Session V31: Focus Session: Topological Insulators: Synthesis & Characterization - ARPES

Topological Surface States in Ternary Spin-Orbit Insulators: An ARPES Viewpoint

Utilization of topological surface states is expected to lead to new vistas in electronics and fundamental physics. However, most of the known topological insulators either do not feature necessary band structure conditions (location of Dirac point with respect to the bulk band) or lack topological invariants essential for certain class of applications. Using angle-resolved photoemission spectroscopy (ARPES), we discuss the electronic band structure topology of a family of ternary spin-orbit insulators some of which feature functional electronic structure with in-gap Dirac point while others feature novel topological invariants (weak Z2 invariants) in crystalline form. We also present some of our recent results on ternary topological insulators.   

Rashba Spin-Splitting Control at the Surface of the Topological Insulator Bi$_2$Se$_3$

The electronic structure of Bi$_2$Se$_3$ is studied by angle-resolved photoemission and density functional theory. We show that the instability of the surface electronic properties, observed even in ultra-high-vacuum conditions, can be overcome via {\it in situ} potassium deposition. In addition to accurately setting the carrier concentration, new Rashba-like spin-polarized states are induced, with a tunable, reversible, and highly stable spin splitting. {\it Ab initio} slab calculations reveal that these Rashba states are derived from the 5-quintuple-layer quantum-well states. While the K-induced potential gradient enhances the spin splitting, this may be present on pristine surfaces due to the symmetry breaking of the vacuum-solid interface. Phys.Rev.Lett. 107 186405 (2011)   

Recent Results on Topological Phase Transition and Texture Inversion in Tunable Topological Insulators

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, we report the observation of a phase transition in a tunable spin-orbit system, BiTl(S$_{1-\delta}$Se$_{\delta}$)$_2$, in which the topological state formation is visualized (S.-Y. Xu \textit{et al}., \textit{Science} (2011)). 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 ($\pm$e/2) and other fractional topological phenomena. We also present some of our recent results that reveal further novel spin and electronic properties of the system close to the critical point of the topological phase transition.   

Surface Termination of Cleaved Bi$_{2}$Se$_{3}$ Investigated by Low Energy Ion Scattering

The 3D Topological Insulator, Bismuth Selenide (Bi$_{2}$Se$_{3})$, is investigated with low energy ion scattering (LEIS). Se vacancies are believed to be responsible for the metallic behavior in transport, and LEIS is uniquely sensitive to the outermost atomic layer composition. Bi$_{2}$Se$_{3}$ is comprised of Se-Bi-Se-Bi-Se quintuple layers (QLs). Since the van der Waals bonds between QLs is weaker than the covalent bonds within each QL, it has been assumed that it is Se-terminated when cleaved. This assumption has been used in previous surface studies, such as STM or ARPES, which do not provide the composition of the surface atoms. 3 keV Na$^{+}$ ions were scattered from single crystal Bi$_{2}$Se$_{3}$ cleaved in ultra-high vacuum. At room temperature, the spectra indicate a surface terminated with Bi, rather than Se, although some Se is still present. The samples display a sharp 1x1 LEED pattern, indicative of an ordered material. We conclude that Bi$_{2}$Se$_{3}$ cleaves between the QLs, but that the surface Se quickly desorbs, likely as Se$_{2}$ or Se$_{4}$. To test this, the Se:Bi ratio was monitored by LEIS after a sample was cleaved at liquid nitrogen temperature. It was found that the ratio starts out high, but decreases over the course of hours until it reaches the same value as that of a room temperature cleave.   

