Pairing in the Hubbard model: the Cu5O4 cluster versus the Cu-O plane
Istituto Nazionale di Fisica della Materia, Dipartimento di Fisica,
Universita' di Roma Tor Vergata, Via della Ricerca Scientifica, 1, 00133
Published online: 15 March 2000
We study the Cu5O4 cluster by exact diagonalization of a three-band Hubbard model and show that bound electron or hole pairs are obtained at appropriate fillings, and produce superconducting flux quantization. The results extend earlier cluster studies and illustrate a canonical transformation approach to pairing that we have developed recently for the full plane. The quasiparticles that in the many-body problem behave like Cooper pairs are W=0 pairs, that is, two-hole eigenstates of the Hubbard Hamiltonian with vanishing on-site repulsion. The cluster allows W=0 pairs of d symmetry, due to a spin fluctuation, and s symmetry, due to a charge fluctuation. Flux quantization is shown to be a manifestation of symmetry properties that hold for clusters of arbitrary size.
PACS: 74.20.Mn – Nonconventional mechanisms (spin fluctuations, polarons and bipolarons, resonating valence bond model, anyon mechanism, marginal Fermi liquid, Luttinger liquid, etc.) / 71.10.Li – Excited states and pairing interactions in model systems / 74.72.-h – High-Tc compounds
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2000