Frequency Shifts and Linewidth Changes of Infrared-Active Phonons in Double-Layered High-Temperature Superconductors

We calculate frequency shifts and changes in linewidths of infrared-active phonons within a shell model for the bare phononic system coupled to an electronic double-layer structure with inter-layer charge transfer. The theoretical concept is applied to YBaCuO yielding a good description of experimental results in the normal state as well as at the transition to the superconducting state.