Charge Excitations in Cuprate and Nickelate in Resonant Inelastic X-Ray Scattering

TL;DR

This paper models RIXS spectra in La2CuO4 and La2NiO4 using a combination of ab initio, Hartree-Fock, and RPA methods, revealing that observed peaks are band-to-band excitations influenced by electron correlations.

Contribution

It introduces a comprehensive theoretical framework combining ab initio, Hartree-Fock, and RPA to interpret RIXS spectra in cuprates and nickelates, challenging the bound exciton interpretation.

Findings

Calculated spectra match experimental features

RIXS peaks are band-to-band excitations, not bound excitons

Differences between La2CuO4 and La2NiO4 explained by model

Abstract

We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Cu and Ni K edges in La2CuO4 and La2NiO4, respectively. We make use of the Keldysh-Green-function formalism, in which the RIXS intensity is described by a product of incident-photon-dependent factor and density-density correlation function in the 3d states. The former factor is calculated using the $4p$ density of states given by an ab initio band structure calculation and the latter using the wavefunctions given by a Hartree-Fock calculation of a multi-orbital tight-binding model. The initial state is described within the Hartree-Fock approximation and the electron correlations on charge excitations are treated within the random phase approximation. The calculated RIXS spectra well reproduce several characteristic features in the experiments. Although several groups have interpreted the RIXS peaks as bound excitons, our calculation indicates that they should be interpreted as band-to-band excitations augmented by electron correlations. The difference in RIXS spectra between La2CuO4 and La2NiO4 is explained from this point of view.