Localization and interaction of indirect excitons in GaAs coupled   quantum wells
  We introduced an elevated trap technique and exploited it for lowering the effective temperature of indirect excitons. We observed narrow photoluminescence lines which correspond to the emission of individual states of indirect excitons in a disorder potential. We studied the effect of exciton-exciton interaction on the localized and delocalized exciton states and found that the homogeneous line broadening increases with density and dominates the linewidth at high densities. 
Alexander High, Aaron Hammack, Leonid Butov, Leonidas Mouchliadis,   Alexei Ivanov, Micah Hanson, Arthur Gossard