Coulomb drag in topological insulator films

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 rho(D) and find that rho(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 rho(D) with graphene, identifying qualitative and quantitative differences, and we discuss the multi-valley case, ultra thin films and electron-hole layers. (C) 2015 Elsevier B.V. All rights reserved.