Evidence for Exciton Crystals in a 2D Semiconductor Heterotrilayer

Two-dimensional (2D) transition metal dichalcogenides (TMDC) and their moiré interfaces have been demonstrated for correlated electron states, including Mott insulators and electron/hole crystals commensurate with moiré superlattices 1–7 . Here we present spectroscopic evidences for ordered bosons interlayer exciton crystals in a WSe 2 /MoSe 2 /WSe 2 trilayer, where the enhanced Coulomb interactions over those in heterobilayers have been predicted to result in exciton ordering 8–10 . While the dipolar interlayer excitons in the heterobilayer may be ordered by the periodic moiré traps 11–14 , their mutual repulsion results in de-trapping at exciton density n ex ≥ 10 11 cm -2 to form mobile exciton gases and further to electron-hole plasmas, both accompanied by broadening in photoluminescence (PL) peaks and large increases in mobility 15 . In contrast, ordered interlayer excitons in the trilayer are characterized by negligible mobility and by sharper PL peaks persisting to n ex ≥ 10 12 cm -2 . We find that an optically dark state attributed to the predicted quadrupolar exciton crystal transitions to the bright dipolar excitons either with increasing n ex or by an applied electric field. These ordered interlayer excitons may serve as models for the exploration of quantum phase transitions and quantum coherent phenomena. Fermions and bosons can undergo quantum phase transitions and form ordered ground states. These processes are determined by changes to fundamental parameters in the system thermal phase TMDC