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.Mx (Mx 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.
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We acknowledge the following people for discussions: P. W. Anderson, B. Altshuler, L. Balents, M. R. Beasley, B. A. Bernevig, C. Callan, J. C. Davis, H. Fertig, E. Fradkin, L. Fu, D. Gross, D. Haldane, K. Le Hur, B. I. Halperin, D. A. Huse, C. L. Kane, C. Kallin, E. A. Kim, R. B. Laughlin, D.-H. Lee, P. A. Lee, J. E. Moore, A. J. Millis, A. H. Castro Neto, J. Orenstein, P. Phillips, S. Sachdev, Dan C. Tsui, A. Vishwanath, F. Wilczek, X.-G. Wen and A. Yazdani. The spin-resolved and spin-integrated ARPES measurements using synchrotron X-ray facilities and theoretical computations are supported by the Basic Energy Sciences of the US Department of Energy (DE-FG-02-05ER46200, AC03-76SF00098 and DE-FG02-07ER46352) and by the Swiss Light Source, Paul Scherrer Institute. Materials growth and characterization are supported by the NSF through the Princeton Center for Complex Materials (DMR-0819860) and Princeton University. M.Z.H. acknowledges additional support from the A. P. Sloan Foundation, an R. H. Dicke fellowship research grant and the Kavli Institute of Theoretical Physics at Santa Barbara.
Author Contributions D.H., Y.X. and D.Q. contributed equally to the experiment with the assistance of L.W. and M.Z.H.; D.G., Y.S.H. and R.J.C. provided critically important high quality single crystal samples; J.G.C. and N.P.O. performed the transport measurements; J.H.D., F.M., J.O., L.P. and A.V.F. provided beamline assistance; H.L. and A.B. carried out the theoretical calculations; M.Z.H. conceived the design to reach the topological transport regime and was responsible for the overall project direction, planning, and integration among different research units.
This file contains Supplementary Figures S1- S6 with Legends, Supplementary Data and Supplementary References.
About this article
npj Quantum Materials (2019)