Some Considerations on the Additional Absorption Peak in the c-axis Conductivity of Bilayer Cuprate Superconductors: Interpretation of the Changes Induced by a Parallel Magnetic Field and the Role of Bilayer Splitting

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 (Delta E comparable to 2Delta_{max}) the spectra of the superconducting state exhibit an additional mode which is due to the condensate and similar to the one of earlier phenomenological approaches.