The extreme quantum regime of 2D electron and hole systems

Abstract

The ground state of a 2D electron system (2DES) in a high magnetic field (Laughlin liquid versus Wigner solid) is probed by low temperature measurements of the intrinsic optical spectra of GaAs single heterojunctions (SHJs) unperturbed by the presence of deliberately introduced impurity layers. The objective is to understand the relationship of the signature observed in natural recombination optics to the quantum Hall effect (QHE) and magnetically-induced Wigner solid (MIWS) regions of the ground state phase diagram. Once understood, the photoluminescence (PL) signature then offers the potential to elicit quantitative information on the nature of the ground states. Recombination dynamics provide essential information on the nature of optical processes and time-resolved PL (TRPL) data are presented which identify the importance of excitonic recombination mechanisms in standard SHJ samples at high magnetic fields. A coherent model is put forward which accounts for the observed PL and TRPL signature throughout the magnetic field range-integer, fractional QHE and MIWS regimes. Our results are consistent with a mechanism in which the spectral feature associated with crystallisation derives from the trapping of excitons within a few Bohr radii ( z separation) of the heterointerface and their subsequent ( x, y) localisation by the presence of a 2D electron lattice at this interface. The development of an insulated, optically-transparent top gate architecture that extends the PL studies in the MIWS regime to examine the additional perturbation of well-defined patterns in the 2DES, located up to 700 nm below the semiconductor surface in our low density high quality samples, is described. A summary is also given of preliminary electrical transport data for the 2D hole system in p-type GaAs heterojunctions, in intense pulsed magnetic fields (to 52 T) at low temperatures (0.3 K), which examine competition of fractional QHE ground states and an insulating phase in the extreme quantum limit that has been associated with a hole solid.