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Cited by 1,979Topological Materials and PhenomenaAdvanced Condensed Matter PhysicsMagnetic properties of thin films

A tunable topological insulator in the spin helical Dirac transport regime

Open access
Hsieh, D., Xia, Y., Qian, D., Wray, L., Dil, J. H., Meier, F., Osterwalder, J., Patthey, L. +9 more·2009·Nature
DOI 10.1038/nature08234
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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.

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Written from the paper’s full text.

TL;DR

The paper reports spin- and angle-resolved photoemission evidence for a spin-momentum-locked, nearly 100% spin-polarized Dirac surface state in Bi₂Se₃/Bi₂Te₃ and shows that Ca bulk compensation plus NO₂ surface dosing can tune the surface Fermi level to the Dirac point, realizing a topological transport regime stable to 300 K. It establishes room-temperature topological order and proposes future transport, spintronic, Majorana, and fractional-charge experiments.

Key findings

  • Spin-resolved ARPES on Bi₂Te₃ at binding energy −20 meV along Γ–M found a clear y-component polarization of equal magnitude and opposite sign for opposite momenta; numerical fits yield 100(±15)% polarized spins pointing along k × z, while x and z components show no clear signal within statistical error.
  • Ca substitution in Bi₂Se₃ tunes the bulk Fermi level: resistivity peaks at 0.25% Ca and the Hall carrier density changes sign; E_F shifts by about 0.3 eV between δ = 0 and δ = 0.0025, placing E_F about 0.2 eV below the bulk conduction band minimum, consistent with the known ~0.35 eV bulk gap.
  • NO₂ surface dosing raises the surface Dirac point from E_B ≈ −0.3 eV at 0 L to −0.15 eV at 0.1 L and to charge neutrality (E_B = 0 eV) at 2 L; 0.1 L removes approximately 0.0066 electrons per surface unit cell, and an excess 2 L removes an additional ~0.005 electrons per unit cell, reducing the Fermi surface to a point.
  • The charge-neutral Dirac ground state is robust up to 300 K over days-long measurement times; angle-integrated intensity near the Dirac point decreases linearly to zero, and the same topological number ν₀ = 1 is inferred for Bi₁.₉₉₇₅Ca₀.₀₀₂₅Se₃.NO₂.

Methods

  • Spin-integrated ARPES was performed at Advanced Light Source beamlines 12.0.1 and 7.0.1 with 29–100 eV photons; typical energy and momentum resolutions were 15 meV and 1% of the surface Brillouin zone, and 50 meV and 2% of the surface Brillouin zone, respectively.
  • Spin-resolved ARPES was performed at the SIS beamline, Swiss Light Source, using the COPHEE spectrometer with two 40 kV classical Mott detectors measuring all three spatial spin components; measurements used 20–22 eV photons with 80 meV and 3% of the surface Brillouin zone resolutions.
  • Samples were cleaved in ultra-high vacuum better than 5 × 10⁻¹¹ torr and held at 10 K unless otherwise specified, with spin-resolved data collected at 50 K; NO₂ adsorption used controlled exposures to NO₂ gas, and calculations used the LAPW method in slab geometry with WIEN2K.

Limitations

  • Stated: the surface-state dispersion shows a pronounced time dependence after cleavage, so data collection times were only long enough to ensure sufficient statistics for measuring the spin-polarized character; the surface Fermi level rises back over a typical time scale of 18 hours due to surface band bending, meaning bulk Ca doping alone does not change E_F relative to the surface Dirac point in the ground state.
  • Stated: increasing NO₂ adsorption causes increasing surface disorder, making measured spectra progressively more diffuse and gradually reducing total photoemission intensity from the buried Bi₂₋δCaδSe₃ surface; non-Lorentzian spin-resolved lineshapes and their non-exact merger at large |k_x| are attributed to time evolution of the surface band dispersion, the dominant source of statistical uncertainty.
  • Evident: no direct transport measurement of the topological transport regime is reported; the paper states that results “pave the way for future transport based studies” and that direct detection of surface-edge states “would be possible” using transport methods. Also stated: the helical nature of surface electrons is most clearly resolved in Bi₂Te₃ because Bi₂Se₃ has an additional bulk conduction-band contribution around Γ.

