Millikelvin magneto-optical studies of two-dimensional hole systems

We present magnetophotoluminescence (PL) investigations of a range of very high-mobility low-density two-dimensional hole systems in GaAs-${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$As quantum wells, obtained using excitation both above and below the ${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ga}}_{\mathit{x}}$As band gap. At low two-dimensional hole densities, we observe two peaks at zero magnetic field and an analysis of the temperature and excitation-power dependences of these suggests that they originate from the recombination of neutral and positively charged excitons. At higher densities, the peaks evolve smoothly into a structure related to the two-dimensional hole density of states. Their intensity dependences and their evolution in magnetic field support this description. In high magnetic fields at low densities, we observe a new PL line at lower energy whose behavior is similar to PL features reported recently as being an optical signature of the magnetically induced hole Wigner solid. However, examination of the effects of illumination on hole density leads us to conclude that our observations are not associated with the Wigner solid. In the higher-density regime, we observe minima in the PL intensity and shifts in the PL energy at integer and fractional Landau-level filling factors, and a more complicated PL structure at very low filling factors. © 1996 The American Physical Society.