Description of Resonant Inelastic X-Ray Scattering in Correlated Metals

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 FeL3absorption resonance of the benchmark materialBaFe2As2, we show that the RIXS response inBaFe2As2is dominated by the direct-channel contribution, including the Raman-like response below threshold. Calculations are initially performed within the first-principles Bethe-Salpeter equation (BSE) framework, which we then significantly improve by invoking a quasiboson model to describe the secondary excitations within the intermediate state. This enhancement allows the many-electron RIXS signal to be approximated as a convolution of BSE-calculated spectra with effective spectral functions. We construct these spectral functions, 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 process allows us to evaluate the indirect RIXS response from first principles, accounting for the full periodicity of the crystal structure and with full dependence on the momentum transfer.

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Plain Language Summary

Strongly correlated materials—in which interactions among electrons cannot be ignored—display a plethora of fascinating phenomena offering promise for applications including high-temperature superconductivity, data manipulation, and quantum information science. However, understanding and leveraging the complex behavior driving these phenomena presents considerable challenges. In recent years, resonant inelastic x-ray scattering (RIXS) has grown into a powerful experimental tool for deciphering the complexity of correlated materials. Despite these advances, the application of RIXS to most correlated metals remains limited by experimental and theoretical challenges, leaving a fascinating sector of correlated materials untapped. Here, we present a blueprint for calculating spectra of strongly correlated metals that will significantly open RIXS to these important systems.

In RIXS, one measures the energy and momenta of x rays scattered off a material to probe its electronic properties. Adding or removing an electron from a material initiates a correlated electronic response that manifests in x-ray photoemission spectra as unique structures, constituting the indirect contribution to RIXS. We demonstrate how to calculate these correlated contributions to RIXS spectra from first principles, thereby improving on the ability to interpret these spectra in experiments.

The ability to accurately calculate RIXS spectra of correlated metals from first principles will allow detailed, joint experimental and computational investigations that can provide deeper insight into itinerant magnetism and the behavior of “bad metals”—metals with atypical electronic behavior—on the verge of superconductivity.