Electronic Structure of Clean and Adsorbate-Covered Bi$_{2}$Se$_{3}$

The electronic structure of the topological insulator Bi$_{2}$Se$_{3}$ was probed using angle-resolved photoemission spectroscopy (ARPES). The electronic properties of clean and adsorbate-covered samples were studied for a combination of different bulk dopings and adsorbates. Due to contamination on the surface, the Dirac point of the topological surface states moves to higher binding energies, indicating an increasingly strong downward bending of the bands near the surface. This time-dependent band bending can be accelerated by intentionally exposing the surface to carbon monoxide, alkali atoms and other species. For a sufficiently strong band bending, new spectral features in the energy region of both the valence band and the conduction band are found. These changes are explained by a confinement of the states and the formation of quantum well states. This interpretation is supported by simple calculations of the band bending effects and by photon energy-dependent ARPES measurement that show how the band dispersion in the direction perpendicular to the surface is lost and non-dispersing, sharp states appear within the energy regions of the projected bulk bands. For a strong band bending, the conduction band quantum well states are strongly Rashba split.   

Sharp Dirac cone at a buried superconductor/topological insulator interface

We have studied the fully buried interface of a topological insulator, Bi2Se3, with the conventional superconductor Nb. We characterized the chemical reactivity at the interface using core level photoemission spectroscopy. Using ARPES we are able to observe the sharp Dirac cone at the buried interface and characterize its details; including energy positions, band dispersion, and electronic scattering rates. All these measurements are carried out over a wide range of coverages, and set the stage for more advanced studies of this interface.   

Mapping the Orbital Texture of the Topological Insulator Bi2Se3

The orbital texture of the topological insulator Bi2Se3 was observed with ARPES using linearly polarized light. The topological state features a superposition of all three p orbitals. We compare the measured orbitals to the existing density functional theory calculations in the literature. This illustrates some of the unusual properties of this topological state and helps clarify the origin of the currently conflicting results from the spin resolved ARPES measurements.   

Spin resolved photoemission on surface doped topological insulator Bi$_2$Se$_3$

Topological insulators (TL) have attracted much attention because of their exotic properties. Bi$_2$Se$_3$ is a model TL with a relative large bulk gap and a simple surface state structure. By depositing various impurities on the surface, we were able to fill the topological surface state and higher lying Rashba splitting surface states. The spin texture of the surface electronic structure was determined in spin resolved photoemission measurement.   

Controlling the Bulk-surface Spectroscopic Separation in 3D Topological ``Metals'' Exploiting Hybridization

How to separately probe the surface state in 3D topological insulators (TI) with chemical potential crossing conduction bands, i.e. topological metals, is a key challenge in exploiting the topological nature of surface states. The advent of MBE grown thin films of TI's with varying thickness only adds to the importance of the issue. We study the effects of hybridization in the spirit of Fano model for the low-energy effective four-band model of $Bi_2Se_3$ on a slab. We find that the apparent bulk-surface spectroscopic separation in the ARPES data on 3D TI can be viewed as {\it a consequence of hybridization} rather than the evidence for the absence of hybridization. We describe how the separation depends on the film thickness and propose ways to control the separation using strain.   

Topological insulators in the quaternary chalcogenide compounds and ternary famatinite compounds

We present first-principles calculations to predict several three-dimensional (3D) topological insulators in quaternary chalcogenide compounds of compositions I$_2$-II-IV-VI$_4$ and ternary famatinite compounds of compositions I$_3$-V-VI$_4$. Among the large number of members of these two families, we give examples of naturally occurring compounds that are mainly Cu-based chalcogenides. We show that these materials are candidates for 3D topological insulators or can be tuned to obtain topologically interesting phases by manipulating the atomic number of the various cations and anions. A band inversion can occur at a single point $\Gamma$ with large inversion strength, in addition to the opening of a bulk bandgap throughout the Brillouin zone. We discuss how the two investigated families of compounds are related to each other by cross-substitution of cations in the underlying tetragonal structure. Work supported by the US DOE.   

