Anomalous flux quantization in a Hubbard ring with correlated hopping

We solve exactly a generalized Hubbard ring with twisted boundary conditions. The magnitude of the nearest-neighbor hopping depends on the occupations of the sites involved and the term which modifies the number of doubly occupied sites t(AB) = 0. Although eta-pairing states with off-diagonal long-range order are part of the degenerate ground state, the behavior of the energy as a function of the twist rules out superconductivity in this limit. A small t(AB) breaks the degeneracy and for moderate repulsive U introduce superconducting correlations which lead to ''anomalous'' flux quantization.