A theory for colors of strongly correlated electronic systems

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.

Hyperbolic exciton polaritons in a van der Waals magnet

“…Simultaneously, the onsite d-d components of the exciton wavefunction are reduced in the ordered AFM phase (Fig. 5f ), leading to a larger oscillator strength 45 , 46 . Together with reduced scattering rates at low temperature, the increased spectral weight from magnetically-induced exciton delocalization allows for robust HEPs in CrSBr.…”

Optically Addressing Exciton Spin and Pseudospin in Nanomaterials for Spintronics Applications

“…While this principle is correct for systems with ferromagnetic chains, − this is not true universally. Acharya et al shows that magnetic disordering significantly enhances oscillator strength of the excitons in bulk AFM insulators like NiO and MnF 2 , and in such cases the exact nature of the electronic hopping that contributes to the exciton spectral weight is less trivial in nature compared to the aforementioned scenario of FM chains.…”