Some considerations on the additional absorption peak in the c-axis infrared conductivity of bilayer cuprate superconductors

Changes of the 400 cm−1 peak in the c-axis conductivity of underdoped YBa2Cu3O6.6 upon application of a parallel magnetic field reported by Kojima et al. are shown to be consistent with the model where the peak is due to the superfluid. Results of our calculations of the c-axis response of bilayer compounds with well defined bilayer–split bands are presented and discussed. For moderate values of the bilayer splitting (Δε comparable to 2Δmax) the spectra of the superconducting state exhibit an additional mode which is due to the condensate and similar to the one of earlier phenonomenological approaches.