Ab-initio order parameter fluctuations in the excitonic insulator transition

There has been much recent interest in excitonic insulators (EIs), a correlated phase of material with coherent electron-hole pairings. Although excitonic condensation shares formal similarities with superconductivity, unlike Cooper pairing, electron-hole pairing conserves charge, resulting in different symmetry groups.

Previous studies have explored EIs in both model Hamiltonians and real materials--but typically within a mean-field framework. Our work expands upon the mean-field analysis of real materials by incorporating order parameter fluctuations, which are important for determining the transition temperature T_c and characterizing the behavior of the EI phase near T_c. More specifically, we study the possible EI phase transition of candidate materials such as 1T’-MoS₂ and Ta₂Pd₃Te₅ using an ab-initio GW+BSE approach. This will be contrasted with mean-field results to assess the importance of order parameter fluctuations.