X-ray spectroscopy on infinite-layer Nd_(1-x)Sr_(x)NiO_2

University of Bristol

Since their experimental realisation in 2019 [1] infinite-layer nickelates have emerged as a new route to study unconventional superconductivity. From the 2D planes of NiO₂ formed of spin-½ Ni¹⁺ 3d⁹ ions, the Ni 3d_x²-y² band at the Fermi level, and strong superexchange evidenced by collective spin excitations in the undoped parent compound [2], it is tempting to draw analogies to the cuprate family of high T_c superconductors. However, progress has been limited by challenging material growth. For example, superconductivity has only yet been achieved in 10 nm thin films. While this has limited many experimental probes, resonant inelastic x-ray scattering has proved to be a powerful tool to probe the electronic structure and collective excitations. Here we present resonant inelastic x-ray scattering measurements on infinite layer nickelates, and demonstrate the presence of a lattice symmetry-breaking modulation characteristic of a charge density wave (CDW) in the undoped parent compound [3], potentially further highlighting similarities with the cuprates. Next, we show there is a pronounced effect of growth conditions on the normal state CDW signals and the collective spin excitations. Finally, we show the superconducting state is very sensitive to growth conditions, and corresponds to a narrow region of intercalated hydrogen concentration [4]. These results highlight the sensitivity of the normal and superconducting state to growth conditions.

[1] D. Li et al. Nature 514 624-627 (2019).
[2] H. Lu et al. Science 373 213-216 (2021).
[3] C. C. Tam et al., Nature Materials 21 1116-1120 (2022).
[4] X. Ding, C. C. Tam et al., Nature 615, 50-55 (2023).