An extended multiband Hubbard model of the two-dimensional Cu
O2 planes of high-
Tc superconductors is examined by exact diagonalization of the Hamiltonian for small clusters with periodic boundary conditions. Holes are shown to bind in the limit of large Cu-site and nearest-neighbor Cu-O Coulomb repulsion. A simple explanation of the source of the binding energy is given in this strong-coupling limit. The more realistic case of relatively weak nearest-neighbor repulsion is also examined, and suggestive evidence is presented that the effective attraction will also exist in the thermodynamic limit for this case.
We consider the multiband, purely repulsive Hubbard model on a class of systems in which there are two (or more) kinds of orbitals per unit cell such that one metallic band interacts with the other insulating band. From both perturbational canonical transformation and quantum Monte Carlo studies we have shown that superconductivity due to nonretarded attraction (negative U) is a generic feature of this class of metal-insulator composite many-body systems. Remarkably, the superconductivity arises, under appropriate conditions, for various types of insulating bands (a charge-transfer insulator, Mott-Hubbard insulator, etc.). Superconductivity is unambiguously identified from the pairing correlation function, which agrees with that of the attractive Hubbard model surprisingly well even in the nonperturbative regime. The superconductivity revealed here is in a sense an opposite limit to conventional superconductivity with retarded attraction from boson-exchange mechanisms. We also show that the present mechanism is distinct from existing multiband models such as d-${\mathit{p}}_{\mathrm{\ensuremath{\sigma}}}$ models. As we increase the magnitude of the attraction in the present model by changing some parameters (e.g., to make the accompanying band metallic), the similarity with the attractive Hubbard model becomes eventually degraded, so that the optimum condition for superconductivity is determined by this kind of tradeoff. We also discuss the relevance of the present model to high-
Tc superconductivity in cuprates and fullerenes.
Phase diagram of the two-band Fermi gas with repulsive intraband and interband interactions is obtained with the bosonization technique. In the forward-scattering model, there is a region for the superconductivity (SC) within a band due to charge fluctuations in the accompanying band. When the backward scattering is relevant the SC region becomes wider, while when the umklapp scattering is relevant the system is renormalized into two independent single bands, which is considered to cover the regime where superconductivity a\ifmmode\acute\else\textasciiacute\fi{} la single-band attractive Hubbard model is found from quantum Monte Carlo results in the multiband model on discrete lattices.
High-temperature superconductivity emerges in many different quantum materials, often in regions of the phase diagram where the electronic kinetic energy is comparable to the electron-electron repulsion. Describing such intermediate-coupling regimes has proven challenging as standard perturbative approaches are inapplicable. Here, we employ quantum Monte Carlo methods to solve a multiband Hubbard model that does not suffer from the sign problem and in which only repulsive interband interactions are present. In contrast to previous sign-problem-free studies, we treat magnetic, superconducting, and charge degrees of freedom on an equal footing. We find an antiferromagnetic dome accompanied by a metal-to-insulator crossover line in the intermediate-coupling regime, with a smaller superconducting dome appearing in the metallic region. Across the antiferromagnetic dome, the magnetic fluctuations change from overdamped in the metallic region to propagating in the insulating region. Our findings shed new light on the intertwining between superconductivity, magnetism, and charge correlations in quantum materials.
The leading superconducting instabilities of the two-dimensional extended repulsive one-band Hubbard model within spin-fluctuation pairing theory depend sensitively on electron density, band, and interaction parameters. We map out the phase diagrams within a random-phase-approximation spin- and charge-fluctuation approach, and find that while
B1g (${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$) and
B2g (
dxy) pairing dominates in the absence of repulsive longer-range Coulomb interactions
VNN, the latter induces pairing in other symmetry channels, including, e.g.,
A2g (
g-wave), nodal
A1g (extended
s-wave), or nodal
Eu (
p-wave) spin-triplet superconductivity. At the lowest temperatures, transition boundaries in the phase diagrams between symmetry-distinct spin-singlet orders generate complex time-reversal symmetry broken superpositions. By contrast, we find that boundaries between singlet and triplet regions are characterized by first-order transitions. Finally, motivated by recent photoemission experiments, we have determined the influence of an additional explicitly attractive nearest-neighbor interaction,
VNN<0, on the superconducting gap structure. Depending on the electronic filling, such an attraction boosts
Eu (
p-wave) spin-triplet or
B1g (${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$) spin-singlet ordering.