-pairing superconductivity
in the spin-polarized strong-coupling negative-U Hubbard model,
Phys. Rev. B 46 (1992) 8409
J. Stein, R. Oppermann:
-pairing superconductivity
in the spin-polarized strong-coupling negative-U Hubbard model,
Phys. Rev. B 46 (1992) 8409
We have calculated the effect of a single unpaired electron on
superconductivity in the strong coupling negative-U Hubbard model,
employing a diagram technique with generalized Matsubara frequencies.
Using a generalization of the Hamiltonian to n orbitals per site and
applying a geometry-controlled approximation in the limit of high
coordination numbers z, we perform a loop expansion
of the model. This allows to incorporate fluctuation corrections to the
mean field solution which emerges as result in lowest order of the
loop expansion.
We show that in the limit U -> infinity the mean field solution
exhibits a phase
transition to an antiferrosuperconducting state which is
characterized by a "staggered" local superconducting order parameter
with its sign alternating from site to site.
This superconducting state (referred to as
"-pairing" state by a
number of authors)
for the attractive case is related to the Nagaoka ground state of the
repulsive Hubbard model by a partial particle-hole transformation.
We have found that the model in the slightly spin-polarized case
(i.e. when a
single unpaired electron is present) for highly coordinated lattices
and in lowest loop order
is equivalent to a pair hopping model with
temperature dependent repulsive coupling.
In this paper we report the mean field results for the transition temperature,
the order parameter, the chemical potential,
the upper critical magnetic field Hc2
and excitation spectra
both above and below Tc and demonstrate that the system
exhibits a complete Meissner effect.