Doping Dependence of Collective Spin and Orbital Excitations in Spin 1 Quantum Antiferromagnet La$_{2-x}$Sr$_x$NiO$_4$ Observed by X-rays

TL;DR

This study demonstrates that resonant inelastic x-ray scattering (RIXS) can detect collective magnetic excitations in $S=1$ nickelate systems, revealing doping-dependent changes in spin and orbital excitations and highlighting RIXS's potential for probing magnetic interactions.

Contribution

It is the first empirical evidence that RIXS can probe collective magnetic excitations in $S=1$ systems, showing doping effects on spin and orbital excitations in La$_{2-x}$Sr$_x$NiO$_4$.

Findings

Zone boundary magnon energy decreases with doping faster than in cuprates.

Asymmetric magnetic excitation peaks indicate single and multi spin-flip processes.

Orbital character of doped holes influences magnetic excitation behavior.

Abstract

We report the first empirical demonstration that resonant inelastic x-ray scattering (RIXS) is sensitive to \emph{collective} magnetic excitations in $S=1$ systems by probing the Ni $L_3$-edge of La$_{2-x}$Sr$_x$NiO$_4$ ($x = 0, 0.33, 0.45$). The magnetic excitation peak is asymmetric, indicating the presence of single and multi spin-flip excitations. As the hole doping level is increased, the zone boundary magnon energy is suppressed at a much larger rate than that in hole doped cuprates. Based on the analysis of the orbital and charge excitations observed by RIXS, we argue that this difference is related to the orbital character of the doped holes in these two families. This work establishes RIXS as a probe of fundamental magnetic interactions in nickelates opening the way towards studies of heterostructures and ultra-fast pump-probe experiments.