Optical and plasmonic properties of twisted bilayer graphene: Impact of interlayer tunneling asymmetry and ground-state charge inhomogeneity

Twisted bilayer graphene (TBG) at twist angles ??1? has recently attracted a great deal of interest for its rich transport phenomenology. We present a theoretical study of the local optical conductivity, plasmon spectra, and thermoelectric properties of TBG at different filling factors and twist angles ?. Our calculations are based on the electronic band structures obtained from a continuum model that has two tunable parameters u0 and u1 which parametrize the intrasublattice interlayer and intersublattice interlayer tunneling rate, respectively. In this article we focus on two key aspects: (i) we study the dependence of our results on the value of u0, exploring the whole range 0<=u0<=u1; and (ii) we take into account effects arising from the intrinsic charge density inhomogeneity present in TBG, by calculating the band structures within the self-consistent Hartree approximation. At zero filling factor, i.e., at the charge neutrality point, the optical conductivity is quite sensitive to the value of u0 and twist angle, whereas the charge inhomogeneity brings about only modest corrections. On the other hand, away from zero filling, static screening dominates and the optical conductivity is appreciably affected by the charge inhomogeneity, the largest effects being seen on the intraband contribution to it. These findings are also reflected by the plasmonic spectra. We compare our results with existing ones in the literature, where effects (i) and (ii) above have not been studied systematically. As natural byproducts of our calculations, we obtain the Drude weight and Seebeck coefficient. The former displays an enhanced particle-hole asymmetry stemming from the inhomogeneous ground-state charge distribution. The latter is shown to display a broad sign-changing feature even at low temperatures (?5K) due to the reduced slope of the bands, as compared to those of single-layer graphene.