1. Joseph Henry Laboratories of Physics, Department of Physics,


2. Department of Chemistry,


3. Princeton Center for Complex Materials,


4. Princeton Institute for Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA


5. Department of Physics, Shanghai Jiao Tong University, Shanghai 200030, China


6. Swiss Light Source, Paul Scherrer Institute, CH-5232, Villigen, Switzerland


7. Physik-Institute, Universitat Zurich-Irchel, 8057 Zurich, Switzerland


8. Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, California 94720, USA


9. Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA

Correspondence to: M. Z. Hasan^1,3,4 Correspondence and requests for materials should be addressed to M.Z.H. (Email: mzhasan@Princeton.edu).

Helical Dirac fermions—charge carriers that behave as massless relativistic particles with an intrinsic angular momentum (spin) locked to its translational momentum—are proposed to be the key to realizing fundamentally new phenomena in condensed matter physics^{1, 2, 3, 4, 5, 6, 7, 8, 9}. Prominent examples include the anomalous quantization of magneto-electric coupling^{4, 5, 6}, half-fermion states that are their own antiparticle^{7, 8}, and charge fractionalization in a Bose–Einstein condensate⁹, all of which are not possible with conventional Dirac fermions of the graphene variety¹⁰. Helical Dirac fermions have so far remained elusive owing to the lack of necessary spin-sensitive measurements and because such fermions are forbidden to exist in conventional materials harbouring relativistic electrons, such as graphene¹⁰ or bismuth¹¹. It has recently been proposed that helical Dirac fermions may exist at the edges of certain types of topologically ordered insulators^{3, 4, 12}—materials with a bulk insulating gap of spin–orbit origin and surface states protected against scattering by time-reversal symmetry—and that their peculiar properties may be accessed provided the insulator is tuned into the so-called topological transport regime^{3, 4, 5, 6, 7, 8, 9}. However, helical Dirac fermions have not been observed in existing topological insulators^{13, 14, 15, 16, 17, 18}. Here we report the realization and characterization of a tunable topological insulator in a bismuth-based class of material by combining spin-imaging and momentum-resolved spectroscopies, bulk charge compensation, Hall transport measurements and surface quantum control. Our results reveal a spin-momentum locked Dirac cone carrying a non-trivial Berry's phase that is nearly 100 per cent spin-polarized, which exhibits a tunable topological fermion density in the vicinity of the Kramers point and can be driven to the long-sought topological spin transport regime. The observed topological nodal state is shown to be protected even up to 300 K. Our demonstration of room-temperature topological order and non-trivial spin-texture in stoichiometric Bi₂Se₃.M_x (M_x indicates surface doping or gating control) paves the way for future graphene-like studies of topological insulators, and applications of the observed spin-polarized edge channels in spintronic and computing technologies possibly at room temperature.