  A three-dimensional (3D) topological insulator (TI) is a quantum state of
matter with a gapped insulating bulk yet a conducting surface hosting
topologically-protected gapless surface states. One of the most distinct
electronic transport signatures predicted for such topological surface states
(TSS) is a well-defined half-integer quantum Hall effect (QHE) in a magnetic
field, where the surface Hall conductivities become quantized in units of
(1/2)e2/h (e being the electron charge, h the Planck constant) concomitant with
vanishing resistance. Here, we observe well-developed QHE arising from TSS in
an intrinsic TI of BiSbTeSe2. Our samples exhibit surface dominated conduction
even close to room temperature, while the bulk conduction is negligible. At low
temperatures and high magnetic fields perpendicular to the top and bottom
surfaces, we observe well-developed integer quantized Hall plateaus, where the
two parallel surfaces each contributing a half integer e2/h quantized Hall (QH)
conductance, accompanied by vanishing longitudinal resistance. When the bottom
surface is gated to match the top surface in carrier density, only odd integer
QH plateaus are observed, representing a half-integer QHE of two degenerate
Dirac gases. This system provides an excellent platform to pursue a plethora of
exotic physics and novel device applications predicted for TIs, ranging from
magnetic monopoles and Majorana particles to dissipationless electronics and
fault-tolerant quantum computers.
