Electronic Correlations and Topology in Graphene Moiré Multilayers and InAs/GaSb-Derivative Systems

Citation

Polski, Robert Michael (2023) Electronic Correlations and Topology in Graphene Moiré Multilayers and InAs/GaSb-Derivative Systems. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/yhws-0f08. https://resolver.caltech.edu/CaltechTHESIS:01052023-230400021

Abstract

Twisted bilayer graphene (TBG) near the magic angle exhibits a wide variety of correlated and topological phases such as superconductivity, correlated insulators, and orbital ferromagnetism. We show using electrical transport measurements that adding a layer of tungsten diselenide in proximity to twisted bilayer graphene stabilizes superconductivity to twist angles significantly below the magic angle despite the disappearance of correlated insulators and insulators at full moiré filling. These findings--along with our report of a relationship between superconductivity and symmetry breaking Fermi surface reconstruction--suggest constraints on theories of the origin of superconductivity in TBG. In the context of this TBG-tungsten diselenide system, we study how the correlated phases evolve over a wide twist angle range and classify them into a hierarchy based on where they occur relative to the magic angle (or where bands have been maximally flattened). While effects such as orbital ferromagnetism near one electron per moiré unit cell and gapped correlated insulators only exist in close proximity to the magic angle, superconductivity and high-temperature cascade transitions survive in a wider twist angle range.

We also analyze the structures of twisted trilayer, quadrilayer, and pentalayer graphene (and all proximitized to tungsten diselenide) near their respective theoretical magic angles, revealing robust electron- and hole-side superconductivity in each heterostructure. We additionally find previously unreported insulating states in twisted trilayer and quadrilayer graphene along with an enlarged filling range of superconductivity in pentalayer. Our studies on twisted graphene multilayers beyond two layers allow us to generalize the correlated physics found in TBG and consider the role of the additional bands introduced.

In the last part of this thesis, we measure the two-dimensional topological insulator candidate system InAs/GaSb with added stoichiometric impurities. Previous studies in pure InAs/GaSb structures have revealed low bulk resistivity and edge states that arise from trivial effects which can be easily mistaken for topological effects. Due, in part, to the strain effects of Indium impurities added to GaSb, our results show high bulk resistivity. We also, due to the wide gate-tunability in our devices, are able to measure the expected spin-orbit-split valence band structure. Our development of highly tunable InAs/GaSb-derivative structures paves the way for another look at two-dimensional topological insulator behavior in these systems and for their integration into superconducting devices.

Thesis Files