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We performed Cu {\it K}-edge X-ray absorption fine structure measurements onT'-type La$_{1.8}$Eu$_{0.2}$CuO$_4$ (LECO) and Nd$_2$CuO$_4$ (NCO) toinvestigate the variation in the electronic state associated with the emergenceof superconductivity due to annealing. The X-ray absorption near-edge structurespectra 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. Wefound a significant variation of the electronic state at the Cu sites in LECOdue to annealing. The electron density after annealing ($n_{\rm AN}$) wasevaluated for both superconducting LECO and non-superconducting NCO and foundto be 0.40 and 0.05 electrons per Cu, respectively. In LECO but not in NCO,extended X-ray absorption fine structure analysis revealed a softening in thestrength of the Cu-O bond in the CuO$_2$ plane due to annealing, which isconsistent with the screening effect on phonons in the metallic state. Sincethe amounts of oxygen loss due to annealing ($\delta$) for LECO and NCO arecomparable with each other, these results suggest distinct electron-dopingprocesses in the two compounds. That electron-doping in NCO approximatelyfollows the relation $n_{\rm AN}=2\delta$ can be understood if electrons aredoped through oxygen deficiency, but the anneal-induced metallic nature andlarge $n_{\rm AN}$ of LECO suggest a variation of the electronic band structurecauses self-doping of carriers. The origin of the difference in dopingprocesses due to annealing is discussed in connection with the size of thecharge transfer gap.

Distinct variation of electronic states due to annealing in La$_{1.8}$Eu$_{0.2}$CuO$_4$ and Nd$_{2}$CuO$_4$