Signatures of collective photon emission and ferroelectric ordering of excitons near their Mott insulating state in a WSe$_2$/WS$_2$ heterobilayer

TL;DR

This paper reports the discovery of collective photon emission linked to ferroelectric ordering of excitons in a WSe2/WS2 heterobilayer, revealing strong dipolar interactions and phase transitions in moiré quantum materials.

Contribution

It provides the first evidence of ferroelectric ordering and collective radiance in moiré excitons, highlighting their potential for novel optical phenomena.

Findings

Observation of in-plane ferroelectric phase of dipolar moiré excitons

Detection of accelerated photon emission at low densities

Evidence of strong dipolar inter-site interactions

Abstract

Spontaneous symmetry breaking, arising from the competition of interactions and quantum fluctuations, is fundamental to understanding ordered electronic phases. Although electrically neutral, optical excitations like excitons can interact through their dipole moment, raising the possibility of optically active ordered phases. The effects of spontaneous ordering on optical properties remain largely unexplored. Recent observations of the excitonic Mott insulating state in semiconducting moir\'e crystals make them promising for addressing this question. Here, we present evidence for an in-plane ferroelectric phase of dipolar moir\'e excitons driven by strong exciton-exciton interactions. We discover a surprising speed-up of photon emission at late times and low densities in excitonic decay. This counterintuitive behavior is attributed to collective radiance, linked to the transition between disordered and symmetry-broken ferroelectric phases of moir\'e excitons. Our findings provide first evidence for strong dipolar inter-site interactions in moir\'e lattices, demonstrate collective photon emission as a probe for moir\'e quantum materials, and pave the way for exploring cooperative optical phenomena in strongly correlated systems.