Quasiparticle band structures of $\beta$-HgS, HgSe, and HgTe

The electronic structures of mercury chalcogenides in the zinc-blende strucrure have been calculated by the LDA, $GW$ (one-shot, $G_{0}W_{0}$) and quasi-particle self-consistent $GW$ ($QSGW$) approximations including spin-orbit coupling (SO). The slight tendency to overestimation of the band gaps by $QSGW$ is avoided by using a $hybrid$ scheme (20$\%$ LDA and 80 $\%$ $QSGW$. The results of $G_{0}W_{0}$ depend strongly starting wave functions and are thus quite different from those from $QSGW$. Within $QSGW$ HgS is found to be a semiconductor, with a $\Gamma_{6}$ s-like conduction-band minimum state above the valence-band top $\Gamma_{7}$ and $\Gamma_{8}$ (``negative'' SO splitting). HgSe and HgTe have ``negative'' gaps (inverted band structure). In HgTe the $\Gamma_{7}$ state is below $\Gamma_{6}$ due to the large Te SO splitting, in contrast HgSe where $\Gamma_{6}$ is below $\Gamma_{7}$.   

Exceptionally Weak Electron-Phonon Coupling on the Surface of the Topological Insulator Bi$_2$Se$_3$ - A Promise for Room Temperature Applications

Gapless surface states on topological insulators are protected from elastic scattering on non-magnetic impurities which makes them promising candidates for low-power electronic applications. However, for wide-spread applications, these states should have to remain coherent at ambient temperatures. Here, we studied temperature dependence of the electronic structure and the scattering rates on the surface of a model topological insulator, Bi$_2$Se$_3$, by high resolution angle-resolved photoemission spectroscopy. We found an extremely weak broadening of the topological surface state with temperature and no anomalies in the state's dispersion, indicating exceptionally weak electron-phonon coupling. Our results demonstrate that the topological surface state is protected not only from elastic scattering on impurities, but also from scattering on low-energy phonons, suggesting that topological insulators could serve as a basis for room temperature electronic devices.   

Persistence of Topological Order and Formation of Quantum Well States in Topological Insulators B2(Se,Te)3 under Ambient Conditions

We report high resolution angle-resolved photoemission measurements on the surface state of the prototypical topological insulators, Bi2Se3, Bi2Te3 and Bi2Se0.4Te2.6, upon exposing to ambient conditions. We find that the topological order persists even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the surface of the topological insulators after the exposure which depends sensitively on the original composition, x, in Bi2Se3-xTex. These rich information are crucial in utilizing the surface state and in probing its physical properties under ambient conditions.   

Strong Warping Effects and Angular Momentum Structures of Topological Insulator

We performed angle resolved photoemission (ARPES) studies on Bi2Te3 with circularly polarized light. The the alignment of OAM is found to have a strong binding energy dependence. OAM close to Dirac point has an ideal chiral structure (sin$\theta$) without out-ofplane component. As the binding energy decreases, warping effect comes in and circular dichroism along the constant energy contour cannot be explained by a simple sin$\theta$ function but requires a sin 3$\theta$ term. When the warping effect becomes even stronger near the Fermi energy, circular dichroism has sin 6$\theta $ symmetry. Such behavior is found to be compatible with the theoretically predicted spin structure.   

Three-Dimensional Massive Dirac Fermion in the Bulk Band Structure of Cubic Inverse Perovskite Ca$_3$PbO

The band structure of a cubic inverse perovskite Ca$_3$PbO, which has been proposed as a candidate for a topological insulator, is analyzed by means of the first-principles calculation. It turns out that Ca3PbO is actually not a topological insulator, but a close observation of the calculated band structure near the Fermi energy reveals that there exist Dirac fermions in the bulk, instead of the surface, band structure. The Dirac fermion in this material is found on the $\Gamma$-X line in the momentum space and remarkably exactly at the Fermi energy in the energy space. It should also be noted that the discovered Dirac fermion is three-dimensional and massive with a very small mass of about $10^{-2}$ of the bare electron mass. The origin of the Dirac fermion in Ca$_3$PbO and the band structure of the materials related to Ca$_3$PbO will also be discussed.