High-field magneto-spectroscopy of quasi-2D electrons and holes in in As/Al_xGa_1-xSb single quantum wells

Far-infrared magneto-transmission studies on two high mobility InAs/Al_xGa_1-xSb (x = 0.1 and 0.2) type-II single quantum wells in magnetic fields up to 30 T show that a magnetic-field-induced semimetal-semiconductor (SM-SC) transition occurs in the x = 0.2 sample between 8 and 12 T, and in the x = 0.1 sample in the vicinity of 28 T. The so-called X-lines vanish at high magnetic fields and temperatures, consistent with their assignment to internal transitions of stable, spatially separated excitons. A large splitting (∼ 30 cm^-1) between cyclotron-resonance-like transitions for 13 T ≤ B ≤ 23 T for the x = 0.1 sample is attributed to enhanced spin splitting due to conduction-valence-band Landau-level mixing across the interface.