Boson-fermion model: An exact diagonalization study

The main features of a generic boson-fermion scenario for electron pairing in a many-body correlated fermionic system are (i) a crossover from a poor metal to an insulator and finally a superconductor as the temperature decreases; (ii) the buildup of a finite amplitude of local electron pairing below a certain temperature T-*, followed by the onset of long-range phase correlations among electron pairs below a second characteristic temperature T-phi; and (iii) the opening of a pseudogap in the density of states of the electrons below T-*, rendering these electrons poorer and poorer quasi-particles as the temperature decreases, with the electron transport becoming ensured by electron pairs rather than by individual electrons. A number of these features have been obtained so far on the basis of different many-body techniques, all of which have their built-in shortcomings in the intermediate coupling regime, which is of interest here. In order to substantiate these features, we investigate them on the basis of an exact diagonalization study on rings up to eight sites. Particular emphasis has been put on the possibility of having persistent currents in mesoscopic rings tracking the change-over from single- to two-particle transport as the temperature decreases and the superconducting state is approached.