Repulsion-sustained supercurrent and flux quantization in rings of symmetric Hubbard clusters

We test the response to a threading magnetic field of rings of five-site C4v-symmetric repulsive Hubbard clusters connected by weak intercell links; each five-site unit has the topology of a CuO4 cluster and a repulsive interaction is included on every site. In a numerical study of the three-unit ring with eight particles, we take advantage of an exact-diagonalization technique which can be generally applied to many-fermion problems. For O-O hopping we find a superconducting flux quantization (SFQ), but for purely Cu-Cu links bound pair propagation is hindered by symmetry. The results agree with W=0 pairing theory.