Orbital and spin character of doped carriers in infinite-layer nickelates

The recent discovery of superconductivity in Nd_1-xSr_xNiO₂ has drawn significant attention in the field. A key open question regards the evolution of the electronic structure with respect to hole doping. Here we exploit x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS) to probe the doping-dependent electronic structure of Nd_1-xSr_xNiO₂. Upon doping, a high-energy feature in Ni L₃-edge XAS develops in addition to the main absorption peak, while XAS at the O K-, Nd M₃- and Nd M₅-edge exhibits a much weaker response. This implies that doped holes are mainly introduced into Ni 3d states. By comparing our data to atomic multiplet calculations including D_4h crystal field, the doping-induced feature in Ni L₃-edge XAS is consistent with a d⁸ spin-singlet state in which doped holes reside in the 3d_x2-y2 orbitals. This is further supported by the softening of RIXS orbital excitations due to doping, corroborating with the Fermi level shift associated with increasing holes in the Ni 3d_x2-y2 orbital.