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 $t_{\mathrm{AB}}=0\mathrm{}$. 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 $t_{\mathrm{AB}}$ breaks the degeneracy and for moderate repulsive $U$ introduce superconducting correlations which lead to “anomalous” flux quantization.