Magneto-photoluminescence fine structures of a two-dimensional electron gas in the extreme quantum limit

We study magneto-photoluminescence (PL) spectra of a two-dimensional electron system in an ultrahigh-mobility GaAs/AlGaAs single quantum well at a very low temperature ∼ 50 mK. In the extreme quantum limit up to 7.0 T (corresponding to Landau level filling factor ν∼0.29), we observe an unexpectedly large number of sharp PL peaks. These fine structures are split from fundamental optical transitions assigned to negatively charged excitons (trions) and neutral excitons. They are found to exhibit unconventional circular polarization characteristics. The emergence of circularly polarized PL fine structures cannot be described by non-interacting band structures, indicating significant effects of many-body interactions.