Abstract

To fully capitalize on the potential and versatility of resonant inelastic x-ray scattering (RIXS), it is essential to develop the capability to interpret different RIXS contributions through calculations, including the dependence on momentum transfer, from first-principles for correlated materials. Toward that objective, we present new methodology for calculating the full RIXS response of a correlated metal in an unbiased fashion. Through comparison of measurements and calculations that tune the incident photon energy over a wide portion of the Fe L$_3$ absorption resonance of the example material BaFe$_2$As$_2$, we show that the RIXS response in BaFe$_2$As$_2$ is dominated by the direct channel contribution, including the Raman-like response below threshold, which we explain as a consequence of the finite core-hole lifetime broadening. Calculations are initially performed within the first-principles Bethe-Salpeter framework, which we then significantly improve by convolution with an effective spectral function for the intermediate-state excitation. We construct this spectral function, also from first-principles, by employing the cumulant expansion of the Green's function and performing a real-time time dependent density functional theory calculation of the response of the electronic system to the perturbation of the intermediate-state excitation. Importantly, this allows us to evaluate the indirect RIXS response from first-principles, accounting for the full periodicity of the crystal structure and with dependence on the momentum transfer.

Highlights

  • Calculations are initially performed within the first-principles BetheSalpeter equation (BSE) framework, which we significantly improve by invoking a quasiboson model to describe the secondary excitations within the intermediate state

  • As we summarize in Appendix A, the many-body x-ray absorption coefficient can be reduced to a convolution of the BSE absorption coefficient and a spectral function that corresponds to the imaginary part of an effective twoparticle Green’s function for the exciton

  • We have developed new methodology to calculate RIXS spectra from first principles with full momentum dependence for moderately correlated metallic materials

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Summary

INTRODUCTION

Resonant inelastic x-ray scattering (RIXS) is a highly versatile and powerful probe of elementary excitations in materials owing to its sensitivity to all electronic (i.e., spin, charge, and orbital) and lattice degrees of freedom,. Contrary to model Hamiltonians, cluster calculations, or approaches that reduce the electronic structure to Wannier orbitals of the correlated bands, the DFT BSE includes all bands associated with all sites and orbital shells on an equal footing This inclusion reduces the arbitrariness of the method, avoids parameterization, naturally accounts for hybridization effects, and is applicable to the transition metal and ligand edges. V and VI that this approximation enables us to combine our BSE calculations and the rt-TDDFT-derived effective excitonic spectral function to calculate both band-structure and correlation contributions to RIXS spectra This combination allows for a practical, first-principles method of calculating the RIXS response of correlated metals with full momentum dependence, including secondary electronic excitations and indirect loss contributions

X-RAY ABSORPTION AND EMISSION
DIRECT-CHANNEL RESONANT INELASTIC X-RAY SCATTERING
QUASIBOSON MODEL FOR SECONDARY EXCITATIONS
INCORPORATING SECONDARY EXCITATIONS WITHIN RIXS CALCULATIONS
OBSERVATION OF dd EXCITATIONS IN BaFe2As2
SUMMARY AND OUTLOOK

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