Parallel field magnetoresistance in topological insulator thin films

We report that the finite thickness of three-dimensional topological insulator (TI) thin films produces an observable magnetoresistance (MR) in phase coherent transport in parallel magnetic fields. The MR data of Bi sub(2)Se sub(3) and (Bi sub(1-x)Sb sub(x)) sub(2)Te sub(3) thin films are compared with existing theoretical models of parallel field magnetotransport. We conclude that the Tl thin films bring parallel field transport into a unique regime in which the coupling of surface states to bulk and to opposite surfaces is indispensable for understanding the observed MR. The beta parameter extracted from parallel field MR can in principle provide a figure of merit for searching TI compounds with more insulating bulk than existing materials.