Polaron spectroscopy of a bilayer excitonic insulator

TL;DR

This paper investigates how excitons in bilayer materials form interlayer polarons, revealing their spectral features and how these are affected by layer separation, using advanced quantum Monte Carlo and BCS theories.

Contribution

It introduces a novel scheme for probing excitonic insulators via polaron spectroscopy and combines quantum Monte Carlo with BCS theory to analyze excitations and screening effects.

Findings

Interlayer polaron formation affects optical spectra.

Increasing layer separation modifies excitonic binding energies.

Predicted spectral signatures of excitonic insulator excitations.

Abstract

Recent advances in fabrication of two dimensional materials and their moir\'e heterostructures have opened up new avenues for realization of ground-state excitonic insulators, where the structure spontaneously develops a finite interlayer electronic polarization. We propose and analyze a scheme where an optically generated intralayer exciton is screened by excitations out of the excitonic insulator to form interlayer polarons. Using Quantum Monte-Carlo calculations we first determine the binding energy of the biexciton state composed of inter- and intralayer excitons, which plays a central role in understanding polaron formation. We describe the excitations out of the ground-state condensate using BCS theory and use a single interacting-quasiparticle-pair excitation Ansatz to describe dynamical screening of optical excitations. Our predictions carry the hallmarks of the excitonic insulator excitation spectrum and show how changing the interlayer exciton binding energy by increasing the layer separation modifies the optical spectra.