David Hsieh

Observation of a large-gap topological-insulator class with a single Dirac cone on the surface

Y. Xia, 2 D. Qian, 3 D. Hsieh, 2 L. Wray, A. Pal, H. Lin, A. Bansil, D. Grauer, Y. S. Hor, R. J. Cava, and M. Z. Hasan 2 Department of Physics, Princeton University, Princeton, NJ 08544, USA Princeton Center for Complex Materials, Princeton University, Princeton, NJ 08544, USA Department of Physics, Shanghai Jiao Tong University, Shanghai 200030, China Department of Physics, Northeastern University, Boston, MA 02115, USA Department of Chemistry, Princeton University, Princeton, NJ 08544, USA (Dated: Submitted for publication in December 2008)

A tunable topological insulator in the spin helical Dirac transport regime

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 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.Princeton University, Princeton, NJ 08544, USA Department of Physics, Shanghai Jiao Tong University, Shanghai 200030, China Swiss Light Source, Paul Scherrer Institute, CH-5232, Villigen, Switzerland Physik-Institut, Universität Zürich-Irchel, 8057 Zürich, Switzerland Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley, CA 94720, USA Department of Physics, Northeastern University, Boston, MA 02115, USA Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Princeton Center for Complex Materials, Princeton University, Princeton NJ 08544, USA

Observation of Unconventional Quantum Spin Textures in Topological Insulators

D. Hsieh, Y. Xia, L. Wray, D. Qian, A. Pal, J. H. Dil, 3 F. Meier, 3 J. Osterwalder, G. Bihlmayer, C. L. Kane, Y. S. Hor, R. J. Cava, and M. Z. Hasan 7 Joseph Henry Laboratories of Physics, Princeton University, Princeton, NJ 08544, USA Swiss Light Source, Paul Scherrer Institute, CH-5232, Villigen, Switzerland Physik-Institut, Universität Zürich-Irchel, 8057 Zürich, Switzerland Institut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germany Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA Department of Chemistry, Princeton University, Princeton, NJ 08544, USA Princeton Center for Complex Materials, Princeton University, Princeton, NJ 08544, USA (Dated: First submitted to Science on July 22, 2008)A topologically ordered material is characterized by a rare quantum organization of electrons that evades the conventional spontaneously broken symmetry–based classification of condensed matter. Exotic spin-transport phenomena, such as the dissipationless quantum spin Hall effect, have been speculated to originate from a topological order whose identification requires a spin-sensitive measurement, which does not exist to this date in any system. Using Mott polarimetry, we probed the spin degrees of freedom and demonstrated that topological quantum numbers are completely determined from spin texture–imaging measurements. Applying this method to Sb and Bi1–xSbx, we identified the origin of its topological order and unusual chiral properties. These results taken together constitute the first observation of surface electrons collectively carrying a topological quantum Berrys phase and definite spin chirality, which are the key electronic properties component for realizing topological quantum computing bits with intrinsic spin Hall–like topological phenomena.

Topological Surface States Protected from Backscattering by Chiral Spin Texture

Topological insulators are a new class of insulators in which a bulk gap for electronic excitations is generated because of the strong spin–orbit coupling inherent to these systems. These materials are distinguished from ordinary insulators by the presence of gapless metallic surface states, resembling chiral edge modes in quantum Hall systems, but with unconventional spin textures. A key predicted feature of such spin-textured boundary states is their insensitivity to spin-independent scattering, which is thought to protect them from backscattering and localization. Recently, experimental and theoretical efforts have provided strong evidence for the existence of both two- and three-dimensional classes of such topological insulator materials in semiconductor quantum well structures and several bismuth-based compounds, but so far experiments have not probed the sensitivity of these chiral states to scattering. Here we use scanning tunnelling spectroscopy and angle-resolved photoemission spectroscopy to visualize the gapless surface states in the three-dimensional topological insulator Bi1-xSbx, and examine in detail the influence of scattering from disorder caused by random alloying in this compound. We show that, despite strong atomic scale disorder, backscattering between states of opposite momentum and opposite spin is absent. Our observations demonstrate that the chiral nature of these states protects the spin of the carriers. These chiral states are therefore potentially useful for spin-based electronics, in which long spin coherence is critical, and also for quantum computing applications, where topological protection can enable fault-tolerant information processing.

