Observation of an exciton crystal in a moir\'e excitonic insulator

TL;DR

This paper reports the first observation of a stable exciton crystal in a moiré excitonic insulator, demonstrating long-lived exciton crystallization and correlated insulating phases in a tunable electron-hole bilayer system.

Contribution

It provides experimental evidence of an exciton crystal in a moiré superlattice, advancing understanding of bosonic crystallization in solid-state systems.

Findings

Observation of exciton crystallization at one exciton per three moiré sites

Detection of strong Umklapp scattering peaks indicating crystalline order

Identification of correlated insulating phases with doped exciton and charge states

Abstract

Strong Coulomb interactions can drive electrons to crystallize into a Wigner lattice. Achieving the bosonic analogue - a crystal of excitons - has remained elusive due to their short lifetimes and weaker interactions. Here, we report the observation of a thermodynamically stable exciton crystal in an excitonic insulator coupled to a moir\'e potential. Using an electron-hole bilayer composed of a monolayer MoSe2 and a WS2/WSe2 moir\'e superlattice, we construct a tunable extended Bose-Hubbard model with electrical control over exciton and charge doping in thermal equilibrium. Optical spectroscopy reveals spontaneous crystallization of long-lived excitons at one exciton filling per three moir\'e sites, evidenced by strong Umklapp scattering peaks in the optical spectrum. Exciton transport measurements further show a pronounced exciton resistance peak at the same filling, consistent with suppressed exciton hopping in a crystalline phase. When doped away from net charge neutrality, this moir\'e electron-hole bilayer can host new correlated insulating phases where dipolar excitonic insulators form on top of the background of a hole Mott insulator or generalized Wigner crystals in the moir\'e superlattice. These findings establish moir\'e excitonic insulators as a versatile platform for realizing correlated crystalline phases of bosons and fermions.