Eur. Phys. J. B 34, 455-466 (2003)
https://doi.org/10.1140/epjb/e2003-00244-x

Symmetric Hubbard systems with superconducting magnetic response

Istituto Nazionale per la Fisica della Materia, Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica, I-00133 Roma, Italy

Corresponding author: ^a cini@roma2.infn.it

Received: 13 May 2003
Revised: 14 July 2003
Published online: 9 September 2003

Abstract

In purely repulsive, -symmetric Hubbard clusters a correlation effect produces an effective two-body attraction and pairing; the key ingredient is the availability of W=0 pairs, that is, two-body solutions of appropriate symmetry. We study the tunneling of bound pairs in rings of 5-site units connected by weak intercell links; each unit has the topology of a CuO₄ cluster and a repulsive interaction is included on every site. Further, we test the superconducting nature of the response of this model to a threading magnetic field. We present a detailed numerical study of the two-unit ring filled with 6 particles and the three-unit ring with 8 particles; in both cases a lower filling yields normal behavior. In previous studies on 1d Hubbard chains, level crossings were reported (half-integer or fractional Aharonov-Bohm effect) which however cannot be due to superconducting pairs. In contrast, the nontrivial basis of clusters carrying W=0 pairs leads to genuine Superconducting Flux Quantization (SFQ). The data are understood in terms of a cell-perturbation theory scheme which is very accurate for weak links. This low-energy approach leads to an effective hard core boson Hamiltonian which naturally describes itinerant pairs and SFQ in mesoscopic rings. For the numerical calculations, we take advantage of a recently proposed exact diagonalization technique which can be generally applied to many-fermion problems and drastically reduces the size of the matrices to be handled.