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

Neutron scattering experiments using La2-xSrxCu1-yNiyO4 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; Ni2+ and Ni(2+α)+. It indicates that a strong hole localization occurs around Ni, resulting in an effective spin-12 value at Ni sites. Therefore, a charge impurity nature of Ni is most likely realized when x⩾y.