L. Wray

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.

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.

Observation of a topological crystalline insulator phase and topological phase transition in Pb 1− x Sn x Te

A topological insulator protected by time-reversal symmetry is realized via spinorbit interaction driven band inversion. The topological phase in the Bi1−xSbx system is due to an odd number of band inversions. A related spin-orbit system, the Pb1−xSnxTe, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry protected topological crystalline insulator phase in the Pb1−xSnxTe class of materials which has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite-Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi1−xSbx. Our observation of the spin-polarized Dirac surface states in the inverted Pb1−xSnxTe and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.A topological insulator protected by time-reversal symmetry is realized via spin-orbit interaction-driven band inversion. The topological phase in the Bi(1-x)Sb(x) system is due to an odd number of band inversions. A related spin-orbit system, the Pb(1-x)Sn(x)Te, has long been known to contain an even number of inversions based on band theory. Here we experimentally investigate the possibility of a mirror symmetry-protected topological crystalline insulator phase in the Pb(1-x)Sn(x)Te class of materials that has been theoretically predicted to exist in its end compound SnTe. Our experimental results show that at a finite Pb composition above the topological inversion phase transition, the surface exhibits even number of spin-polarized Dirac cone states revealing mirror-protected topological order distinct from that observed in Bi(1-x)Sb(x). Our observation of the spin-polarized Dirac surface states in the inverted Pb(1-x)Sn(x)Te and their absence in the non-inverted compounds related via a topological phase transition provide the experimental groundwork for opening the research on novel topological order in quantum devices.

Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator

Two types of bulk insulator are realized in the same family of compounds through chemical doping. The recently discovered three-dimensional or bulk topological insulators are expected to exhibit exotic quantum phenomena. It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction or the crystal lattice, driving the system through a topological quantum phase transition. By directly measuring the topological quantum numbers and invariants, we report the observation of a phase transition in a tunable spin-orbit system, BiTl(S1–δSeδ)2, in which the topological state formation is visualized. In the topological state, vortex-like polarization states are observed to exhibit three-dimensional vectorial textures, which collectively feature a chirality transition as the spin momentum–locked electrons on the surface go through the zero carrier density point. Such phase transition and texture inversion can be the physical basis for observing fractional charge (±e/2) and other fractional topological phenomena.

Single-Dirac-cone topological surface states in the TlBiSe(2) class of topological semiconductors.

We investigate several strong spin-orbit coupling ternary chalcogenides related to the (Pb,Sn)Te series of compounds. Our first-principles calculations predict the low-temperature rhombohedral ordered phase in TlBiTe₂, TlBiSe₂, and TlSbX₂ (X=Te, Se, S) to be topologically nontrivial. We identify the specific surface termination that realizes the single Dirac cone through first-principles surface state computations. This termination minimizes effects of dangling bonds, making it favorable for photoemission experiments. In addition, our analysis predicts that thin films of these materials could harbor novel 2D quantum spin Hall states, and support odd-parity topological superconductivity.

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

We report the first photoemission study of Fe

Topological surface states and Dirac point tuning in ternary topological insulators

_{1+x}

Momentum dependence of superconducting gap, strong-coupling dispersion kink, and tightly bound Cooper pairs in the high- Tc (Sr,Ba) 1-x(K,Na)xFe2As2 superconductors

Te - the host compound of the newly discovered iron-chalcogenide superconductors (maximum T