Evidence for Exciton Crystals and Quantum Phase Transitions in a 2D Semiconductor Trilayer

Two-dimensional (2D) transition metal dichalcogenides (TMDC) and their moire interfaces have been demonstrated for correlated electron states, including Mott insulators and electron/hole crystals commensurate with moire superlattices. Here we present spectroscopic evidence for ordered bosons - interlayer exciton crystals in a WSe₂/MoSe₂/WSe₂ trilayer, where the enhanced Coulomb interactions over those in heterobilayers have been predicted to result in exciton ordering. While the dipolar interlayer excitons in the heterobilayer may be ordered by the periodic moire traps, their mutual repulsion results in de-trapping at exciton density larger than 10¹¹ 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. In contrast, ordered interlayer excitons in the trilayer are characterized by negligible mobility and by sharper PL peaks persisting to n_ex larger than 10¹² 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.