Suppression of Superconductivity in High Temperature Superconductors: Electronic Structure Effects.

I have studied, using x-ray absorption spectroscopy (XAS), the electronic structure of the copper oxide high temperature superconductors RBa_2Cu _3O_{7-x} , where R = Eu and Y. The objective of the research was to study the electronic state of the copper and oxygen ions as the system was chemically driven from the superconducting regime to the non-superconducting regime. Specifically the oxygen content was reduced by adjusting the annealing conditions, Zn was substituted for Cu, and Pr and Ce were introduced as dopants on the rare-earth sites. Measurements of the Cu-1s and Cu-2p edge structure in high-T_{rm c} superconducting compounds can be consistently interpreted by assuming Cu to be in a 2+ state. Unoccupied states of Cu-3d character were detected in these compounds. More important, measurements of the O-1s edge indicate empty states of O-2p symmetry exist at the Fermi level. When the oxygen content is reduced, and superconductivity is suppressed, there is clear evidence for the formation of Cu^{1+} ions. In addition, a decrease in the intensity of transitions to unoccupied states of Cu-3d and O-2p character is observed. When Zn sufficient to suppress superconductivity is substituted for Cu there is little change in the electronic states of the Cu and O ions. Microprobe analysis of Pr-doped samples indicate that Pr is completely soluble in the Eu 1-2-3 compound. An analysis of the Cu-1s edge of Pr-doped samples show that Cu^{1+} develops in response to the dopant. We interpret this result as an indication that Pr acts primarily to reduce the oxygen content. X -ray absorption spectroscopy of the Pr-2p_ {3/2} edge showed the Pr valence was 3.1. We conclude that charge transfer from the Pr dopant to the Cu-O planes does not account for the suppression of superconductivity in these compounds. Microprobe analysis determined the Eu based 1-2-3 chemical matrix does not support Ce as a dopant.