Model-written summary. Check it against the paper before relying on it.

Integrity signals

Registry

Crossref · 10.1038/nature08234

Cites

13 of 13 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 7 cited sources checked · 0 supported · 0 flagged

  • Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    When a 0.25% concentration of Ca is introduced, EF is dramatically lowered to lie near the Dirac node.

    “When a 0.25% concentration of Ca is introduced, E F is dramatically lowered to lie near the Dirac node (Fig. 2(d))”
    Quote in the paperSubstantial claim
  • Findingnone given
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    The observed topological nodal Dirac ground state is protected even up to room temperature (300 K).

    “The observed topological nodal Dirac ground state is found to be protected even up to room temperature (300 K).”
    Quote in the paperSubstantial claim
  • Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    Hole carriers can be systematically introduced into the surface of Bi2-δCaδSe3 by dosing with NO2 molecules.

    “we show that hole carriers can be systematically introduced into the surface by dosing with NO 2 molecules”
    Quote in the paperSubstantial claim
  • Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    The surface electrons of both Bi2Se3 and Bi2Te3 exhibit a finite density of states near EF.

    “Although the surface electrons of both Bi 2 Se 3 and Bi 2 Te 3 exhibit a finite density of states near E F (Figs 1(a)-(d))”
    Quote in the paperSubstantial claim
  • Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    The paper reports the first realization of a tunable topological insulator in a bismuth based class by combining spin- and momentum-resolved spectroscopies, bulk charge compensation and surface control.

    “Here we report the first realization of a tunable topological insulator in a bismuth based class by combining spin- and momentum-resolved spectroscopies, bulk charge compensation and surface control.”
    Quote in the paperSubstantial claim
  • May combine two claims. 11

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    As Ca concentration is increased further, by δ=0.01, EF is located clearly below the Dirac node and intersects the hole-like bulk valence band.

    “As the Ca concentration is increased further, the position of E F continues a downward trend such that by δ = 0.01, it is located clearly below the Dirac node (Fig. 2(e)) and intersects the hole-like bulk valence band.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    0.1 L of NO2 removes approximately 0.0066 electrons per surface unit cell of Bi2-δCaδSe3(111), and an excess of 2 L reduces the FS to a single point within experimental resolution, with an additional 0.005 electrons per unit cell removed.

    “We find that 0.1 L of NO 2 removes approximately 0.0066 electrons per surface unit cell of Bi 2 -δ Ca δ Se 3 (111), and an excess of 2 L reduces the FS to a single point within our experimental resolution, which has an additional 0.005 electrons per unit cell removed.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingcitation
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    The direct demonstration of spin-polarized edge channels and room temperature operability of chemically gated stoichiometric Bi2Se3 or Bi2-δCaδSe3 was not achieved in purely 2D topological systems such as Hg(Cd)Te quantum wells.

    “Our direct demonstration of spin-polarized edge channels and room temperature operability of chemically gated stoichiometric Bi 2 Se 3 or Bi 2 -δ Ca δ Se 3 , not achieved in purely 2D topological systems such as Hg(Cd)Te quantum wells [29]”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [29] Konig, M. et al. Quantum spin Hall insulator state in HgTe quantum wells. Science 318, 766–770 (2007) source

      Not checked in the free run.

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    For measured Ca concentrations below and above 0.25%, electrical conduction is supported by electron and hole carriers respectively.

    “which shows that for measured Ca concentrations below and above 0.25%, electrical conduction is supported by electron and hole carriers respectively.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    Bulk Ca doping succeeds in tuning EF between the bulk valence and conduction bands, but does not change the position of EF relative to the surface Dirac point in the ground state.

