GaAs Hole Systems Research Articles

Collective cyclotron modes in high-mobility two-dimensional hole systems in GaAs - (Ga, Al)As heterojunctions: I. Experiments at low magnetic fields and theory

The cyclotron resonance of very high-mobility two-dimensional holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied over the frequency range 30 to 200 GHz. Although the presence of two hole spin subbands in the samples suggests that two cyclotron resonances should be observed, in practice only a single resonance occurs for a wide range of conditions (temperature, field) and sample properties (hole density, mobility). Furthermore, the cyclotron resonance spectra often exhibit a strong temperature dependence. In the case of a single, sharp cyclotron resonance, the resonance field may shift by as much as 20% when warming the sample from 1.4 to 4.2 K. In the case of spectra containing multiple cyclotron resonances, similar changes in temperature shift the resonance positions together to form a single cyclotron absorption. This behaviour is explained in terms of two interacting hole subsystems with different effective masses formed by the two spin subbands. An analytical expression for the contribution to the high-frequency conductivity due to coupled cyclotron motion of the two hole systems is derived and shown to encompass previous theories developed for more restricted ranges of conditions. The expression predicts the complex behaviour of the experimental spectra very well, and enables hole masses, hole - lattice scattering rates and hole - hole scattering rates to be extracted. Comparisons between theory and data also show that a reactive interaction dominates the coupling between the spin subsystems at low temperatures. This is the first of two papers dealing with correlated hole cyclotron resonance; the second shows that the model derived in this work can also be used to treat cyclotron resonance data recorded at very high magnetic fields .

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{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum wells, obtained using excitation both above and below the ${\mathrm{Al}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\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. \textcopyright{} 1996 The American Physical Society.