Interlayer Coupling and the Metal-Insulator Transition in Pr-substituted Bi2Sr2CaCu2O8+y

Substitution of rare earth ions for Ca in Bi2Sr2CaCu2O8+y is known to cause a metal-insulator transition. Using resonant photoemission we study how this chemical substitution affects the electronic structure of the material. For the partial Cu-density of states at EF and in the region of the valence band we observe no significant difference between a pure superconducting and an insulating sample with 60 % Pr for Ca substitution. This suggests that the states responsible for superconducting are predominately O-states. The Pr-4f partial density of states was extracted utilizing the Super-Coster-Kronig Pr 4d-4f resonance. It consists of a single peak at 1.36 eV binding energy. The peak shows a strongly asymmetric Doniach-Šunjic lineshape indicating the presence of a band of electronic states with a cut off at EF even in this insulating sample. This finding excludes a band gap in the insulating sample and supports the existence of a mobility gap caused by spatial localization of the carriers. The presence of such carriers at the Pr-site between the CuO2-planes shows that the electronic structure is no