Electron Spectroscopy of Single-Layer (n = 1) Bi2Sr2?xLaxCuO6+δ Crystals at Optimal Doping

On single crystals of the single-layer (n = 1) high-T _c superconductor Bi₂Sr_2?x La _x CuO_6+δ at optimal doping (x = 0.4), the electron spectroscopies x-ray absorption (XAS) and high-resolution angle-resolved photoemission (ARPES) were performed. The XAS gives the intensity of the so-called prepeak of the O 1s line what is due to the unoccupied part of the Zhang–Rice (ZR) singlet band. For ARPES, the advantages of single-layer material are the absence of bilayer effects and the possibility to study the electronic properties of the normal state at a sample temperature where the thermal broadening is extremely small (<10 meV). The controlled variation of the polarization vector of the synchrotron radiation made it possible to resolve a distinct fine-structure of the occupied part of the ZR singlet band at the Fermi level. These observations have enormous consequences for line shape analyses and the determination of pseudogaps, and thus the mechanism of high-T _c superconductivity.