The effects of low temperature illumination and annealing on fractional quantum Hall (FQH) characteristics of a GaAs/AlGaAs quantum well are investigated. Illumination alone, below 1 K, decreases the density of the 2DEG electrons by more than an order of magnitude and resets the sample to a repeatable initial state. Subsequent thermal annealing at a few Kelvin restores the original density and dramatically improves FQH characteristics. A reliable illumination and annealing recipe is developed that yields an energy gap of 600 mK for the 5/2 state.

Reentrant integer quantum Hall (RIQH) states are believed to be correlated electron solid phases, although their microscopic description remains unclear. As bias current increases, longitudinal and Hall resistivities measured for these states exhibit multiple sharp breakdown transitions, a signature unique to RIQH states. A comparison of RIQH breakdown characteristics at multiple voltage probes indicates that these signatures can be ascribed to a phase boundary between broken-down and unbroken regions, spreading chirally from source and drain contacts as a function of bias current and passing voltage probes one by one. The chiral sense of the spreading is not set by the chirality of the edge state itself, instead depending on electron- or holelike character of the RIQH state.

A new method of creating indirect excitons in asymmetric double quantum wells, based on mass filtering, is introduced in this paper. It is shown that different tunneling times of electrons and holes from the narrow well to the wide one in connection with fine tuning of the carrier density by a small barrier photoexcitation allow the creation of an electrically neutral system of indirect excitons. Resonant microwave absorption is investigated in such a double-layer system which detects both size-dependent plasma resonances for the case of a charged electron-hole system and size-independent indirect exciton 1S-2S transitions in the case of a neutral system. The exciton transition energy is measured as a function of the layer separation, and a contradiction with theoretical predictions is established which indicates a strong dielectric screening of excitonic states due to overlap of their wavefunctions.

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