Symmetry-adapted BCS-type trial wave functions for mesoscopic rings - art.no. 155308

In mesoscopic systems, the symmetry of the wave function cannot be broken a nd must be considered. We construct symmetry-adapted trial wave functions o f the Bardeen-Cooper-Schrieffer-type for mesoscopic; rings described by an extended-Hubbard model at half-filling. The comparison with exact numerical (Lanczos) diagonalization in small rings indicates that this variational a pproach is reasonably accurate. Within this approach, we demonstrate a cros sover from a weak-correlation regime to a strong correlation one. Particula rly interesting is the behavior of the lowest-excitation energy, which swit ches from a highest-occupied-molecular-orbital-lowest-unoccupied-molecular- orbital gap to a splitting energy related to a collective tunneling of elec trons.