Ni-Impurity Effect in High-$T_{\rm c}$ Cuprates Studied by Neutron Scattering and XAFS Spectroscopy

Neutron scattering experiments using La$_{2-x}$Sr$_x$Cu$_{1-y}$Ni$_{y}$O$_{4}$ 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$^{(2+\alpha)+}$. It indicates that a strong hole localization occurs around Ni, resulting in an effective spin-$1/2$ value at Ni sites. Therefore, a charge dopant nature of Ni is most likely realized when $x \ge y$.