Coulomb drag in topological insulator films

Abstract We study Coulomb drag between the top and bottom surfaces of topological insulator films. We derive a kinetic equation for the thin-film spin density matrix containing the full spin structure of the two-layer system, and analyze the electron–electron interaction in detail in order to recover all terms responsible for Coulomb drag. Focusing on typical topological insulator systems, with a film thicknesses d up to 6 nm, we obtain numerical and approximate analytical results for the drag resistivity ρ D and find that ρ D is proportional to T 2 d − 4 n a − 3 / 2 n p − 3 / 2 at low temperature T and low electron density n a , p , with a denoting the active layer and p the passive layer. In addition, we compare ρ D with graphene, identifying qualitative and quantitative differences, and we discuss the multi-valley case, ultra thin films and electron–hole layers.