A theory for colors of strongly correlated electronic systems

Swagata Acharya; Dimitar Pashov; Cedric Weber; Mark van Schilfgaarde; Alexander I Lichtenstein; Mikhail I Katsnelson

doi:10.1038/s41467-023-41314-6

A theory for colors of strongly correlated electronic systems

Nat Commun. 2023 Sep 9;14(1):5565. doi: 10.1038/s41467-023-41314-6.

Authors

Swagata Acharya^{1

2}, Dimitar Pashov³, Cedric Weber⁴, Mark van Schilfgaarde⁵, Alexander I Lichtenstein^{6

7}, Mikhail I Katsnelson⁸

Affiliations

¹ Institute for Molecules and Materials, Radboud University, Nijmegen, 6525 AJ, The Netherlands. swagata.acharya@nrel.gov.
² National Renewable Energy Laboratory, Golden, 80401, CO, USA. swagata.acharya@nrel.gov.
³ Theory and Simulation of Condensed Matter, King's College London, The Strand, London, WC2R2LS, UK.
⁴ Quantum Brilliance Pty, The Australian National University, Gould Building (116), Daley Road, Canberra, ACT 2600, Australia.
⁵ National Renewable Energy Laboratory, Golden, 80401, CO, USA.
⁶ Institute of Theoretical Physics, University of Hamburg, Hamburg, 20355, Germany.
⁷ European X-Ray Free-Electron Laser Facility, Holzkoppel 4, Schenefeld, 22869, Germany.
⁸ Institute for Molecules and Materials, Radboud University, Nijmegen, 6525 AJ, The Netherlands.

Abstract

Many strongly correlated transition metal insulators are colored, even though they have band gaps much larger than the highest energy photons from the visible light. An adequate explanation for the color requires a theoretical approach able to compute subgap excitons in periodic crystals, reliably and without free parameters-a formidable challenge. The literature often fails to disentangle two important factors: what makes excitons form and what makes them optically bright. We pick two archetypal cases as examples: NiO with green color and MnF₂ with pink color, and employ two kinds of ab initio many body Green's function theories; the first, a perturbative theory based on low-order extensions of the GW approximation, is able to explain the color in NiO, while the same theory is unable to explain why MnF₂ is pink. We show its color originates from higher order spin-flip transitions that modify the optical response, which is contained in dynamical mean-field theory (DMFT). We show that symmetry lowering mechanisms may determine how 'bright' these excitons are, but they are not fundamental to their existence.