Following the successful isolation of a continuously growing number of layeredmaterials in monolayer forms, these can now be assembled into stacks, referred toas van der Waals heterostructures. In this thesis, we investigate theoretically theelectronic properties of heterostructures based on bilayer graphene, two coupledlayers of carbon. We first study the minibands of bilayer graphene placed on asemiconducting substrate with a unit cell about, but not exactly, three times largerthan that of graphene. While the former introduces asymmetry in the distributionof the electronic wave function between the layers and opens a band gap in theelectronic spectrum, the latter generates a long wavelength moire perturbationthat couples states in inequivalent graphene Brillouin zone corners. We show that,depending on the details of the moire perturbation, the miniband structure canbe tuned to a situation where a single narrow miniband is separated from therest of the spectrum by small gaps. We then discuss electron tunneling betweenbilayer and monolayer graphene across a hexagonal boron nitride barrier in thepresence of a strong perpendicular magnetic field. We demonstrate that in sucha device, valley-polarization close to unity can be produced in fields of order 1 T.Finally, we discuss electron transport in a van der Waals tunnelling transistor inwhich the electronic density of states in one of the electrodes has been modulatedby a superlattice perturbation. Using the example of twisted bilayer grapheneand a similar system of monolayer graphene on hexagonal boron nitride, we showthat negative differential resistance is possible in such transistors as a consequenceof the moire-induced changes in the density of states and without the need ofmomentum conserving tunnelling.
|Date of Award||20 Sep 2018|
|Supervisor||Marcin Mucha-Kruczynski (Supervisor)|
- Moire Patterns
- van der Waals Heterostructures
- 2D materials
- hexagonal boron-nitride