Quantum properties of few-layer graphene and van der Waals heterostructures

Abstract

In this thesis, we study the quantum properties of semimetallic few-layer graphene, and its van der Waals (vdW) heterostructures with insulating hexagonal boron nitride (hBN) and the ferromagnetic semiconducting chromium trihalides (CrX$_3 $, X=Cl,Br,I). Monolayer graphene is an atomically thin two-dimensional (2D) material, which was first isolated from bulk graphite at the University of Manchester 2004. It is famed for its physical properties, including its large tensile strength and electrical and thermal conductivities. Constructing van der Waals (vdW) heterostructures of graphene and different materials allows for novel two-dimensional materials synergising their properties, and the interference of the incommensurate lattices of each layer gives a long-range moir\'e superlattice (mSL). This reduces the density scale of effects such as the umklapp scattering of electrons off a periodic lattice potential. We start with a study into engineering the topological properties of bilayer graphene using strain and a vertical bias, predicting that the topological magnetic moment is enhanced by two orders of magnitude with modest variation of the parameters. We also show how this manifests in the Landau level spectrum and the anomalous contribution to the Hall conductivity. Then, we consider a highly-aligned hBN/graphene/hBN heterostructure, which features a Kagom\'e network of chiral channels between regions of different topology. We derive the Aharonov-Bohm oscillations in the electrical conductivity of coherent electron wave packets propagating through this network, which is a signature of structural inversion symmetry breaking. We continue with an experimental study into the opening of a bandgap in bilayer graphene resulting from the charge transfer to a CrX$_3 $ substrate. We demonstrate excellent agreement with our theoretical predictions, and show that electrons in the conduction band of the CrX$_3 $ layer are strongly correlated. After this, we discuss the electrical properties of a 3D material composed of alternating stacks of commensurate graphene and hBN. This exhibits semimetallic and semiconducting behaviour, depending on the coupling between the graphene layers and the relative stacking of the hBN layers. We conclude with a study into the contribution of umklapp electron-electron scattering to the $ p $-doped resistivity of a bilayer graphene/hBN heterostructure. This has distinct features from the corresponding monolayer graphene/hBN heterostructure, with the resistivity rapidly growing with hole density above a threshold before reaching a peak value, both of which are determined by the size of the mSL.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Vladimir Falko (Supervisor)