimensional Hubbard Model

Charge dynamics of the two-dimensional Hubbard model is investigated. Lancz$\ddot{\rm o}$s-diagonalization results for the optical conductivity and the Drude weight of this model are presented. Near the Mott transition, large incoherence below the upper-Hubbard band is obtained together with a remarkably suppressed Drude weight in two dimensions while the clearly coherent character is shown in one dimension. The two-dimensional results are consistent with previous results from quantum Monte Carlo calculations indicating that the Mott transition in this two-dimensional model belongs to the universality class characterized by the dynamical exponent of $z=4$.Comment: 4 pages LaTeX including 2 PS figures, to appear in J. Phys. Soc. Jp