Anomalous magneto-optical properties of a two-dimensional electron gas around integer filling factors

Light emission from the lowest Landau level of a GaAs/AlGaAs single hetero-structure, in the integer quantum Hall regime, is observed only in the presence of spin-down electrons in the last occupied Landau level. At low temperatures the light emission starts abruptly at filling factors ν=2,4,6. For increasing magnetic field the emission energy remains essentially constant up to a critical field. Beyond it, the emission peak tends progressively to the usual Landau-level-like behavior. Simultaneous transport measurements demonstrate that the critical field corresponds to the onset of extended states at the Fermi level. The results are qualitatively interpreted as due to the formation of complex bound-states with the photo-created hole.