PATH-INTEGRAL APPROACH TO COOPER PAIRS AND CHARGED BOSONS

With the functional-integral technique, equations for the excitation spectrum of fermions with an arbitrary strong attraction are obtained. The evolution of the BCS excitation spectrum in the weak-coupling regime to the excitations of a Bose gas in the strong-coupling regime is discussed. To obtain the Bogolubov sound in the strong-coupling regime, four-particle correlations are taken into account. The functional integration allows a discussion of the excitations at a finite temperature and a controlled treatment of the problem of maximum attainable T(c).