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Session Q11 - HTSC: PHOTOEMISSION
Mixed session, Friday morning, March 24, 8:00
Room B3, San Jose Convention Center
In Bi-2212 the substitution of divalent Ca with rare earth ions like Pr is known to cause a transition from a high-temperature superconductor to an insulator. Using resonant photoemission we study the influence of this metal-insulator transition on the electronic structure. For an insulating sample with x_Pr = 0.6 we extract the partial Pr-4f density of states. It consists of a single asymmetric peak at E_B = 1.36 eV, with a Doniach-\^Sunjic lineshape. The asymmetric shape indicates the presence of a continuum of electronic states at the Pr-site with a sharp cut-off at E_F , even in this insulating sample. It thus excludes the possibility of a genuine band gap, but is compatible with a mobility gap caused by spatially localized electronic states. We have searched for resonance enhancements in the photoemission crossection when scanning the photon energy across the Pr-4d core-level. Such resonance enhancements are observed for the Pr-4f state, the two Cu-many body features and for the states at E_F . The resonances of the Cu-many body features indicate a coupling between these states in the Cu O_2 plane and those of the substituted Pr-ions. Such a coupling to randomly substituted Pr-ions creates a random electrostatic potential in the Cu O_2 plane and can lead to spatial localization of the electronic states.