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
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.
PACS: 71.27.+a – Strongly correlated electron systems; heavy fermions / 74.20.Mn – Nonconventional mechanisms / 73.22.-f – Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals
© EDP Sciences, Società Italiana di Fisica, Springer-Verlag, 2003