  Topological insulators are a novel class of quantum matter with a gapped
insulating bulk yet gapless spin helical Dirac fermion conducting surface
states. Here, we report local and non-local electrical and magneto transport
measurements in dual-gated BiSbTeSe2 thin film topological insulator devices,
with conduction dominated by the spatially separated top and bottom surfaces,
each hosting a single species of Dirac fermions with independent gate control
over the carrier type and density. We observe many intriguing quantum transport
phenomena in such a fully-tunable two-species topological Dirac gas, including
a zero-magnetic-field minimum conductivity close to twice the conducatance
quantum at the double Dirac point, a series of ambipolar two-component
half-integer Dirac quantum Hall states and an electron-hole total filling
factor zero state (with a zero-Hall plateau), exhibiting dissipationless
(chiral) and dissipative (non-chiral) edge conduction respectively. Such a
system paves the way to explore rich physics ranging from topological
magnetoelectric effects to exciton condensation.
