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 lower [bending motion along c of the planar oxygen, O(3)] to the higher [vibrations of the apical oxygen, O(1)]. This doping-induced transfer of oscillator strength to the apical oxygen, accompanied by a sudden increase in asymmetry in the normal-state line shape of metallic samples which exhibit superconductivity at low temperatures, suggests that this process may play a role in the mechanism leading to the superconducting pairing. The linewidth of these two phonons shows a maximum as a function of doping near which, upon entry into the superconducting state, there is further anomalous broadening. A third phonon, attributed to vibrations of oxygen within the O(2)-Cu(1)-O(2) ‘‘sticks’’ and thus spatially removed from the CuO2 planes, shows no unusual behavior. These observations are consistent with a strong interaction between the phonons and the carriers of the structural block containing the CuO2 planes.