Spin-fluctuation exchange study of superconductivity in two- and three-dimensional single-band Hubbard models (original) (raw)
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Journal of the Physical Society of Japan, 2000
We have systematically studied superconducting instabilities in the repulsive Hubbard model for d-wave and p-wave pairing in various 2D and 3D lattices. Using fluctuation exchange approximation, we consider 3D face centered cubic lattice, 3D body centered cubic lattice, 3D simple cubic lattice, 2D square lattice and 2D triangular lattice, where either strong ferro-or antiferromagnetic spin fluctuation is present. We show that (i) d-wave instability mediated by antiferromagnetic spin fluctuations is stronger than p-wave instability mediated by ferromagnetic spin fluctuations both in 2D and 3D, and (ii)d-wave instability in 2D is much stronger than that in 3D. These amount that the "best" situation is the antiferromagnetic-fluctuation mediated in 2D as far as the single-band Hubbard model on ordinary lattices are concerned.
Physica B: Condensed Matter, 2005
In order to explore how superconductivity arises when charge fluctuations and spin fluctuations coexist, we have obtained a phase diagram against the off-site repulsion V and band filling n for the extended, repulsive Hubbard model on the square lattice with the fluctuation exchange approximation. We have found the existence of (i) a transition between dxy and d x 2 −y 2 pairing symmetries, (ii) f-pairing in between the d x 2 −y 2 and CDW phases for intermediate 0.5 < n < 1.0 and large V , and (iii) for anisotropic cases the pairing symmetry changing, in agreement with a previously proposed "generic phase diagram", as d → f → s when V (hence the charge fluctuations) are increased. All these are consequences of the structure in the charge and spin susceptibilities, which have peaks habitating at common or segregated positions in k space.
Interplay between magnetic and superconducting fluctuations in the doped 2d Hubbard model
2019
We study the Hubbard model on a square lattice, using the dynamical vertex approximation and the parquet approximation. These methods allow us to describe the mutual interference of spin-fluctuations in the particle-hole channel and superconducting fluctuations in the cooperon channel in an unbiased way. For small dopings we find predominant commensurable antiferromagnetic spin- and d-wave superconducting fluctuations; for larger doping incommensurate antiferromagnetic spin fluctuations are concomitant to triplet s-wave superconducting fluctuations.
Unconventional superconductivity on the triangular lattice Hubbard model
Physical Review B, 2013
Using large-scale dynamical cluster quantum Monte Carlo simulations, we explore the unconventional superconductivity in the hole-doped Hubbard model on the triangular lattice. Due to the interplay of electronic correlations, geometric frustration, and Fermi surface topology, we find a doubly degenerate singlet pairing state at an interaction strength close to the bare bandwidth. Such an unconventional superconducting state is mediated by antiferromagnetic spin fluctuations along the Γ-K direction, where the Fermi surface is nested. An exact decomposition of the irreducible particle-particle vertex further confirms the dominant component of the effective pairing interaction comes from the spin channel. Our findings suggest the existence of chiral d + id superconductivity in hole-doped Hubbard triangular lattice in strongly correlated regime, and provide insight to the superconducting phases of the water-intercalated sodium cobaltates NaxCoO2 • yH2O, as well as the organic compounds κ-(ET)2X and Pd(dmit)2.
Superconducting Fluctuations in the Normal State of the Two-Dimensional Hubbard Model
Physical review letters, 2015
We compute the two-particle quantities relevant for superconducting correlations in the two-dimensional Hubbard model within the dynamical cluster approximation. In the normal state we identify the parameter regime in density, interaction, and second-nearest-neighbor hopping strength that maximizes the d_{x^{2}-y^{2}} superconducting transition temperature. We find in all cases that the optimal transition temperature occurs at intermediate coupling strength, and is suppressed at strong and weak interaction strengths. Similarly, superconducting fluctuations are strongest at intermediate doping and suppressed towards large doping and half filling. We find a change in sign of the vertex contributions to d_{xy} superconductivity from repulsive near half filling to attractive at large doping. p-wave superconductivity is not found at the parameters we study, and s-wave contributions are always repulsive. For negative second-nearest-neighbor hopping the optimal transition temperature shift...
