Crystallization of an exciton superfluid

Indirect excitons--pairs of electrons and holes spatially separated in semiconductor bilayers or quantum wells--are known to undergo Bose-Einstein condensation and to form a quantum fluid. Here we show that this superfluid may crystallize upon compression. However, further compression results in quantum melting back to a superfluid. This unusual behavior is explained by the effective interaction potential between indirect excitons, which strongly deviates from a dipole potential at small distances due to many-particle and quantum effects. Based on first-principles path-integral Monte Carlo simulations, we compute the complete phase diagram of this system and predict the relevant parameters necessary to experimentally observe exciton crystallization in semiconductor quantum wells.