Nature of oxygen dopant-induced states in high-temperature Bi₂Sr₂CaCu₂O_8+x superconductors: A photoemission investigation

We investigate the nature of oxygen dopant-induced electronic states in the high-temperature Bi₂Sr₂CaCu₂O_8+x superconductor by measuring photoemission spectra of underdoped, optimally doped, and overdoped samples using a wide photon energy range (15–100 eV). We find a small and broad nondispersive peak at −0.8 eV associated with oxygen dopants. Surprisingly, the detailed analysis of the resonance profile suggests a mixing with Cu states. Particularly, the A_1g symmetry revealed by polarization-dependent experiments and the comparison with the single-layered Bi₂Sr₂CuO_6+x indicates that the oxygen dopant-induced states are mixed to the superconducting CuO₂ planes through the oxygen dopant—apical oxygen O2p_z—Cu3d_3z²−r² channel.