In an Eliashberg superconductor at zero temperature, there is a clear gap edge as well as phonon structure in the quasiparticle density of states at an energy equal to the gap plus the phonon energy. Here we study how gap and phonon structure shift and get altered with finite temperature and with the introduction of anisotropy when dealing with a layered structure as in the copper oxides. For moderate coupling, gap and phonon structure follow closely the usual BCS temperature variation of the gap, but this no longer holds in very strong coupling where the concept of gap is lost as temperature is increased.

• Received 20 September 1991

DOI:https://doi.org/10.1103/PhysRevB.45.7368