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arXiv:cond-mat/0306163 · Physical Review B

Possibility of superconductivity in the repulsive Hubbard model on the Shastry-Sutherland lattice

T. Kimura, Kazuhiko Kuroki, Ryotaro Arita, Hideo Aoki

Published Feb 6, 2004• 15 citations
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Abstract

Possibility of superconductivity from electron repulsion in the Shastry-Sutherland lattice, which has a spin gap at half filling, is explored with the repulsive Hubbard model in the fluctuation-exchange approximation. We find that, while superconductivity is not favored around the half filling, superconductivity is favored around the quarter filling. Our results suggest that the Fermi-surface nesting is more important than the spin dimerization for superconductivity.

DOI
10.1103/physrevb.69.054501

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Possibility of superconductivity from electron repulsion in the Shastry-Sutherland lattice, which has a spin gap at half filling, is explored with the repulsive Hubbard model in the fluctuation-exchange approximation. We find that, while superconductivity is not favored around the half filling, superconductivity is favored around the quarter filling. Our results suggest that the Fermi-surface nesting is more important than the spin dimerization for superconductivity.
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We explore how the superconductivity arising from the on-site electron-electron repulsion changes when the repulsion is made long-ranged, 1∕r1∕r1∕r-like interaction by introducing an extended Hubbard model with the repulsion extended to distant (12th) neighbors. With a simplified fluctuation-exchange approximation, we have found for the square lattice that: (i) As the band filling becomes dilute enough, the charge susceptibility becomes comparable with the spin susceptibility, where ppp- and then sss-wave pairings become relatively dominant, in agreement with the result for the electron gas by Takada, while (ii) the ddd wave, which reflects the lattice structure, dominates well away from the half-filling. All of these can be understood in terms of the spin and charge structures along with the shape and size of the Fermi surface.
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The two-dimensional Hubbard model near half-filling is studied using a configuration-interaction method. Superconducting order-parameter and spin-spin correlation functions are calculated as functions of distance to probe for superconductivity and antiferromagnetism. Consistent with our previous results, these more extensive calculations point toward d-wave superconductivity in the ground state of the two-dimensional repulsive Hubbard model. The d-wave correlations tend to increase with interaction strength, size of the state space used in the calculation, and doping away from half-filling. The superconducting correlations on the Fermi surface are interpreted as d-wave pairing between spin density-wave quasiparticles.
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-wave 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 d pairing.
An electronic Hubbard-type model with bond-charge interaction and on-site repulsion U is studied in one and two dimensions. Evidence is provided in favor of superconductivity, without phase separation, for UUc{\mathit{U}}_{\mathit{c}}Uc​(n), although no explicit attraction between particles is present in the Hamiltonian. The presence of a bound state for two particles in vacuum in D=1 and 2 (but not for D=3) suggests that, at low density and low dimensionality, superconductivity is due to condensation of preexistent pairs (dimers). The one-dimensional (1D) phase diagram is studied analytically as well as numerically and shows a transiiton between a Luttinger liquid and a strong-coupling phase with diverging superconducting susceptibility. Estimates of critical exponents are given at low density and at quarter filling for several values of the parameters. The 2D model at quarter filling is analyzed by Lanczos diagonalization on a 4\ifmmode\times\else\texttimes\fi{}4 cluster. The numerical results are consistent with the phase diagram obtained by BCS mean-field theory and is qualitatively similar to the 1D case: a ground state with spin gap and off-diagonal long-range order at UUc{\mathit{U}}_{\mathit{c}}Uc​(n); and a Fermi liquid above Uc{\mathit{U}}_{\mathit{c}}Uc​ with no sign of phase separation.