Origin of the Anomalously Strong Influence of Out-of-Plane Disorder on High-T_cSuperconductivity

The electronic structure of Bi2Sr2-xRxCuOy (R = La, Eu) near (pi, 0) of the first Brillouin zone was studied by angle-resolved photoemission spectroscopy (ARPES). The temperature T* above which the pseudogap structure in the ARPES spectrum disappears was found to have an R dependence that is opposite to that of the superconducting transition temperature T-c. This indicates that the pseudogap state competes with high-T-c superconductivity, and the large suppression of T-c caused by out-of-plane disorder is due to the stabilization of the pseudogap state.