  Weak antilocalization (WAL) and linear magnetoresistance (LMR) are two most
commonly 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 bulk
states (often carrying significant conduction in many TIs) and are observable
even in non-TI materials. Here, we demonstrate back-gated ambipolar TI
field-effect transistors in (Bi0.04Sb0.96)2Te3 thin films grown by molecular
beam epitaxy on SrTiO3(111), exhibiting a large carrier density tunability (by
nearly 2 orders of magnitude) and a metal-insulator transition in the bulk
(allowing effectively switching off the bulk conduction). Tuning the Fermi
level from bulk band to TSS strongly enhances both the WAL (increasing the
number 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 by
a strongly nonlinear Hall effect, suggesting important roles of charge
inhomogeneity (and a related classical LMR), although existing models of LMR
cannot capture all aspects of our data. Our systematic gate and temperature
dependent magnetotransport studies provide deeper insights into the nature of
both MR phenomena and reveal differences between bulk and TSS transport in TI
related materials.
