Master's Defense: Emily Hajigeorgiou

Quantum point contacts with complex gate geometry in high-mobility GaAs

Quantum point contacts (QPCs) are often referred to as fundamental building blocks for condensed matter experiments. Traditionally, a QPC consists of two gates patterned on the surface of a GaAs/AlGaAs heterostructure. In this thesis, we fabricate and study two complex gate geometry QPC devices consisting of seven and twelve gates. Our motivation for complicating what would normally be a conventional two-gate device, arises from its inherent limitation in fine tuning the potential of the ballistic channel. Specifically, our interest lies in the fractional quantum Hall regime, where QPCs exhibit unique capabilities to locally manipulate edge channels, providing a platform to explore exotic fractional quantum Hall states. However, recent studies have highlighted the sensitivity of these states to device geometry, indicating that merely two gates might not be enough to bring us in the regime of interest. More complex devices that consisted of two gate layers including a screening layer of gates were designed, fabricated and measured in this work. We were able to show that by pixelating the QPC channel with gates, we can enable spatial movement of quantum Hall edge states. Furthermore, we show evidence that with our unique device design, that allows for spatial movement of states as well as the ability of having a top gate over each channel, certain fractional quantum Hall states appear in our devices.