Superconductivity in the Hubbard model with pair hopping
Physical Review B, 1999
The phase diagrams and superconducting properties of the extended Hubbard model with pair hopping interaction, i.e. the Penson-Kolb-Hubbard model are studied. The analysis of the model is performed for d-dimensional hypercubic lattices, including d = 1 and d = ∞, by means of the (broken symmetry) Hartree-Fock approximations and, for d = ∞, by the slave-boson mean-field method. For d = 1, at half-filling the phase diagram is shown to consist of nine different phases including two superconducting states with center-of-mass momentum q = 0 and q = Q (η-pairing), site and bond-located antiferromagnetic and charge-density wave states as well as three mixed phases with coexisting site and bond orderings. The stability range of the bond-type orderings is shrank with increasing lattice dimensionality d and for d = ∞ the corresponding diagram consists of four phases only, involving exclusively site-located orderings. Comparing the pair hopping model with the attractive Hubbard model we found in the both cases gradual evolution from the BCS-like limit to the tightly bound pairs regime and a monotonic increase of the gap in the excitation spectrum with increasing coupling. However, the dynamics of electron pairs in both models is qualitatively different, which results in different dependences of condensation energies and critical temperatures on interaction parameters as well as in different electrodynamic properties, especially in a strong coupling regime. 71.27.+a, 75.30.Fr, 71.45.Lr
Spin-Triplet Superconductivity Induced by Charge Fluctuations in Extended Hubbard Model
Journal of the Physical Society of Japan, 2005
The pairing symmetry in the electron mechanism for superconductivity is explored when charge fluctuations coexist with spin fluctuations. The extended Hubbard model is adopted to obtain, with the fluctuation exchange approximation, a phase diagram against the on-site Coulomb repulsion U and the off-site repulsion V for the square lattice with second-neighbor hopping t ′. We have found that (i) for large U (> 9) a triplet superconductivity with a sin(kx + ky) symmetry can appear just below the charge density wave phase. The pairing is degenerate with sin(kx − ky), so a chiral sin(kx+ky)+i sin(kx−ky) that breaks the time reversal symmetry should result, which is a candidate for the gap function on the γ band of Sr2RuO4 and is consistent with a recent measurement of the specific heat. (ii) By systematically deforming the Fermi surface with varied t ′ , we have identified the region where the triplet pairing is most favored to be the region where the Fermi surface traverses the van Hove singularity with the charge susceptibility strongly enhanced.
Magnetic and superconducting correlations in the two-dimensional Hubbard model
physica status solidi (b), 2006
The interplay and competition of magnetic and superconducting correlations in the weakly interacting two-dimensional Hubbard Model is investigated by means of the functional renormalization group. At zero temperature the flow of interactions in one-loop approximation evolves into a strong coupling regime at low energy scales, signalling the possible onset of spontaneous symmetry breaking. This is further analyzed by a mean-field treatment of the strong renormalized interactions which takes into account magnetic and superconducting order simultaneously. The effect of strong correlations on single-particle properties in the normal phase is studied by calculating the flow of the self-energy.
Superconductivity in a generalized Hubbard model
Physica C: Superconductivity, 1997
We consider a Hubbard model in the square lattice, with a generalized hopping between nearest-neighbor sites for spin up (down), which depends on the total occupation n b of spin down (up) electrons on both sites. We call the hopping parameters tAA, tAB, and tBB for n b = 0, 1 or 2 respectively. Using the Hartree-Fock and Bardeen-Cooper-Schrieffer mean-field approximations to decouple the two-body and three-body interactions, we find that the model exhibits extended s-wave superconductivity in the electron-hole symmetric case tAB > tAa = tBB for small values of the Coulomb repulsion U or small band fillings. For moderate values of U, the antiferromagnetic normal (AFN) state has lower energy. The translationally invariant d-wave superconducting state has always larger energy than the AFN state.