Prototypical Topological Insulator Research Articles

Proximity-induced magnetism and an anomalous Hall effect in Bi2Se3/LaCoO3: a topological insulator/ferromagnetic insulator thin film heterostructure.

Inducing magnetism in a topological insulator (TI) by exchange coupling with a ferromagnetic insulator (FMI) will break the time-reversal symmetry of topological surface states, offering possibilities to realize several predicted novel magneto-electric effects. Seeking suitable FMI materials is crucial for the coupling of heterojunctions, and yet is challenging as well and only a few kinds have been explored. In this report, we introduce epitaxial LaCoO3 thin films on a SrTiO3 substrate, which is an insulating ferromagnet with a Curie temperature of TC ∼ 85 K, to be combined with TIs for proximity coupling. Thin films of the prototype topological insulator, Bi2Se3, are successfully grown onto the (001) surface of LaCoO3/SrTiO3, forming a high-quality TI/FMI heterostructure with a sharp interface. The magnetic and transport measurements manifest the emergence of a ferromagnetic phase in Bi2Se3 films, with additional induced moments and a suppressed weak antilocalization effect, while preserving the carrier mobility of the intrinsic Bi2Se3 films at the same time. Moreover, a signal of an anomalous Hall effect is observed and persists up to temperatures above 100 K, paving the way towards spintronic device applications.

ARPES study of the epitaxially grown topological crystalline insulator SnTe(111)

SnTe is a prototypical topological crystalline insulator, in which the gapless surface state is protected by a crystal symmetry. The hallmark of the topological properties in SnTe is the Dirac cones projected to the surfaces with mirror symmetry, stemming from the band inversion near the L points of its bulk Brillouin zone, which can be measured by angle-resolved photoemission. We have obtained the (111) surface of SnTe film by molecular beam epitaxy on BaF2(111) substrate. Photon-energy-dependence of in situ angle-resolved photoemission, covering multiple Brillouin zones in the direction perpendicular to the (111) surface, demonstrate the projected Dirac cones at the Γ¯ and M¯ points of the surface Brillouin zone. In addition, we observe a Dirac-cone-like band structure at the Γ point of the bulk Brillouin zone, whose Dirac energy is largely different from those at the Γ¯ and M¯ points.

Observation of antiphase coherent phonons in the warped Dirac cone ofBi2Te3

In this Rapid Communication we investigate the coupling between excited electrons and phonons in the highly anisotropic electronic structure of the prototypical topological insulator Bi$_2$Te$_3$. Using time- and angle-resolved photoemission spectroscopy we are able to identify the emergence and ultrafast temporal evolution of the longitudinal-optical A$_{1g}$ coherent-phonon mode in Bi$_2$Te$_3$. We observe an antiphase behavior in the onset of the coherent-phonon oscillations between the $\overline{\Gamma K}$ and the $\overline{\Gamma M}$ high-symmetry directions that is consistent with warping. The qualitative agreement between our density-functional theory calculations and the experimental results reveals the critical role of the anisotropic coupling between Dirac fermions and phonon modes in the topological insulator Bi$_2$Te$_3$.

Nonmagnetic band gap at the Dirac point of the magnetic topological insulator (Bi(1-x)Mn(x))2Se3.

Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. Epitaxial (Bi1−xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap of ∼100 meV. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection.

Two-dimensional bismuth-rich nanosheets through the evaporative thinning of Se-doped Bi2Te3

High bulk conductance obscures the behavior of surface states in the prototypical topological insulators Bi2Te3 and Bi2Se3. However, ternary phases of Bi2Te3−ySey with balanced donor and acceptor levels may lead to large bulk resistivity, allowing for the observation of the surface states. Additionally, the contribution of the bulk conductance may be further suppressed by nanostructuring, increasing the surface-to-volume ratio. Herein we report the synthesis of a ternary tetradymite newly confined to two dimensions. Ultra-thin large-area stable nanosheets were fabricated via evaporative thinning of a Bi2Te2.9Se0.1 original phase. Owing to vapor pressure differences, a compositional shift to a final Bi-rich phase is observed. The Se/Te ratio of the nanosheet increases tenfold, due to the higher stability of the Bi–Se bonds. Hexagonal crystal symmetry is maintained despite dramatic changes in thickness and stoichiometry. Given that small variations in stoichiometry of this ternary system can incur large changes in carrier concentration and switch majority carrier type, the large compositional shifts found in this case imply that compositional analysis of similar CVD and PVD grown materials is critical to correctly interpret topological insulator performance. Further, the characterization techniques deployed, including STEM-EDS and ToF-SIMS, serve as a case study in determining such compositional shifts in two-dimensional form.

