Inducing a topological transition in graphene nanoribbons superlattices by external strain

Abstract

The purpose of this thesis is to study the existence of localized states in superlattices, which can be used to create nanoelectronic devices. These ideas are born with the observation of localized states in polyacetylene, which provided a simple example of topological insulators: insulating materials that, under certain conditions, present low energy localized states at their edges, meanwhile being insulating in the bulk. This model was proposed by Su, Schrieffer and Heeger (SSH). On the other hand, a material that has a set of symmetries similar to polyacetylene is graphene. A two-dimensional material in which carbon atoms are arranged in a hexagonal or honeycomb lattice. Furthermore, the 2010 Nobel Prize was awarded to Geim and Novoselov for synthesizing this material. This thesis focus on the electronic and mechanical consequences of joining different segments of graphene nanoribbons (GNR). In certain cases the GNR segments form a superlattice reproducing the SSH model at energies close to the Fermi level. We found a general mechanism to control the SSH-like bonding patterns with external strain, even a topological transition can be induced at moderated strain values.\cite{flores2022inducing} In particular, for a specific combination of GNRs, we found a topological transition between ≈ 3-5 of strain. The SSH-like states are surprisingly robust, even close to the fracture of the material.