Phase diagram of the two-dimensional extended Hubbard model: pairing from charge and spin fluctuations (original) (raw)
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Physical Review B, 1999
In order to identify the most favorable situation for superconductivity in the repulsive single-band Hubbard model, we have studied instabilities for d-wave pairing mediated by antiferromagnetic spin fluctuations and p-pairing mediated by ferromagnetic fluctuations with the fluctuation exchange approximation in both two dimensions and three dimensions. By systematically varying the band filling and band structure we have shown that (i) d-pairing is stronger in two dimensions than in three dimensions, and (ii) p-pairing is much weaker than the d-pairing.
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.
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.
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.
Evolution of the superconductivity dome in the two-dimensional Hubbard model
Physical Review B, 2013
In a recent publication [Chen et al., Phys. Rev. B 86, 165136 (2012)], we identified a line of Lifshitz transition points separating the Fermi liquid and pseudogap regions in the hole-doped two dimensional Hubbard model. Here we extend the study to further determine the superconducting transition temperature in the phase diagram. By means of large-scale dynamical cluster quantum Monte Carlo simulations, we are able to identify the evolution of the d-wave superconducting dome in the hole-dope side of the phase diagram, with next-nearest-neighbor hopping (t ′), chemical potential and temperature as control parameters. To obtain the superconducting transition temperature Tc, we employ two-particle measurements of the pairing susceptibilities. As t ′ goes from positive to negative values, we find the d-wave projected irreducible pairing vertex function is enhanced, and the curvature of its doping dependence changes from convex to concave, which fixes the position of the maximum superconducting temperature at the same filling (n ≈ 0.85) and constraints the dome from precisely following the Lifshitz line. We furthermore decompose the irreducible vertex function into fully irreducible, charge and spin components via the parquet equations, and consistently find that the spin component dominates the pairing vertex function in the doping range where the dome is located. Our investigations deepen the understanding of the phase diagram of the two dimensional Hubbard model, and more importantly pose new questions to the field. For example, we found as t ′ goes from positive to negative values, the curvature of the pairing strength as a function of doping changes from convex to concave, and the nature of the dominant fluctuations changes from charge degree of freedom to spin degree of freedom. The study of these issues will lead to further understanding of the phase diagram of the two dimensional Hubbard model and also the physics of the hole-doped cuprate high temperature superconductors.
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...
Disorder effects on superconducting tendencies in the checkerboard Hubbard model
Physical Review B, 2013
The question of whether spatially inhomogeneous hopping in the two dimensional Hubbard model can lead to enhancement of superconductivity has been tackled by a number of authors in the context of the checkerboard Hubbard model (CHM). We address the effects of disorder on superconducting properties of the CHM by using exact diagonalization calculations for both potential and hopping disorder. We characterize the superconducting tendencies of the model by focusing on the pair binding energy, the spin gap, and d-wave pairing order parameter. We find that superconducting tendencies, particularly the pair binding energy, are more robust to disorder when there is inhomogeneous hopping than for the uniform Hubbard model. We also study all possible staggered potentials for an eight site CHM cluster and relate the behaviour of these configurations to the disordered system.
Superconductivity with s and p symmetries in an extended Hubbard model with correlated hopping
The European Physical Journal B, 1998
We consider a generalized Hubbard model with on-site and nearest-neighbour repulsions U and V respectively, and nearest-neighbour hopping for spin up (down) which depends on the total occupation n b of spin down (up) electrons on both sites involved. The hopping parameters are tAA, tAB and tBB for n b = 0, 1, 2 respectively. We briefly summarize results which support that the model exhibits s-wave superconductivity for certain parameters and extend them by studying the Berry phases. Using a generalized Hartree-Fock(HF) BCS decoupling of the two and three-body terms, we obtain that at half filling, for tAB < tAA = tBB and sufficiently small U and V the model leads to triplet p-wave superconductivity for a simple cubic lattice in any dimension. In one dimension, the resulting phase diagram is compared with that obtained numerically using two quantized Berry phases (topological numbers) as order parameters. While this novel method supports the previous results, there are quantitative differences.
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.