Ab initio optical absorption spectra for excitonic insulators

Excitonic insulators are a strongly correlated phase of matter whose electronic ground state is a condensate of electron-hole pairs, which may be viewed as a BCS-like state in a certain limit. In this talk, we present an ab initio framework for determining the optical absorption spectrum for this phase. Here we start with the electronic band structure of the normal phase within the GW approximation, and then utilize an ab initio electron-hole interaction kernel for an iterative computation of the BCS gap function. Within this framework, the interaction kernel matrix elements are the same ones used to compute excitonic effects on the optical absorption of the normal phase in standard GW plus Bethe Salpeter Equation (GW-BSE) calculations. Finally, the excitonic insulator optical response is evaluated from the resultant excitonic insulator band structure.

As a first study, this method is applied to a Kagome lattice 4-triangulene covalent organic framework, which was recently conjectured to be an excitonic insulator candidate, using the BerkeleyGW code for normal phase and interaction kernel calculations.