Line Defects as Valley Filters in Graphene

Abstract

Hexagonal two-dimensional crystals typically feature two identical but inequivalent conduction band minima—so-called valleys—at the corners of the Brillouin zone. Exploiting this additional quantum number to process information—the goal of the field of valleytronics—requires directly manipulating the valley quantum number. Line defects emerging at grain boundaries between regions of pristine graphene could potentially act as valley filters. Quantum transport is quantitatively simulated through two such typical defects in a fully ab initio parameterized tight-binding model to assess the validity of such a strategy. Pronounced valley polarization effects are found in transport through the line defect, that sensitively depend on the type and electronic structure of the line defect. In particular, a 5-8-5 defect features several strongly localized states breaking the valley symmetry, and, accordingly, leads to pronounced valley filtering effects. By contrast, a comparatively similar 5-7 defect does not feature localized states, instead showing pronounced valley filtering at large angles of incidence. A much weaker energy dependence can still be exploited to select a specific valley. These results highlight the potential of defect engineering for building valleytronic devices, yet also the importance of realistic, quantitative models for judging the properties of tailored defects.