Nonequilibrium spin texture within a thin layer below the surface of current-carrying topological insulatorBi2Se3: A first-principles quantum transport study

We predict that unpolarized charge current injected into a ballistic thin film of prototypical topological insulator (TI) Bi$_2$Se$_3$ will generate a {\it noncollinear spin texture} $\mathbf{S}(\mathbf{r})$ on its surface. Furthermore, the nonequilibrium spin texture will extend into $\simeq 2$ nm thick layer below the TI surfaces due to penetration of evanescent wavefunctions from the metallic surfaces into the bulk of TI. Averaging $\mathbf{S}(\mathbf{r})$ over few \AA{} along the longitudinal direction defined by the current flow reveals large component pointing in the transverse direction. In addition, we find an order of magnitude smaller out-of-plane component when the direction of injected current with respect to Bi and Se atoms probes the largest hexagonal warping of the Dirac-cone dispersion on TI surface. Our analysis is based on an extension of the nonequilibrium Green functions combined with density functional theory (NEGF+DFT) to situations involving noncollinear spins and spin-orbit coupling. We also demonstrate how DFT calculations with properly optimized local orbital basis set can precisely match putatively more accurate calculations with plane-wave basis set for the supercell of Bi$_2$Se$_3$.

Intrinsic surface dipole in topological insulators

We calculate the local density of states of two prototypical topological insulators (Bi2Se3 and Bi2Te2Se) as a function of distance from the surface within density functional theory. We find that, in the absence of disorder or doping, there is a 2 nm thick surface dipole the origin of which is the occupation of the topological surface states above the Dirac point. As a consequence, the bottom of the conduction band is bent upward by about 75 meV near the surface, and there is a hump-like feature associated with the top of the valence band. We expect that band bending will occur in all pristine topological insulators as long as the Fermi level does not cross the Dirac point. Our results show that topological insulators are intrinsic Schottky barrier solar cells.

Giant Spin Pumping and Inverse Spin Hall Effect in the Presence of Surface and Bulk Spin-Orbit Coupling of Topological Insulator Bi2Se3.

Three-dimensional (3D) topological insulators are known for their strong spin-orbit coupling (SOC) and the existence of spin-textured surface states that might be potentially exploited for "topological spintronics." Here, we use spin pumping and the inverse spin Hall effect to demonstrate successful spin injection at room temperature from a metallic ferromagnet (CoFeB) into the prototypical 3D topological insulator Bi2Se3. The spin pumping process, driven by the magnetization dynamics of the metallic ferromagnet, introduces a spin current into the topological insulator layer, resulting in a broadening of the ferromagnetic resonance (FMR) line width. Theoretical modeling of spin pumping through the surface of Bi2Se3, as well as of the measured angular dependence of spin-charge conversion signal, suggests that pumped spin current is first greatly enhanced by the surface SOC and then converted into a dc-voltage signal primarily by the inverse spin Hall effect due to SOC of the bulk of Bi2Se3. We find that the FMR line width broadens significantly (more than a factor of 5) and we deduce a spin Hall angle as large as 0.43 in the Bi2Se3 layer.

Controlling the Spin Texture of Topological Insulators by Rational Design of Organic Molecules.

We present a rational design approach to customize the spin texture of surface states of a topological insulator. This approach relies on the extreme multifunctionality of organic molecules that are used to functionalize the surface of the prototypical topological insulator (TI) Bi2Se3. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin texture of Bi2Se3. The theoretical predictions are then verified in angular-resolved photoemission experiments. We show that, by tuning the strength of molecule-TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created. These tailored interface properties-passivation, spin-texture tuning, and creation of hybrid interface states-lay a solid foundation for interface-assisted molecular spintronics in spin-textured materials.

