Motivated by the short coherence length of the high-temperature superconductors, we study in the negative-U Hubbard model the crossover from Cooper pairs for U≪t to composite bosons for U≫t. We compute the collective-mode spectrum using a generalized random-phase-approximation analysis within the equations-of-motion formalism. We find a smooth evolution of the Anderson mode for weak coupling into the Bogoliubov sound mode for hard-core bosons.

• Received 8 August 1991

DOI:https://doi.org/10.1103/PhysRevB.45.5087