Anomalous temperature- and doping-induced changes in the c-axis apical-oxygen phonon mode of Pb2Sr2RCu3O8

We observe via polarized far-infrared-reflectance measurements of single-crystal samples of Pb2Sr2RCu3O8, where R is a rare-earth element, significant doping and temperature-induced changes in two c-axis phonons assigned to motion within the structural block containing the double CuO2 planes. In the normal state both phonons shift to lower frequency as the level of doping increases, with a monotonic transfer of oscillator strength from the …