Magneto-optical probing of weak disorder in a two-dimensional hole gas
  In two-beam magneto-photoluminescence spectra of a two-dimensional valence hole gas we identify the three-level energy spectrum of a free positive trion with a field-induced singlet-triplet transition. The recombination spectrum of acceptor-bound trions is also detected, including a cyclotron replica corresponding to the hole shake-up process. The emergence of a shake-up peak at low temperature is shown to be a sensitive probe of the presence of a small number of impurities inside the high-mobility quantum well, and its relative position is directly related to the hole cyclotron mass. 
Leszek Bryja, Arkadiusz Wojs, Jan Misiewicz, Marek Potemski, Dirk   Reuter, and Andreas Wieck