A tunable topological insulator in the spin helical Dirac transport regime

Abstract

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. Prominent examples include the anomalous quantization of magneto-electric coupling, half-fermion states that are their own antiparticle, and charge fractionalization in a Bose-Einstein condensate, all of which are not possible with conventional Dirac fermions of the graphene variety. 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 graphene or bismuth. It has recently been proposed that helical Dirac fermions may exist at the edges of certain types of topologically ordered insulators-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. However, helical Dirac fermions have not been observed in existing topological insulators. 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 Bi(2)Se(3).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.

Topological Insulator Nanowires and Nanoribbons

“…[3][4][5][6][7] Single crystalline nanostructure, on the other hand, offers an attractive alternative system to study the surface states, for example, by transport measurements or surface scanning techniques. As the sample * Corresponding author.…”

“…Angleresolved photoemission spectroscopy (ARPES) measurements on bulk single crystals of BixSb1-x, Bi2Se3, and Bi2Te3 have verified the existence of the 3D TI phase. [3][4][5][6][7] In particular, the surface states of Bi2Se3 forms a single Dirac cone inside a large bulk band gap of 0.3 eV, thus being suggested as the reference material for the 3D TIs. 2,4,8 The unique properties of the Bi2Se3 TI may pave the way for dissipationless quantum electronics and room temperature spintronics applications.…”

Ultrathin Topological Insulator Bi2Se3 Nanoribbons Exfoliated by Atomic Force Microscopy

“…It is likely that the chemical potential is actually located in the bulk conduction band. 11,13 For the 4-6 QLs, even if the outmost 1-2 QLs are affected by the environment and become insulating, there remain enough Bi 2 Se 3 QLs in the middle that contribute to the small R s .DC transport measurements are also performed on ultra-thin nanoribbons. A device with 4-probe geometry is fabricated from an AFM-exfoliated Bi 2 Se 3 nanoribbon with 5 QLs by standard electron beam lithograp hy and thermal evaporation of Cr/Au contacts (Figure 5b).…”

Interface electronic structure at the topological insulator–ferrimagnetic insulator junction

“…Although TIs show bulk insulating performance, they exhibit Dirac-like gapless bands at their surfaces [4][5][6][7][8][9][10] . The surface state is ensured by time-reversal symmetry (TRS) and the spin polarization of the surface state electrons is locked to its momentum.…”