A tunable topological insulator in the spin helical Dirac transport regime

Helical Dirac fermions are relativistic particles which, unlike conventional Dirac fermions in graphene, have a net intrinsic angular momentum (spin) interlocked with their translational momentum, a property desirable for spintronic and computing technologies. Recently, it was proposed that such helical Dirac systems could be realized in so-called topological insulators — materials in which strong spin–orbit coupling gives rise to a bulk insulating gap and surface states protected against scattering by time-reversal symmetry. Hsieh et al. combine spin- and momentum-resolved spectroscopic imaging techniques to report the experimental realization of such a system in a bismuth-based material, where the experiments reveal nearly 100% spin polarization even up to room temperature. Crucially, the paper reports tunability of the fermion density, via doping, enabling the authors to drive the system to the so-called topological transport regime, which is believed to facilitate spin transport without heat dissipation. Helical Dirac fermions are charge carriers that behave as massless relativistic particles with an intrinsic angular momentum (spin) locked to their translational momentum, a property desirable for spintronic and computing technologies. It has recently been proposed that such fermions may exist at the edges of certain types of topologically ordered insulators. Here, the realization and characterization of such a system is reported; the results reveal nearly 100 per cent spin polarization, even up to room temperature. 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 physics1,2,3,4,5,6,7,8,9. Prominent examples include the anomalous quantization of magneto-electric coupling4,5,6, half-fermion states that are their own antiparticle7,8, 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 insulators3,4,12—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 regime3,4,5,6,7,8,9. However, helical Dirac fermions have not been observed in existing topological insulators13,14,15,16,17,18. 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.