Search and design of nonmagnetic centrosymmetric layered crystals with large local spin polarization

Until recently, spin-polarization in nonmagnetic materials was the exclusive territory of non- centrosymmetric structures. It was recently shown that a form of hidden spin polarization (named the Rashba-2 or R-2 effect) could exist in globally centrosymmetric crystals provided the individual layers belong to polar point group symmetries. This realization could considerably broaden the range of materials that might be considered for spin-polarization spintronic applications to include the hitherto forbidden spintronic compound that belong to centrosymetric symmetries. Here we take the necessary steps to transition from such general, material-agnostic condensed matter theory arguments to material-specific design principles that could aid future laboratory search of R-2 materials. Specifically, we (i) classify different prototype layered structures that have been broadly studied in the literature in terms of their expected R-2 behavior, including the Bi2Se3-structure type (a prototype topological insulator), MoS2-structure type (a prototype valleytronic compound) and LaBiOS2-structure type (a host of superconductivity upon doping); (ii) formulate the properties that ideal R-2 compounds should have in terms of combination of their global unit cell symmetries with specific point group symmetries of their constituent sectors; (iii) use first-principles band theory to search for compounds from the prototype family of LaOBiS2-type structures that satisfy these R-2 design metrics. We initially consider both stable and hypothetical compounds to establish an understanding of trends of R-2 with composition, and then indicate the predictions that are expected to be stable and synthesizable. We predict large spin splittings (up to ~ 200 meV for holes in LaOBiTe2) as well as surface Rashba states. Experimental testing of such predictions is called for.

Graphene-like conjugated π bond system in Pb1−xSnxSe

Following the identification of the π bond in graphene, in this work, a π bond constructed through side-to-side overlap of half-filled 6pz orbitals was observed in a non-carbon crystal of Pb1–xSnxSe (x ∼ 0.34) (PSS), a prototype topological crystalline insulator and thermoelectric material with a high figure-of-merit. PSS compounds with a rock-salt type cubic crystal structure were found to consist of σ bond connected covalent chains of Pb(Sn)-Se with an additional π bond that is shared as a conjugated system among the four nearest neighbor Pb pairs in square symmetry within all {001} monoatomic layers per cubic unit cell. The π bond formed with half-filled 6pz orbitals between Pb atoms is consistent with the calculated results from quantum chemistry. The presence of π bonds was identified and verified with electron energy-loss spectroscopy through plasmonic excitations and electron density mapping via an inverse Fourier transform of X-ray diffraction.

Atomic structure ofBi2Se3andBi2Te3(111) surfaces probed by photoelectron diffraction and holography

Understanding how topologically protected surface states behave at surfaces and interfaces requires knowledge of the atomic structure. Whether the (111) surfaces of the prototypical topological insulators ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ are Bi or chalcogen terminated is the subject of current controversies. We employ photoelectron diffraction and holography, combining the advantages and avoiding the disadvantages of the contesting techniques previously used. We find bulklike chalcogen termination with a very small surface relaxation $(l1%)$ in agreement with density functional theory simulations. We prove the chalcogen termination for cleaved crystals and epitaxial films which shows the robustness of our conclusions.

Ab initiostudies of adatom- and vacancy-induced band bending inBi2Se3

We investigate the influence of potassium adsorption and selenium vacancies in the surface layer on the electronic properties of the prototypical topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. These modifications of the surface give rise to oscillations in the charge density that extend deep into the crystal. They result in a long-ranged potential perpendicular to the surface (also referred to as band bending) and new states in the band structure that are reminiscent of the states of a two-dimensional electron gas. Very similar effects have been observed in several experiments. The reorganization of the charge deep inside the crystal as a reaction to the surface modification constitutes a remarkable property of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and is closely related to its layered structure. The emergence of the long-ranged potential as a direct consequence of the charge reorganization turns out to be a generic property of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. However, calculations without spin-orbit coupling show that the band bending is not related to the nontrivial topological character of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.

Ion beam modification of topological insulator bismuth selenide

We demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.

Anisotropic effect of warping on the lifetime broadening of topological surface states in angle-resolved photoemission fromBi2Te3

Using angle resolved photoemission measurements, the authors detect a strong anisotropy of the lifetime broadening of the topological surface-state electrons of bismuth telluride, hence, demonstrating, for the first time, that the theoretically predicted strong band warping in this prototypical topological insulator results in a strong anisotropy of the electron scattering rates.

