The TRIXS Project

The aim of the TRIXS project is to develop the theory and software for the ab initio predictive description, without adjustable parameters, of Resonant Inelastic X-ray Spectroscopy (RIXS). RIXS is an important experimental tool to probe elementary excitations in solids. It gives vital information about the (strong) electron correlation, thanks to the large variety of targeted excitations, including

i) charge-transfer excitations (crucial to unravel the nature of an insulator, like charge transfer type or Mott-Hubbard)

ii) crystal-field features such as dd excitations in strongly correlated materials.

Other accessible and well studied excitations are excitons, plasmons, phonons, and magnons. Theory plays a crucial role in the interpretation and understanding of the experimental spectra, in particular to disentangle the many types of excitations in RIXS spectra.

The physical picture of RIXS can be summarized as follows:

i) an incoming photon promotes a core electron to an empty conduction state

ii) a different electron from the valence region fills the core hole.

The net result is a final state with an electron-hole excitation, whose energy and momentum are defined by the conservation laws. Therefore, in RIXS one combines an excitation from a core level and a de-excitation to the same core level, the net result begin an excitation for the valence.

So RIXS is not (only) a measure of core levels, but of valence states.

The project has two main parts:

1. Extend both the theory and the implementation of the Bethe-Salpeter equation to the calculation of RIXS spectra.

2. Describe the electron correlation relevant to RIXS. In particular, RIXS is a powerful tool to study strong correlation. However, current approaches cannot accurately treat strong correlation without using adjustable parameters. To treat strongly correlated systems we will generalize an ab initio method developed by ourselves which has been successful in describing strong correlation.

Throughout the duration of the TRIXS project we will benchmark our results for semiconductors and correlated solids against high-resolution RIXS experimental spectra obtained by ourselves at the SEXTANTS beamline at SOLEIL. Finally, the ab initio numerical tool developed in the TRIXS project will be made available to the scientific community through a open source code.

Toulouse Team

Pina Romaniello

Arjan Berger

Gabriele Riva

Palaiseau Team

Francesco Sottile

Matteo Gatti

Laura Urquiza

SOLEIL Team

Alessandro Nicolaou

All-electron many-body approach to resonant inelastic X-ray scattering Christian Vorwerk, Francesco Sottile and Claudia Draxl, Phys. Chem. Chem. Phys. 24, 17439 (2022)

Photoemission spectral functions from the three-body Green’s function Gabriele Riva, Timothée Audinet, Matthieu Vladaj, Pina Romaniello and J. Arjan Berger, SciPost Phys. 12, 093 (2022)

Photoemission Spectra from the Extended Koopman’s Theorem, Revisited, Stefano Di Sabatino, Jaakko Koskelo, Julien Prodhon, J. Arjan Berger, Michel Caffarel and Pina Romaniello. Frontiers in Chemistry 9, 746735 (2021)

Wigner localization in two and three dimensions: An ab initio approach Miguel Escobar Azor, Estefania Alves, Stefano Evangelisti and J. Arjan Berger, J. Chem. Phys. 155, 124114 (2021)

Accurate ground-state energies of Wigner crystals from a simple real-space approach, Estefania Alves, Gian Luigi Bendazzoli, Stefano Evangelisti and J. Arjan Berger, Phys. Rev. B 103, 245125 (2021)

Photoemission spectrum in paramagnetic FeO under pressure: Towards an ab initio description, Stefano Di Sabatino, Jaakko Koskelo, J. Arjan Berger, and Pina Romaniello, Phys. Rev. Research 3, 013172 (2021)

Potential Energy Surfaces without Unphysical Discontinuities: The Coulomb Hole Plus Screened Exchange Approach J. Arjan Berger , Pierre-François Loos, and Pina Romaniello, J. Chem. Theory Comput. 17, 191 (2021)

Excitation pathways in resonant inelastic x-ray scattering of solids Christian Vorwerk, Francesco Sottile, and Claudia Draxl Phys. Rev. Research 2, 042003(R) (2020)