Quantum and Classical Magnetoresistance in Ambipolar Topological Insulator Transistors with Gate-tunable Bulk and Surface Conduction

摘要

Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two mostcommonly observed magnetoresistance (MR) phenomena in topological insulators(TIs) and often attributed to the Dirac topological surface states (TSS).However, ambiguities exist because these phenomena could also come from bulkstates (often carrying significant conduction in many TIs) and are observableeven in non-TI materials. Here, we demonstrate back-gated ambipolar TIfield-effect transistors in (Bi0.04Sb0.96)2Te3 thin films grown by molecularbeam epitaxy on SrTiO3(111), exhibiting a large carrier density tunability (bynearly 2 orders of magnitude) and a metal-insulator transition in the bulk(allowing effectively switching off the bulk conduction). Tuning the Fermilevel from bulk band to TSS strongly enhances both the WAL (increasing thenumber of quantum coherent channels from one to peak around two) and LMR(increasing its slope by up to 10 times). The SS-enhanced LMR is accompanied bya strongly nonlinear Hall effect, suggesting important roles of chargeinhomogeneity (and a related classical LMR), although existing models of LMRcannot capture all aspects of our data. Our systematic gate and temperaturedependent magnetotransport studies provide deeper insights into the nature ofboth MR phenomena and reveal differences between bulk and TSS transport in TIrelated materials.