We investigate the effects of gap fluctuations on the pair-transfer correlation and spectral functions in nanoscopic superconducting grains at finite temperature, by means of the correlated static path approximation (CSPA). The present approach is able to provide a reliable description of the lowest energy weighted moments of the spectral function of small samples in critical regions, improving both the random-phase approximation (RPA) and the SPA predictions. The results confirm the persistence of pairing effects in the spectral function beyond the BCS critical temperatures and sizes, which is visible through the enhancement of the strength at low energies and the concomitant decrease in the normalized first energy moment. The role played by the zero and the imaginary quasiparticle RPA energies present in the CSPA is also discussed.

• Received 29 July 2002

DOI:https://doi.org/10.1103/PhysRevB.67.144517