Distinct variation of electronic states due to annealing in T\u27-type La1.8Eu0.2CuO4 and Nd2CuO4

We performed Cu K-edge X-ray absorption fine structure measurements on T’ -type La1.8 Eu0.2 CuO4 (LECO) and Nd2 CuO4 (NCO) to investigate the variation in the electronic state associated with the emergence of superconductivity due to annealing. The X-ray absorption near- edge structure spectra of as-sintered (AS) LECO are quite similar to those of AS NCO, indicating that the ground state of AS T′-type LECO is a Mott insulator. We clarified a significant variation in the electronic state at the Cu sites in LECO due to annealing. The electron density after annealing (n_an) was evaluated for both superconducting LECO and non-superconducting NCO and found to be 0.40 and 0.05 electrons per Cu atom, respectively. In LECO but not in NCO, extended X-ray absorption fine structure analysis revealed a reduction in the strength of the Cu-O bond in the CuO2 plane due to annealing, which is consistent with the screening effect on phonons in the metallic state. Since the amounts of oxygen loss due to annealing (δ) for LECO and NCO are comparable, these results suggest distinct electron-doping processes in the two compounds. The electron-doping in NCO approximately follows the relation n_an = 2δ; this can be understood if electrons are doped through oxygen deficiency, but the anneal-induced metallic nature and large n_an of LECO suggest that a variation of the electronic band structure causes the self-doping of carriers. The origin of the difference in doping processes due to annealing is discussed in relation to the size of the charge transfer gap