  The excitations of a two-dimensional electron gas in quantum wells with
intermediate carrier density (~10^{11} cm^{-2}), i.e., between the
exciton-trion- and the Fermi-Sea range, are so far poorly understood. We report
on an approach to bridge this gap by a magneto-photoluminescence study of
modulation-doped (Cd,Mn)Te quantum well structures. Employing their enhanced
spin splitting, we analyzed the characteristic magnetic-field behavior of the
individual photoluminescence features. Based on these results and earlier
findings by other authors, we present a new approach for understanding the
optical transitions at intermediate densities in terms of four-particle
excitations, the Suris tetrons, which were up to now only predicted
theoretically. All characteristic photoluminescence features are attributed to
emission from these quasi-particles when attaining different final states.