    “Therefore, although bulk Ca doping succeeds in tuning E F between the bulk valence and conduction bands, it does not change the position of E F relative to the surface Dirac point in the ground state.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    Starting from EB ∼ -0.3 eV at 0 L, the surface Dirac point rises to -0.15 eV at 0.1 L where the surface bent CBM has completely disappeared, and finally to the charge neutrality point (EB = 0 eV) at 2 L.

    “Starting from E B ∼ -0.3 eV at a 0 Langmuir (L) dose, it rises to -0.15 eV at 0.1 L where the surface bent CBM has completely disappeared, and finally to the charge neutrality point ( E B = 0 eV) at 2 L.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    The fitted spin polarization of forward and backward moving electrons in Bi2Te3 is 100(±15)% and points along the (k × z) direction.

    “The fit results yield 100( ± 15)% polarized (see Fig. 1(h)) spins that point along the ( k × z ) direction”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Findingown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    The Dirac cone exhibits a tunable topological fermion density in the vicinity of the Kramers' point and can be driven to the long-sought topological transport regime.

    “which exhibits a tunable topological fermion density in the vicinity of the Kramers' point and can be driven to the long-sought topological transport regime.”
    Quote in the paperSubstantial claim

  • May combine two claims.

    Interpretationreasoning
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    Because surface doping does not affect the carrier density in the bulk (which thus remains insulating), the energy of the Dirac point is lifted above the bulk VBM; a new time independent electronic ground state is realized that lies in the topological transport regime with EF intersecting the Dirac node.

    “Because surface doping does not affect the carrier density in the bulk (which thus remains insulating), the energy of the Dirac point is lifted above the bulk VBM; a new time independent electronic ground state is realized that lies in the topological transport regime with E F intersecting the Dirac node.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Surface doping with NO2 does not affect the carrier density in the bulk, which remains insulating.

  • May combine two claims.

    Interpretationreasoning
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    Both Ca2+ and NO2- are non-magnetic and so do not break time-reversal symmetry, therefore the same topological number (ν0=1) applies even in the Dirac transport regime realized by our method.

    “Both Ca 2+ and NO 2 -are non-magnetic and so do not break timereversal symmetry, therefore the same topological number ( ν 0 = 1) applies even in the Dirac transport regime (Fig. 4) realized by our method shown here, which is stable with both time and temperature.”
    Quote in the paperSubstantial claim
    • Rests on an assumption: Ca2+ and NO2- are non-magnetic and do not break time-reversal symmetry.

  • May combine two claims.

    Interpretationown data
    § First observation of Spin-Momentum helical locking in Bi 2 Se 3 and Bi 2 Te 3 , demonstration of Topological-Order at 300K and a realization of topological-transport-regime

    Because Ca dopants are present in only trace quantities in Bi1.9975Ca0.0025Se3.NO2, the values of ξ2m(kT) do not deviate from those of pure Bi2Se3, as evidenced by the persistence of a single gapless surface band in both Bi2Se3 and Bi1.9975Ca0.0025Se3.

    “Because Ca dopants are present in only trace quantities in Bi 1 . 9975 Ca 0 . 0025 Se 3 .NO 2 , the values of ξ 2 m ( k T ) do not deviate from those of pure Bi 2 Se 3 , as evidenced by the persistence of a single gapless surface band in both Bi 2 Se 3 and Bi 1 . 9975 Ca 0 . 0025 Se 3 .”
    Quote in the paperSubstantial claim
11 of 11

  • The paper relies on this without support.

    Surface doping with NO2 does not affect the carrier density in the bulk, which remains insulating.

    “Because surface doping does not affect the carrier density in the bulk (which thus remains insulating)”

  • The paper relies on this without support.

    Ca2+ and NO2- are non-magnetic and do not break time-reversal symmetry.

