CROSSOVER BETWEEN BCS AND PREFORMED-BOSON THEORIES WITH INCREASING INTERACTIONS

I review recent work on Fermi systems with attractive interactions of arbitrary strength. First, I describe the results of a functional inte gral formulation to study the crossover from cooperative Cooper pairin g to independent bound state formation and condensation. The inadequac y of a saddle point approximation with increasing coupling is pointed out, and the importance of temporal (quantum) fluctuations for normal state properties at intermediate and strong coupling is emphasized. Ne xt, results from a quantum Monte-Carlo simulation of the two-dimension al (2D) attractive Hubbard model are described. The intermediate coupl ing normal state is found to deviate markedly from a canonical Fermi l iquid: ''spin-gap'' behavior, with 1/T1T approximately chi(T), is foun d in a degenerate Fermi system.