Session U13: Topological Insulators: Bi2Se3 and Bi2Te2Se

Topological dangling bonds with large spin splitting and enhanced spin polarization on the surfaces of Bi$_2$Se$_3$

We investigate the topological surface state properties at various surface cleaves in the topological insulator Bi$_2$Se$_3$, via first principles calculations and scanning tunneling microscopy/spectroscopy (STM/STS). While the typical surface termination occurs between two quintuple layers, we report the existence of a surface termination within a single quintuple layer where dangling bonds form with giant spin splitting owing to strong spin-orbit coupling. Unlike Rashba split states in a 2D electron gas, these states are constrained by the band topology of the host insulator with topological properties similar to the typical topological surface state, and thereby offer an alternative candidate for spintronics usage. We name these new states ``topological dangling-bond states.'' The degree of the spin polarization of these states is greatly enhanced. Since dangling bonds are more chemically reactive, the observed topological dangling-bond states provide a new avenue for manipulating band dispersions and spin-textures by adsorbed atoms or molecules. Work supported by DOE.   

Transient Surface Photoemission Involving Nonlinear Surface Sheet Polarization Developed on the Doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ Topological Insulator

Time- and angle-resolved photoemission spectroscopy is performed on the doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}^{\mathrm{\thinspace }}$topological insulator. We observe unusual variation in the efficiency of photoemission from femto-to-picosecond non-equilibrium particularly when two-dimensional electron gas (2DEG) states are developed on surface, while the surface confinement potential is virtually unchanged. The results indicate that a surface sheet polarization, which is induced nonlinearly by both the photon field and inversion-symmetry-breaking field, grows in magnitude as the 2DEG states become pronounced and opens a so-called surface photoemission channel, \textit{div}\textbf{\textit{A,}} \quad that can be varied transiently. Matrix element effects investigated by linearly-polarized angle-resolved photoemission also supports the presence of \textit{div}\textbf{\textit{A}}. The asymmetric charge distribution developed around vacuum-surface interface is considered as a key to understand and control Rashba splitting of the 2DEG states.   

High mobility topological insulator Bi2Se3 exfoliated devices with hexagonal Boron Nitride dielectrics

We report electronic transport measurements on double-gated topological insulator Bi2Se3 devices. To obtain both top- and bottom-gating, we exfoliate the Bi2Se3 on standard SiO2-capped Si and coat it with an ultrathin layer of hexagonal Boron Nitride (h-BN), which serves as a dielectric for a top gate. Using both top and bottom gates, we are able to identify the individual contributions of both surfaces and the bulk channel, and show that all three channels have mobilities exceeding 1000 cm2/Vs. Our results suggest that the h-BN transfer technique holds potential for providing a future path for high quality TI density-tunable devices.   

Magneto-transport study of magnetically-doped Bi2Se3

The interesting properties of topological insulators (TIs) arise from the zero energy gap at the Dirac point characterizing their surface states. These gapless chiral modes are attributed to spin-orbit coupling (typically very strong in TIs such as Bi2Se3), together with time reversal invariance (TRI). The introduction of magnetic dopants into a TI lattice can break TRI, providing a powerful tool for opening the gap in the Dirac cone, and for studying its consequences. In this paper we explore this phenomenon by introducing magnetic ions Mn and Fe into Bi sites in the Bi2Se3 lattice. A series of such magnetically-doped Bi2Se3 layers were grown by molecular beam epitaxy on GaAs (001) substrates, with the intention of studying the effects of such doping on the magnetic and electronic properties of this TI alloy. We discuss the results of magnetization, X-ray magnetic circular dichroism (XMCD), and extensive magneto-transport studies carried out to explore how the presence of magnetic ions in the TI lattice affects the magnetic and the electronic properties of these materials.   

Tuning Quantum Oscillations of Dirac Surface States on the Topological Insulator Bi$_2$Te$_2$Se by Ionic Liquid Gating

An \emph{in-situ} method to tune the chemical potential near the Dirac Point (DP) of a topological insulator (TI) would greatly facilitate several key experiments. However, in as-grown crystals of Bi-based TIs, the chemical potential $\mu$ lies high above the DP. Using liquid gating on 50-$\mu$m thick crystals of Bi$_2$Te$_2$Se, we demonstrate that $\mu$ can be tuned by a factor of 6 by observing changes to the Shubnikov-de Haas (SdH) period. A surprise is that the SdH amplitudes increase sharply with gating. Liquid gating allows the n=1 Landau level to be accessed, and the $\pi$-Berry phase to be determined with improved accuracy. We will discuss reversibility of liquid gating, and how we may distinguish the purely gating action from chemical reaction.   