    “Both Ca 2+ and NO 2 -are non-magnetic and so do not break timereversal symmetry”
2 of 2

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • The value of ν0 or θ can be determined from spin texture details of the spin-orbit surface states that form when the bulk is terminated.

    “can be determined from spin texture details of the spin-orbit surface states that form when the bulk is terminated as demonstrated by Hsieh et al . ( 6, 7 ).”

    From Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator (2011) · cites this paper as [6]

  • The topological order in TlBi(S1-δSeδ)2 originates from Bi and Se atoms.

    “Because the topological order in TlBi(S 1 -δ Se δ ) 2 originates from Bi and Se atoms and to illustrate an analogy with the known topological insulator Bi 2 Se 3 ( 6, 8, 11 )”

    From Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator (2011) · cites this paper as [6]

  • Spins tangentially arranged on such a circular Fermi surface lead to a Berry's phase of π, which is also a measure of the topological invariant and axion angle (ν0=1=θ/π) of the system.

    “Spins tangentially arranged on such a circular FS lead to a Berry's phase of π , which is also a measure of the topological invariant and the axion angle ( ν 0 = 1 = θ/π ) of the system ( 1, 4, 6 ).”

    From Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator (2011) · cites this paper as [6]

  • Bi2Se3 has spin-polarized topological surface states.

    “This is due to the fact that the topological surface states in Bi 2 Se 3 are spin polarized 8 .”

    From Observation of topological order in a superconducting doped topological insulator (2010) · cites this paper as [8]

  • Non-magnetic disorder created via molecular absorbent NO2 or alkali atom adsorption (K or Na) on the surface leaves the Dirac node intact in both Bi2Se3 and Bi2Te3.

    “Non-magnetic disorder created via molecular absorbent NO 2 or alkali atom adsorption (K or Na) on the surface leaves the Dirac node intact (Fig.5) in both Bi 2 Se 3 and Bi 2 Te 3 (33, 23).”

    From Three-Dimensional Topological Insulators (2011) · cites this paper as [23]

Self-check

Percentages, sums and reported differences re-computed from the paper’s own numbers.

Each contradiction between two passages listed here was confirmed by an independent check.

  • Ca-doped sample is called both most insulating and most metallic-like

    Minor

    The Figure 2 caption gives opposite descriptions of Bi 1 . 9975 Ca 0 . 0025 Se 3 relative to the topological-insulator classes being compared.

