Phase Transitions of Excitons in Two Dimensions

An exciton is a quasi-particle consisting of an electron and a hole bound by the Coulomb potential. It is a solid and excited state analog of the hydrogen atom. Excitons are fundamental to semiconductors and determine a range of processes involving the conversion of light to charge or charge to light. As bosonic particles, excitons can form quantum phases, e.g., Bose-Einstein condensates (BECs). Unlike their 3D counter parts, the Coulomb and exchange interactions are poorly screened in 2D> This results in large exciton binding energies and strong dependences of semiconductor bandgaps on charge carrier concentrations. In 2D transition-metal dichalcogenide (TMDC) heterojunctions, charge separation across the interface results in interlayer excitons with lifetimes five orders of magnitude longer than those of their intralayer counterparts. Both the dipolar nature and long lifetime make interlayer excitons ideal models for the exploration of many body interactions and phase transitions. Moreover, moiré pattern formation and site-specific interlayer hybridization result in periodic potentials for interlayer exciton arrays that may serve as quantum simulators. We show the sensitivity of moiré trapped excitons to local strain, from 0D unstrained QD arrays to 1D strained wires. With increasing exciton density, inter-exciton interaction leads to two distinct phase transitions: from trapped excitons in the moiré-potential to the modestly mobile exciton gas as exciton density increases to n ~ 10¹¹ cm^-2 and from the exciton gas to the highly mobile charge separated electron/hole plasma for n> 10¹² cm^-2. We explore the ordering of interlayer excitons into quantum phases using trilayer structure as predicted in recent theory.