Exploring new electrical and optical properties of two-subband electron systems

We have investigated the physics of a two-subband, two dimensional electron gas (2DEG) system in low temperatures and high magnetic fields. The presence of a second electric subband can have an enormous impact on the physics of such systems. If the second subband is energetically close to the Fermi level, the photoluminescence (PL) intensity can be modulated to oscillate in a similar way as the Shubilikov-de Hass oscillations as the externally applied magnetic field is swept. And the recombination energy displays a step-like behavior. The second set of Landau levels brought about by the populated second subband can mix with the Landau levels of the first subband. This Landau level mixing manifests itself as floating of delocalized states energy in strong magnetic fields and more importantly, re-entrant quantum Hall transition can be observed.; In this thesis, I present first the interesting many-body phenomenon called Fermi-edge enhanced emission. The oscillating emission intensity and the step-like structure in emission energy can be shown to correlate with the relative position of the Fermi level and the second subband. The Fermi edge enhanced emission, however, abruptly ends at nu = 2 and in its place, a new peak with a smaller recombination energy appears suddenly. This new emission peak is not explained by exiting theories regarding PL emissions near low filling factors. We interpret this peak as evidence of a phase transition between an excitonic (bound electron-hole) state to a free (unbound) state. Calculation showing such scenario is plausible is presented. We also present alternative models that shed additional lights on this interesting transition.; Next the physics of Landau level mixing is explored through the theoretical and experimental constructions of a phase diagram on the density vs. magnetic field plane (the n-B plane). This n-B phase diagram shows a distinctly different behavior from the one-subband case. We have observed floating of extended states energy in strong fields and as a consequence, a negative differential Hall resistance (NDHR, drho xy/dB < 0) is observed. We believe this NDHR is a precursor to the long sought re-entrant quantum Hall transition.; Finally we present strong evidence for the re-entrant quantum Hall transition. We have investigated the carrier density, magnetic fields, and temperature evolutions of reentrant integer quantum Hall effects. Our data confirm that whenever Landau levels cross, re-entrant quantum Hall effect can be observed.