1 of 1

Cites

Flagged first, then by citations

Research works printed but not linked: 22

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  2. Moore, J. E. Topological insulators: the next generation. Nature Phys. 5, 378–380 (2009) · DOI · Paper page
  3. Qi, X.-L., Hughes, T. L. & Zhang, S.-C. Topological field theory of time-reversal invariant insulators. Phys. Rev. B 78, 195424 (2008) · DOI · Paper page
  4. Schnyder, A. P. et al. Classification of topological insulators and superconductors. Phys. Rev. B 78, 195125 (2008) · DOI · Paper page
  5. Essin, A., Moore, J. E. & Vanderbilt, D. Magnetoelectric polarizability and axion electrodynamics in crystalline insulators. Phys. Rev. Lett. 102, 146805 (2009) · DOI · Paper page
  6. Akhmerov, A. R., Nilsson, J. & Beenakker, C. W. J. Electrically detected interferometry of Majorana fermions in a topological insulator. Phys. Rev. Lett. 102, 216404 (2009) · DOI · Paper page
  7. Li, L. et al. Phase transitions of Dirac electrons in bismuth. Science 321, 547–550 (2008) · DOI · Paper page
  8. Moore, J. E. & Balents, L. Topological invariants of time-reversal-invariant band structures. Phys. Rev. B 75, 121306(R) (2007) · DOI · Paper page
  9. Hsieh, D. et al. Observation of unconventional quantum spin textures in topological insulators. Science 323, 919–922 (2009) · DOI · Paper page
  10. Noh, H.-J. et al. Spin-orbit interaction effect in the electronic structure of Bi2Te3 observed by angle-resolved photoemission spectroscopy. Europhys. Lett. 81, 57006 (2008) · DOI · Paper page
  11. Nishide, A. et al. Direct mapping of the spin-filtered surface bands of a three-dimensional quantum spin Hall insulator. Preprint at 〈 http://arxiv.org/abs/0902.2251 〉 (2009) · DOI
  12. Checkelsky, J. G., Li, L. & Ong, N. P. Divergent resistance of the Dirac point in graphene: evidence for a transition in high magnetic field. Phys. Rev. B 79, 115434 (2009) · DOI
  13. Zhang, H. et al. Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface. Nature Phys. 5, 438–442 (2009) · DOI · Paper page
  14. Meier, F., Dil, J. H., Lobo-Checa, J., Patthey, L. & Osterwalder, J. Quantitative vectorial spin analysis in angle-resolved photoemission: Bi/Ag(111) and Pb/Ag(111). Phys. Rev. B 77, 165431 (2008) · DOI
  15. Larson, P. et al. Electronic structure of Bi2X3 (X = S, Se, T) compounds: comparison of theoretical calculations with photoemission studies. Phys. Rev. B 65, 085108 (2001) · DOI · Paper page
  16. Mishra, S. K., Satpathy, S. & Jepsen, O. Electronic structure and thermoelectric properties of bismuth telluride and bismuth selenide. J. Phys. Condens. Matter 9, 461–470 (1997) · DOI · Paper page
  17. Black, J., Conwell, E. M., Seigle, L. & Spencer, C. W. Electrical and optical properties of some M2 V-BN3 VI-B semiconductors. J. Phys. Chem. Solids 2, 240–251 (1957) · DOI · Paper page
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  21. Blaha, P. et al. Computer Code WIEN2k (Vienna University of Technology, 2001)
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Authors
Hsieh, D., Xia, Y., Qian, D., Wray, L., Dil, J. H., Meier, F., Osterwalder, J., Patthey, L., Checkelsky, J. G., Ong, N. P., Fedorov, A. V., Lin, H., Bansil, A., Grauer, D., Hor, Y. S., Cava, R. J., & Hasan, M. Z.
Affiliations
  • Princeton University (US)
  • Shanghai Jiao Tong University (CN)
  • University of Zurich (CH)
  • Paul Scherrer Institute (CH)
  • Lawrence Berkeley National Laboratory (US)
  • Northeastern University (US)
Corresponding author
Hasan, M. Z.
ORCID
Venue
Nature
Publisher
Springer Science and Business Media LLC
Published
2009-07-20
Volume
460
Issue
7259
Pages
1101-1105
ISSN
  • 0028-0836 (ISSN-L)
  • 1476-4687
DOI
10.1038/nature08234
OpenAlex
W1589359576
Cited by
1,979
References
13
Citations by year
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FWCI
76.08
Type
journal article
Language
en
Open access
Licence
Locations
  • Nature (publishedVersion)
  • PubMed (publishedVersion)
  • Zurich Open Repository and Archive (University of Zurich) (acceptedVersion, other-oa)
  • arXiv (Cornell University) (submittedVersion)
  • CaltechAUTHORS (California Institute of Technology) (submittedVersion)
  • RePEc: Research Papers in Economics (submittedVersion)
  • DORA PSI (Paul Scherrer Institute) (submittedVersion)
  • Zurich Open Repository and Archive (University of Zurich) (submittedVersion, other-oa)
  • Zurich Open Repository and Archive (University of Zurich)
Topics
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Topological Materials and Phenomena
  • Physical Sciences › Physics and Astronomy › Condensed Matter Physics › Advanced Condensed Matter Physics
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Magnetic properties of thin films
Keywords
topological insulators, bismuth, spin polarization, spin-orbit coupling, topological transport, spintronics, surface states, time-reversal symmetry, relativistic electrons, magnetoelectric coupling, charge fractionalization, Bose-Einstein condensate

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