Ni-impurity effect in high-T_c cuprates studied by neutron scattering and XAFS spectroscopy

Neutron scattering experiments using La_{2 - x} Sr_x Cu_{1 - y} Ni_y O₄ clarified that the parallel spin-density modulations (SDMs) in the superconducting phase are susceptible to Ni, in the same way as the diagonal SDMs in the insulating spin-glass phase. Ni substitution reduces the mobile hole concentration from x down to x - y. Polarized XAFS measurements using Ni K-edge probe two types of Ni valence states; Ni^{2 +} and Ni^{(2 + α) +}. It indicates that a strong hole localization occurs around Ni, resulting in an effective spin-frac(1, 2) value at Ni sites. Therefore, a charge impurity nature of Ni is most likely realized when x ≥ y.