A topological insulator surface under strong Coulomb, magnetic and disorder perturbations

Topological insulators embody a state of bulk matter characterized by spin-momentum-locked surface states that span the bulk bandgap. This highly unusual surface spin environment provides a rich ground for uncovering new phenomena. Understanding the response of a topological surface to strong Coulomb perturbations represents a frontier in discovering the interacting and emergent many-body physics of topological surfaces. Here we present the first controlled study of topological insulator surfaces under Coulomb and magnetic perturbations. We have used time-resolved deposition of iron, with a large Coulomb charge and significant magnetic moment, to systematically modify the topological spin structure of the Bi_2Se_3 surface. We observe that such perturbation leads to the creation of odd multiples of Dirac fermions and that magnetic interactions break time-reversal symmetry in the presence of band hybridizations. We present a theoretical model to account for the observed electron dynamics of the topological surface. Taken collectively, these results are a critical guide in controlling electron mobility and quantum behaviour of topological surfaces, not only for device applications but also in setting the stage for creating exotic particles such as axions or imaging monopoles on the surface.

Control over topological insulator photocurrents with light polarization

Three-dimensional topological insulators represent a new quantum phase of matter with spin-polarized surface states that are protected from backscattering. The static electronic properties of these surface states have been comprehensively imaged by both photoemission and tunnelling spectroscopies. Theorists have proposed that topological surface states can also exhibit novel electronic responses to light, such as topological quantum phase transitions and spin-polarized electrical currents. However, the effects of optically driving a topological insulator out of equilibrium have remained largely unexplored experimentally, and no photocurrents have been measured. Here, we show that illuminating the topological insulator Bi(2)Se(3) with circularly polarized light generates a photocurrent that originates from topological helical Dirac fermions, and that reversing the helicity of the light reverses the direction of the photocurrent. We also observe a photocurrent that is controlled by the linear polarization of light and argue that it may also have a topological surface state origin. This approach may allow the probing of dynamic properties of topological insulators and lead to novel opto-spintronic devices.

Development of ferromagnetism in the doped topological insulator Bi 2 − x Mn x Te 3

The development of ferromagnetism in Mn-doped Bi_2Te_3 is characterized through measurements on a series of single crystals with different Mn content. Scanning tunneling microscopy analysis shows that the Mn substitutes on the Bi sites, forming compounds of the type Bi_(2−x)Mn_xTe_3, and that the Mn substitutions are randomly distributed, not clustered. Mn doping first gives rise to local magnetic moments with Curie-like behavior, but by the compositions Bi_(1.96)Mn_(0.04)Te_3 and Bi_(1.91)Mn_(0.09)Te_3, a second-order ferromagnetic transition is observed, with T_C∼9–12 K. The easy axis of magnetization in the ferromagnetic phase is perpendicular to the Bi2Te3 basal plane. Thermoelectric power and Hall effect measurements show that the Mn-doped Bi_2Te_3 crystals are p-type. Angle-resolved photoemission spectroscopy measurements show that the topological surface states that are present in pristine Bi_2Te_3 are also present at 15 K in ferromagnetic Mn-doped Bi2−xMnxTe3 and that the dispersion relations of the surface states are changed in a subtle fashion.

Measurement of Intrinsic Dirac Fermion Cooling on the Surface of the Topological Insulator Bi2Se3 Using Time-Resolved and Angle-Resolved Photoemission Spectroscopy

We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) Bi(2)Se(3) to study the ultrafast dynamics of surface and bulk electrons after photoexcitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.

Fermi Surface Topology and Low-Lying Quasiparticle Dynamics of Parent Fe1+xTe/Se Superconductor

We report the first photoemission study of Fe

Observation of a warped helical spin texture in Bi2Se3 from circular dichroism angle-resolved photoemission spectroscopy.

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