Softening of $dd$ excitation in the resonant inelastic x-ray scattering spectra as a signature of Hund's coupling in nickelates

TL;DR

This study explores how Hund's coupling influences the RIXS spectra in layered nickelate superconductors, revealing a softening of spectral peaks linked to Hund's coupling strength and doping levels.

Contribution

It provides a detailed analysis of Hund's coupling effects on RIXS spectra in nickelates, highlighting the competition between crystal field and Hund's coupling and proposing experimental tests.

Findings

Hund's coupling dominates in the parent compound.

Spectral peak softening correlates with doping and Hund's coupling.

Theoretical RIXS signals are separated into spin channels.

Abstract

We investigate the effects of Hund's coupling on the resonant X-ray absorption spectra of the recently discovered family of layered nickelate superconductors. We contrast two scenarios depending on the relative strength of the ratio of the effective Hund's coupling ($J_H$) to the crystal fields ($\Delta$) in these systems. We carry out the cluster and DFT+DMFT simulations of the RIXS signal at the Ni $L$-edge for different values of Hund's coupling. We find the latter dominates for the parent compound while the former becomes important for sufficiently large doping. Our results are consistent with the observations of a softening of a RIXS peak as a function of doping by Rossi {\sl et al.}~\cite{PhysRevB.104.L220505}, only when the Hund coupling is sizeable. To interpret the results, we separate the theoretical RIXS signal into spin conserving and non-spin conserving channels and conclude that the infinite layer nickelates are in a regime where $\Delta$ and $J$ compete effectively and suggest further experimental tests of the theory.