Electromechanical Properties Of Atomically Thin Materials
Pearce, Alexander James
Thesis or dissertation
University of Exeter
We discuss the effect of elastic deformations on the electronic properties of atomically thin materials, with a focus on bilayer graphene and MoS2 membranes. In these materials distortions of the lattice translate into fictitious gauge fields in the electronic Dirac Hamiltonian that are explicitly derived here for arbitrary elastic deformations, including in-plane as well as flexural (out-of-plane) distortions. We consider bilayer graphene, where a constant fictitious gauge field causes a dramatic reconstruction of the low energy trigonally warped electronic spectrum inducing topological transitions in the Fermi surface. We then present results of ballistic transport in trigonally warped bilayer graphene with and without strain, with particular focus on noise and the Fano factor. With the inclusion of trigonal warping the Fano factor at the Dirac point is still F = 1/3, but the range of energies which show pseudo diffusive transport increases by orders of magnitude compared to the results stemming out of a parabolic spectrum and the applied strain acts to increase this energy range further. We also consider arbitrary deformations of another two-dimensional membrane, MoS2. Distortions of this lattice also lead to a fictitious gauge field arising within the Dirac Hamiltonian, but with a distinct structure than seen in graphene. We present the full form of the fictitious gauge fields that arise in MoS2. Using the fictitious gauge fields we study the coupling between electronic and mechanical degrees of freedom, in particular the coupling between electrons and excited vibrational modes, or vibrons. To understand whether these effects may have a strong influence on electronic transport in MoS2 we calculate the dimensionless electron-vibron coupling constant for all vibron modes relevant for electronic transport. We find that electron-vibron coupling constant is highly sample specific and that the longitudinal stretching mode is the vibron with the dominant coupling. This however reaches maximum values which are lower than those observed in carbon nanostructures.
PhD in Physics