| This thesis describes the fabrication and low-temperature magnetotransport properties of high-mobility GaAs hole bilayers, with an emphasis on strongly interacting layers with negligible inter-layer tunneling. The study of these two dimensional (2D) systems is stimulated by the existence of many-particle quantum Hall states (QHS) that arise from the inter-layer interaction.; Interacting bilayer systems with negligible inter-layer tunneling need to satisfy certain design requirements, which translate into challenges for sample fabrication: the barrier between the two wells needs to be sufficiently thick to prevent inter-layer tunneling, yet the inter-particle mean distances within the same layer and in opposite layers have to be comparable for strong inter-layer interaction. The GaAs 2D holes that we use in our study have a relatively high effective mass (compared to, e.g., GaAs 2D electrons) which allows us to reduce the barrier thickness while keeping the tunneling relatively small. In samples with low (≃2 x 1010 cm -2) carrier density in each layer, at equal layer density we observe a QHS state at total Landau level filling factor nu = 1 (layer filling factor 1/2). This peculiar bilayer QHS is stabilized when the carriers in each layer pair with the vacancies in the opposite layer forming neutral objects (excitons) which condense at the lowest temperatures. Using independently contacted bilayers in a geometry where equal currents are passed in opposite directions in the two layers (counterflow), we demonstrate that both the longitudinal and the Hall resistivities tend to vanish at low temperatures at nu = 1. This observation demonstrates the pairing of oppositely charged carriers in opposite layers, and implies that the ground state of the system is an excitonic condensate.; Our data show that the nu = 1 QHS is flanked by a reentrant insulating phase at nearby fillings, suggesting the formation of a pinned, bilayer Wigner crystal. As we transfer charge from one layer to another at constant total density, the nu = 1 phase-coherent QHS becomes stronger, evincing its robustness against charge imbalance, while the insulating phase disappears. When imbalanced, our bilayers also display hysteretic magnetoresistance at higher filling factors, suggesting an inter-layer charge instability at these fillings. |