Transport studies in topological insulator Bi$_{2}$Te$_{2}$Se

Recently, 3D topological insulators, featuring spin helical topological surface states (SS), have attracted strong attention in condensed matter physics. Although the SS have been directly revealed and intensively studied by surface sensitive measurements, such as ARPES and STM, transport measurements remain challenging due to coexistence of the surface and bulk conduction channels and the sensitivity of sample surfaces to ambient exposure. We have grown high quality Bi$_{2}$Te$_{2}$Se crystals by the Bridgeman method. Resistance showed an insulating behavior followed by saturation at low temperature, indicating surface conduction. Through magnetotransport measurements, we demonstrated high mobility SS on freshly cleaved crystals. The transport signatures of surface Dirac fermions were uncovered from 2D SdH oscillations and non-linear Hall effect. We have also compared transport properties of the samples before and after exposure to air. A giant cusp in magnetoresistance at zero B field was observed after exposure. Our studies may help understand the interplay between the surface and the bulk conduction channels and the degradation of SS due to environmental exposure. We will also present some experimental results of gate tuning and thermoelectric measurements on Bi$_{2}$Te$_{2}$Se.   

Promising topological surface states with persistent high spin polarization across Dirac point in Bi$_{2}$Te$_{2}$Se and Bi$_{2}$TeSe$_{2}$

Topological insulators (TIs) have attracted a great deal of atteion as key materials for spintronics technology. Among the established TIs, Bi$_{2}$X$_{3}$ (X$=$Se, Te) has been mostly studied because of their relatively large energy gap and the simplest topological surface state (TSS) with helical spin texture. However, an absence of the topological natures of TSS below Dirac point (E$_{\mathrm{D}})$ has been shown by spin- and angle-resolved photoemission spectroscopy (SARPES) and scanning tunneling spectroscopy under perpendicular magnetic field. It could be a disadvantage for extending its spintronic applications. Recently, one of the ternary tetradymite compounds, Bi$_{2}$Te$_{2}$Se was shown to be a TI by the ARPES measurement. Importantly, a highly bulk resistive feature in this compound has successfully led to the observation of its surface-derived quantum oscillations in the magnetotransport experiment. We have unambiguously clarified the spin feature of TSS in Bi$_{2}$Te$_{2}$Se and Bi$_{2}$Se$_{2}$Te for the first time by our novel SARPES. The markedly high spin polarization of topological surface states has been found to be 77{\%} and is persistent in the wide energy range across E$_{\mathrm{D}}$ in those compounds. The finding promises to extend the variety of spintoronic applications.   

Low frequency noise in exfoliated Bi$_{1.5}$Sb$_{0.4}$Te$_{1.7}$Se$_{1.3}$ field effect devices

Topological insulators are a new class of materials which have emerged as the new paradigm to study the exotic topological phases of matter. Electron transport is studied for field effect devices of Bi$_{1.5}$Sb$_{0.4}$Te$_{1.7}$Se$_{1.3}$ thin films, mechanically exfoliated on Si/SiO$_{2}$ substrates. The resistivity initially decreases with decreasing temperature indicating metallic-like behavior. However the resistivity shows an upturn below 13K which can be associated with the weak localization effect. The resistivity as a function of gate voltage shows hysteresis at low carrier densities and is independent of different sweep rates of the gate voltages. In addition to resistivity measurements, we have investigated low frequency noise or ``1/f'' noise as a function of temperature and gate voltage. The magnitude of 1/f noise increases at lower temperatures and with decreasing carrier densities. At lower carrier densities just like resistivity, noise is also saturated indicating long range disorder in the systems due to selenium vacancies. [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010) [2] E. Rossi, J. H. Bardarson, M. S. Fuhrer, and S. Das Sarma, Phys. Rev. Lett. 109, 096801 (2012)   

Scanning tunneling microscopy of topological insulator Bi$_2$Te$_2$Se

Using scanning tunneling microscopy, we study a prototypical topological insulator Bi$_2$Te$_2$Se having suppressed bulk carrier density. Landau level states of its topological surface state remarkably exhibit hysteresis behavior, which shift in energy controllably with the limits of ramping bias, forming hysteresis loops thereafter. The observed hysteresis behavior is attributed to the interplay between a tip-induced gating effect and an impurity-generated random charging effect. This provides a new avenue to controlling the topological surface state.   

