Thermodynamic and tunneling measurements of van der Waals heterostructures

Abstract

In certain electronic systems, strong Coulomb interactions between electrons can favor novel electronic phases that are difficult to anticipate theoretically. Accessing fundamental quantities such as the density of states in these platforms is crucial to their analysis. In this thesis, I explore the application of two measurement techniques towards this goal: capacitance measurements that probe the thermodynamic ground state of an electronic system and planar tunneling measurements that access its quasiparticle excitation spectrum. Both techniques were applied to van der Waals materials, a class of crystals composed of layered atomic sheets with weak interplane bonding which permits the isolation of single and few-layer sheets that can be manually assembled into heterostructures. Capacitance measurements were performed on a material system commonly known as magic-angle twisted bilayer graphene (MATBG).

When two monolayers of graphene, a single sheet of graphite, are stacked on top of one another with a relative twist between their crystal axes, the resultant band structure is substantially modified from the cases of both monolayer graphene and Bernal-stacked (non-twisted) bilayer graphene. At certain magic angles, the low energy bands become extremely flat, quenching the electronic kinetic energy and allowing strong electron-electron interactions to become relevant. Exotic insulating and superconducting phases have been observed using conventional transport measurements. By accessing the thermodynamic density of states of MATBG, we estimate its low energy bandwidth, Fermi velocity, and interaction-driven energy gaps. Time-domain planar tunneling was performed on a heterostructure that consisted of monolayer graphene and hexagonal boron nitride (serving as the dielectric and tunnel barrier) sandwiched between a graphite tunneling probe and metal gate.

Tunneling currents were induced by applying a sudden voltage pulse across the full parallel plate structure. The lack of in-plane charge motion allowed access to the tunneling density of states even when the heterostructure was electrically insulating in the quantum Hall regime. These measurements represent the first application of time-domain planar tunneling to the van der Waals class of materials, an important step in extending the technique to new material platforms.