Dynamical response functions and collective modes of bilayer graphene

Bilayer graphene (BLG) has recently attracted a great deal of attention because of its electrically tunable energy gaps and its unusual electronic structure. In this Rapid Communication we present analytical and semianalytical expressions, based on the four-band continuum model, for the layer-sum and layer-difference density-response functions of neutral and doped BLG. These results demonstrate that BLG density fluctuations can exhibit either single-component massive-chiral character or standard two-layer character, depending on energy and doping.