Looking for magnetic clues: Thin films of the neodymium nickelate NdNiO ₂ doped with strontium have recently been found to be superconducting. This materials class bears structural and electronic similarities to the famed cuprate superconductors, but how far the analogy goes remains unclear. Lu et al. used resonant inelastic x-ray scattering to look for magnetism, which exists in the cuprates, in Nd _1-x Sr _x NiO ₂ films (see the Perspective by Benckiser). The authors observed magnetic modes in the undoped compound that had a doping evolution consistent with the behavior of a doped Mott insulator. Science , abd7726, this issue p. 213 ; see also abi6855, p. 157

The I21 beamline at Diamond Light Source is dedicated to advanced resonant inelastic X-ray scattering (RIXS) for probing charge, orbital, spin and lattice excitations in materials across condensed matter physics, applied sciences and chemistry. Both the beamline and the RIXS spectrometer employ divergent variable-line-spacing gratings covering a broad energy range of 280–3000 eV. A combined energy resolution of ∼35 meV (16 meV) is readily achieved at 930 eV (530 eV) owing to the optimized optics and the mechanics. Considerable efforts have been paid to the design of the entire beamline, particularly the implementation of the collection mirrors, to maximize the X-ray photon throughput. The continuous rotation of the spectrometer over 150° under ultra high vacuum and a cryogenic manipulator with six degrees of freedom allow accurate mappings of low-energy excitations from solid state materials in momentum space. Most importantly, the facility features a unique combination of the high energy resolution and the high photon throughput vital for advanced RIXS applications. Together with its stability and user friendliness, I21 has become one of the most sought after RIXS beamlines in the world.

The recent discovery of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ 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 ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$. Upon doping, a high-energy feature in Ni $L_3$-edge XAS develops in addition to the main absorption peak, while XAS at the O $K$-, Nd $M_3$- and Nd $M_5$-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 $L_3$-edge XAS is consistent with a $d^8$ spin-singlet state in which doped holes reside in the $3d_{x^2-y^2}$ 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 $3d_{x^2-y^2}$ orbital.