Electronic and optical conductivity of Kekulé-patterned graphene: Intravalley and intervalley transport

Abstract

A Kubo formalism is used to calculate the electronic and optical conductivity of a graphene superlattice with Y-shaped kekul\'e bond texture, similar to that visualized in recent experiments. We show that new conduction channels between the valleys in graphene are opened by the kekul\'e distortion. This intervalley contribution to the electronic transport is not present in pristine graphene and here appears due to the folding of the Dirac cones $K$, $K'$ on top of each other. The contribution of intervalley transport to the conductivity of graphene, as well as the modification of the intravalley transport, are analyzed in detail for different frequency, temperature, chemical potential and scattering rate limits. We obtain analytical expressions for the conductivity that reproduce previous expressions used to fit experimental measurements in graphene and compare with direct numerical evaluations of the Kubo formula, finding great agreement. The optical absorption arising from intervalley transitions displays a maximum at a special frequency that can be tuned by doping. Our results show how the single parameter describing the valley coupling in this system could be obtained by measuring graphene's optical absorbance in the region where interband transitions are blocked by the impurities. Finally, we use Fermi's golden rule to independently verify some of the previous results.