Impurity effects on the electronic state in the undoped (Ce-free) superconductor T¤-La_1.8Eu_0.2CuO₄ studied by muon spin relaxation

To investigate the electronic state in the undoped (Ce-free) superconductor TA-La_1.8Eu_0.2CuO₄ (TA-LECO), we have performed muon spin relaxation measurements using impurity-substituted superconducting (SC) reduced samples of TA-La_1.8Eu_0.2Cu₁−_yM_yO₄ (M = Ni, Zn; y ¼ 0; 0:005; 0:01) and also a non-SC as-grown sample of TA-LECO. It has been found that phase separation into SC and magnetically ordered regions takes place and that no magnetic order is developed around M in the SC regions of the M-substituted reduced samples. The magnetic transition temperature in the magnetically ordered regions has been found to increase and decrease with the Ni and Zn substitutions, respectively. Accordingly, it has been concluded that the electronic state in the SC regions of TA-LECO is similar to that in overdoped T-La₂−_xSr_xCuO₄ and understood as a strongly correlated itinerant electron system, while the magnetically ordered regions are due to localized Cu spins. The role of the reduction annealing in the appearance of superconductivity in TA-cuprates has been discussed.