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 $η$-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. PACS numbers: 74.20Mn, 71.27.+a, 71.30+h, 71.28.+d

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