Characterization of surface conducting states in Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ topological insulator single crystals

Topologically protected surface state (TSS) of a topological insulator (TI) can be described in terms of a spin-resolved Dirac band with helical-spin texture. In general, however, as-grown TIs are doped so that the surface conduction can be dominated by the bulk conduction. In this study, we minimized the bulk conduction using high-quality Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ TI thin single crystals, with the Fermi level lying in the bulk gap without gating. We confirmed that the weak anti-localization (WAL) effect and universal conductance fluctuations in our samples arose from the top and bottom surfaces. By back-gate tuning the WAL characteristics, we identified the TSS conducting characteristics and the coupling between the TSS and the topologically trivial two-dimensional electron gas (2DEG) states that emerged due to the band bending near the surface. The ambipolar Hall resistivity of the bottom surface was consistent with the back-gate-voltage dependence of the longitudinal resistance of the TSS. This study provides a highly coherent picture of the surface transport properties of TIs by successfully differentiating the transport of the TSS from those of the bulk conducting state and the topologically trivial 2DEG states.   

Two Dimensional universal conductance fluctuations in topological insulator Bi2Te2Se microribbons

The universal conductance fluctuations (UCFs), one of the most important manifestations of mesoscopic electronic interference, have not yet been demonstrated for the two-dimensional surface state of topological insulators (TIs) to date. Even if one delicately suppresses the bulk conductance of TI crystals, the fluctuation of the bulk conductance still keeps competitive and difficult to be separated from the desired UCFs of the surface carriers. Here we report on the experimental evidence of the UCFs of the two-dimensional surface state in the bulk insulating Bi2Te2Se nanoribbons. The solely-B$\bot $-dependent UCF is achieved and its temperature dependence is investigated. The surface transport is further revealed by weak antilocalizations. Such quantum interference unexpectedly survives through the limited dephasing length of the bulk carriers in the ternary TI crystals. Based on the temperature-dependent scaling behavior, the electron-phonon interaction is addressed as a secondary source of the surface state dephasing. (Scientific Reports, 2, 595 (2012))   

Surface state transport suppression in topological insulators

An unresolved question in experimental research on topological insulators (TI) is the suppression mechanism of a TI's surface state transport. While room temperature ARPES studies reveal clear evidence of surface states, their observation in transport measurements is limited to low temperatures. A better understanding of this suppression is of fundamental interest, and crucial for pushing the boundary of device applications towards room-temperature operation. In this talk, we report the temperature dependent optical properties of the topological insulator Bi$_2$Te$_2$Se (BTS), obtained by infrared spectroscopy and ellipsometry, probing surface and bulk states simultaneously. We see clear evidence of coherent surface state transport at low temperature and find that electron-phonon coupling causes the gradual suppression of surface state transport as temperature rises to 43K. In the bulk, electron-phonon coupling enables the emergence of an indirect band gap transition, which peaks at 43K, and is limited by thermal ionization of the bulk valance band above 43K. For comparison with other resistive TIs, we also discuss the optical properties to BiSbSe$_2$Te.   

High-Temperature Andreev Tunneling in the Surface States of a Topological Insulator

Topological insulators (TIs) are materials with high spin-orbit coupling that possess conductive helical surface states. In order to study the exotic properties of the TI surface states, it is favorable to work with TIs that have a low bulk conductivity and exhibit insulating behavior. Bi2Te2Se has been confirmed to have a high bulk resistivity, and it still shows Shubnikov-de Haas oscillations originating from the two-dimensional surface states. We report the observation of coherent Andreev tunneling into the surface states of Bi2Te2Se in high-temperature superconductor (Bi2Sr2CaCu2O8$+\delta)$/Bi2Te2Se junctions fabricated by mechanical bonding method. The differential conductance measurements will be presented in various temperatures and magnetic fields. The characterization of the zero-bias conductance peak observed, suggests that we are tunneling into the surface states of the TI rather than the bulk states.