We calculate exact ground-state energies for small Hubbard-model clusters (molecules) and show that two electrons added to a molecule can experience an effective attraction despite their bare repulsive interaction (pair binding). This phenomenon takes place IonlyP for intermediate strength of the repulsive interactions, and on intermediate length scales. All the results are in qualitative agreement with those obtained using second-order perturbation theory up to surprisingly large values of the interactions and provide support for a purely electronic mechanism of superconductivity in doped ${\mathrm{C}}_{60}$ .We calculate exact ground-state energies for small Hubbard-model clusters (molecules) and show that two electrons added to a molecule can experience an effective attraction despite their bare repulsive interaction (pair binding). This phenomenon takes place IonlyP for intermediate strength of the repulsive interactions, and on intermediate length scales. All the results are in qualitative agreement with those obtained using second-order perturbation theory up to surprisingly large values of the interactions and provide support for a purely electronic mechanism of superconductivity in doped ${\mathrm{C}}_{60}$.

Physical review. B, Condensed matterMar 1, 199265 citations

Small-cluster calculations for the simple and extended Hubbard models

J. Callaway, D. P. Chen, D. G. Kanhere +1 more

The single-band Hubbard model (both with and without first-neighbor interactions) is studied by exact diagonalization on several one- and two-dimensional clusters, ranging from four to nine sites, including rings and arrangements of squares. Occupancies include the half-filled band (one electron per site) and configurations with one and two holes in the half-filled band. Ground-state properties are determined for a wide range of parameters. We discuss the phase diagram in the (U,V) plane and consider spin, charge, and pairing correlations. The binding of a pair of holes is investigated. Circumstances in which binding and pairing are enhanced by first-neighbor interactions are described.The single-band Hubbard model (both with and without first-neighbor interactions) is studied by exact diagonalization on several one- and two-dimensional clusters, ranging from four to nine sites, including rings and arrangements of squares. Occupancies include the half-filled band (one electron per site) and configurations with one and two holes in the half-filled band. Ground-state properties are determined for a wide range of parameters. We discuss the phase diagram in the (U,V) plane and consider spin, charge, and pairing correlations. The binding of a pair of holes is investigated. Circumstances in which binding and pairing are enhanced by first-neighbor interactions are described.

Physical review. B, Condensed matterJul 1, 199041 citations

{\mathrm{C}}_{60}