Electronic Properties and Measurement Techniques of 2D Heterostructures

Abstract

For the last fifty years, two-dimensional (2D) systems have stood out as model platforms for exploring electronic phenomena. Reduced dimensionality, electrostatic tunability, and enhanced electron-electron interactions enable effects not seen in bulk 3D conductors. Following the isolation of graphene, research on van der Waals (vdW) materials has exploded, revealing a vast landscape of correlated and topological physics. The ability to stack dissimilar materials into heterostructures has enabled control of the carrier density and displacement field of 2D samples while also opening the door for less conventional measurements. The limitless combinations and permutations of 2D heterostructures make for a rich and rewarding field of study. This thesis investigates the niche of 2D semimetals, specifically the electronic transport properties of the hybrid semimetal graphene-WTe2. While graphene hosts massless Dirac fermions that have high mobility and weak intrinsic interactions, WTe2 is dominated by correlated physics. This includesa putative excitonic insulator ground state, a likely unconventional superconducting state, and topological phenomena like the quantum spin Hall effect. By combining these complementary materials, we discover details of interlayer coupling in 2D semimetals as well as measurement and analysis techniques for other 2D heterostructures. I will begin this thesis by presenting historical context for the study of 2D systems, including techniques for the fabrication and measurement of 2D vdW heterostructures. I will then introduce both graphene and WTe2. Through careful analysis of the transport properties of the resulting hybrid, I will recover parameters of physical interest for both constituent materials. I will also present a sensing technique utilizing a remote graphene sensor to probe the chemical potential of the hybrid, shedding further light on its character. It will be shown that the properties of the hybrid can be largely understood from those of the individual monolayers with modifications due to interlayer tunneling and electron-electron interactions. Next, I will present my studies of the 2D dielectrics hexagonal boron nitride (BN) and Bi2SeO5. This work will show a hitherto unreported inverted temperature dependence in the breakdown of BN as well as the potential uses of Bi2SeO5 in 2D devices. Finally, I will discuss the problem of sample rotation and how it can be used to further characterize the electronic properties of matter. I will present my work towards the development of an in-situ two-axis rotator for use in a dilution refrigerator, including some preliminary testing results.