Surface theory of a family of topological Kondo insulators

A low-energy theory for the helical metallic states, residing on the surface of cubic topological Kondo insulators, is derived. Despite our analysis being primarily focused on a prototype topological Kondo insulator, Samarium hexaboride (SmB$_6$), the surface theory derived here can also capture key properties of other heavy fermion topological compounds with a similar underlying crystal structure. Starting from an effective mean-field eight-band model in the bulk, we arrive at a low-energy description of the surface states, pursuing both analytical and numerical approaches. In particular, we show that helical Dirac excitations occur near the $\bar{\Gamma}$ point and the two $\bar{X}$-points of the surface Brillouin zone and generally the energies of the Dirac points display {\it offset} relative to each other. We calculate the dependence of several observables (such as bulk insulating gap, energies of the surface Dirac fermions, their relative position to the bulk gap, etc.) on various parameters in the theory. We also investigate the effect of a spatial modulation of the chemical potential on the surface spectrum and show that this band bending generally results in "dragging down" of the Dirac points deep into the valence band and strong enhancement of Fermi velocity of surface electrons. Comparisons with recent ARPES and quantum oscillation experiments are drawn.

Local structure of the SnTe topological crystalline insulator: Rhombohedral distortions emerging from the rocksalt phase

${\mathrm{A}}^{\mathrm{IV}}{\mathrm{B}}^{\mathrm{VI}}$ crystals are believed to possess a rhombohedral (ferroelectric) structure at low temperature that changes to the rocksalt (paraelectric) structure above the Curie temperature. For GeTe it has been recently demonstrated that locally the structure retains the subsets of the shorter and longer bonds across the ferroelectric-to-paraelectric transition despite acquiring the cubic structure on average. Nothing is known about the existence of local distortions in SnTe, a prototypical topological crystalline insulator, where the crystal symmetry plays a crucial role. In this work we report the results of x-ray absorption measurements. We find that the structure is locally rhombohedrally distorted, and the distortions increase at $Tg100\phantom{\rule{4.pt}{0ex}}\text{K}$, breaking the rocksalt average symmetry. Our density functional theory simulations performed at 0 K indicate that the role of spin-orbit coupling in the formation of the local structure of SnTe at low temperature is negligibly small. The small stochastic distortions do not affect the intrinsic band inversion of SnTe.

Proximity effect between a topological insulator and a magnetic insulator with large perpendicular anisotropy

We report that thin films of a prototype topological insulator, Bi2Se3, can be epitaxially grown onto the (0001) surface of BaFe12O19 (BaM), a magnetic insulator with high Curie temperature and large perpendicular anisotropy. In the Bi2Se3 thin films grown on non-magnetic substrates, classic weak antilocalization (WAL) is manifested as cusp-shaped positive magnetoresistance (MR) in perpendicular magnetic fields and parabola-shaped positive MR in parallel fields, whereas in Bi2Se3/BaM heterostructures the low field MR is parabola-shaped, which is positive in perpendicular fields and negative in parallel fields. The magnetic field and temperature dependence of the MR is explained as a consequence of the suppression of WAL due to strong magnetic interactions at the Bi2Se3/BaM interface.

Tuning Dirac states by strain in the topological insulator Bi2Se3

Bismuth selenide is a prototypical 3D topological insulator; its electronic spectrum features a Dirac cone populated by surface states. Now, it is experimentally and numerically shown that a bandgap forms beyond a certain critical compressive strain, destroying the surface states.

Topological surface states onBi1−xSbx: Dependence on surface orientation, termination, and stability

Topological insulators support metallic surface states whose existence is protected by the bulk band structure. It has been predicted early that the topology of the surface state Fermi contour should depend on several factors, such as the surface orientation and termination and this raises the question to what degree a given surface state is protected by the bulk electronic structure upon structural changes. Using tight-binding calculations, we explore this question for the prototypical topological insulator Bi$_{1-x}$Sb$_x$, studying different terminations of the (111) and (110) surfaces. We also consider the implications of the topological protection for the (110) surfaces for the semimetals Bi and Sb