OPTICAL INVESTIGATION OF THE METAL-INSULATOR-TRANSITION IN THE CA-FREE PB2SR2LCU3O8 (L = Y, DY, EU, SM, ND, AND PR) SERIES

The ab-plane dc resistivity and reflectivity of single crystals of the Ca-free Pb2Sr2LCu3O8 series, where L = Y, Dy, Eu, Sm, Nd, and Pr, has been investigated. A trend is observed from insulating to metallic and superconducting behavior with decreasing size of the rare-earth constituent. A transfer of spectral weight from the Cu-O charge-transfer band to the mid- and far-infrared regions accompanies the progression to metallic character. As the electronic background develops, phonons visible in the far-infrared reflectance of the insulating members (Nd and Pr) become increasingly more screened, while concurrently, there is an unexpected appearance of phonons in the 400-600-cm-1 region of the spectrum. These phonons develop an antiresonantlike line shape at higher doping levels that we attribute to coupling to the mid-infrared continuum. Comparison to Raman and neutron work suggests the presence of a universal band near 400 cm-1.