Session C29: Bi-based Topological Insulators

Nanoscale Andreev Reflection Spectroscopy on Bismuth-Chalcogenide Topological Insulators

Andreev reflection (AR) is the basic mechanism underlying the superconducting proximity effect which, at the \mbox{interface} between a \mbox{topological} insulator (TI) and a spin-singlet \mbox{superconductor}, can \mbox{induce} chiral $p$-wave pairing in the TI. Despite this novel \mbox{importance}, it is not well understood how AR is affected by the unique attributes of a three-dimensional TI, namely the Dirac dispersion and helical spin-polarization of its surface states. In this work, we use both \mbox{$s$-wave} and $d$-wave\footnote{C. S. Turel et al., \textbf{Appl. Phys. Lett.} 99, 192508 (2011)} superconducting tips to perform AR spectroscopy at \mbox{4.2 K} on flux-grown Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ single crystals, as well as \mbox{epitaxial} Bi$_{2}$Se$_{3}$ thin films grown on SrTiO$_{3}$ substrates by molecular beam \mbox{epitaxy}. These AR measurements are complemented by scanning \mbox{tunneling} \mbox{spectroscopy}, in order to characterize the superconducting tip as well as the doping level and surface condition of the TI sample. Our data are \mbox{analyzed} using BTK theory, in light of the characteristic band structure of bismuth chalcogenides, to elucidate how the band structure affects the AR process.   

Pressure-induced Lifshitz and Weyl Semi-metallic Phase Transitions in BiSb

By means of first-principle calculations, we report a non-magnetic stoichiometric crystal structure of BiSb with broken space-inversion symmetry. This structure belongs to the $R3m$ space group and it was obtained after a systematic study of the low-energy phases of Bi$_{1-x}$Sb$_x$ $(0 < x < 1)$ compounds found by using minima hopping structure search method [1]. This structure is insulating in bulk and has non-trivial band topology. We observe pressure-induced Lifshitz and Weyl semi-metallic phases as electronic phase transitions in this system. The obtained Weyl semi-metallic phase exist in the ${4.0-6.0}$ GPa pressure range. We find that a total 12 pairs of Weyl points, 12 monopoles and 12 antimonopoles, exist in the bulk Brillouin zone. The Weyl points with opposite chirality are located at different energy values yielding separate electron and hole Fermi-surfaces, which drives novel topological transport properties in this system. The surface state calculations reveal reminiscence of the Fermi-arcs at (001) surface of the BiSb slab, which further confirm the existence of Weyl semi-metallic phase in BiSb [2-4]. [1] J. Chem. Phys. 120, 9911 (2004) [2] Science 349, 622 (2015) [3] Nat Phys 11, 748 (2015) [4] Phys. Rev. X 5, 031013 (2015)   

Ab initio study of the adsorption, diffusion, and intercalation of alkali metal atoms on the (0001) surface of the topological insulator Bi2Se3

We present the results of an \emph{ab initio} study of the adsorption, diffusion, and intercalation of alkali metal adatoms on the (0001) stepped surface of the topological insulator Bi$_2$Se$_3$ for the case of low coverage. The calculations of the activation energies of the adatoms diffusion on the surface and in the van der Waals gaps near the steps, as well as the estimation of diffusion lengths, show that efficient intercalation through the steps is possible only for Li and Na. Data obtained for K, Rb, and Cs atoms indicate that their thermal desorption at high temperatures can start before intercalation. These results are discussed in the context of the experimental data available \cite{1,2}. \begin{thebibliography}{99} \bibitem{1} Z.-H. Zhu, et al. Phys. Rev. Lett. {\bf 107}, 186405 (2011). \bibitem{2} M. Bianchi, et al. ACS Nano {\bf 6}, 7009 (2012). \end{thebibliography}   

Topological insulators are tunable waveguides for hyperbolic polaritons

We present a theoretical analysis showing that layered topological insulators, for example, Bi$_2$Se$_3$ are optically hyperbolic materials in a range of THz frequencies. As such, these topological insulators possess deeply subdiffractional, highly directional collective modes: hyperbolic phonon-polaritons. We predict that in thin crystals the dispersion of these modes is split into discrete subbands and is strongly influenced by electron surface states. If the surface states are doped, then hybrid collective modes result from coupling of the phonon-polaritons with surface plasmons. The strength of the hybridization can be controlled by an external gate that varies the chemical potential of the surface states. We also show that momentum-dependence of the plasmon-phonon coupling leads to a polaritonic analog of the Goos-H\"anchen effect. Directionality of the polaritonic rays and their tunable Goos-H\"anchen shift are observable via THz nanoimaging.   

ARPES study of the surface states and their aging in a topological insulator, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$

Topological insulators possess time reversal symmetry protected metallic surface states over the insulating bulk, where these surface states are expected to be immune to weak disorder, chemical passivation of the surface or temperature change. However, significant discrepancy from such behavior has been found experimentally in various materials. We studied the detailed electronic structure and its aging of a topological insulator, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ employing high resolution photoemission spectroscopy. Both the band structure results and high resolution angle resolved photoemission data reveal significantly different surface electronic structure for different surface terminations. Furthermore, oxygen impurity on Se terminated surface exhibits an electron doping scenario, while oxygen on Bi terminated surface corresponds to a hole doping scenario. The intensity of the Dirac states reduces with aging indicating fragility of the topological order due to surface impurities. References \begin{enumerate} \item D. Biswas, S. Thakur, K. Ali, G. Balakrishnan,, and K. Maiti, Sci. Rep. (Nature) \textbf{5}, 10260 (2015). \item D. Biswas and K. Maiti, EPL \textbf{110}, 17001 (2015). \item D. Biswas, S. Thakur, G. Balakrishnan, and K. Maiti, Sci. Rep. (Nature) (2015) (to be published). \end{enumerate}   

Interband Spin-Orbit Coupling in Topological Surface States Explored by Photoemission Spectroscopies

Three-dimensional crystals with a topologically non-trivial band gap in the bulk Brillouin zone are typically classified by either Z2 topological invariants or by Chern number topological invariants. The Z2 topological band insulators are said to possess surface states protected by time-reversal-symmetry and topological crystalline insulators possess surface states protected by mirror-symmetry. Here we provide evidence, through spin- and angle-resolved photoemission spectroscopy and first-principles calculations of layered (Bi$_{2})_{m}$(Bi$_{2}$X$_{3})_{n}$ (X $=$ Se, Te) materials, that surfaces of Z2 strong topological insulators can possess states protected by mirror-symmetry alone. The role of interband coupling in producing mirror-protected surface states with novel Fermi contours and spin-textures will be discussed, and an argument for the unification of Z2 and Chern number invariant classifications will be made.   

Strong Zeeman effects in the Landau level spectrum of (In$_{\mathrm{x}}$Bi$_{\mathrm{1-x}})_{\mathrm{2}}$Se$_{\mathrm{3}}$

We investigate the surface states of (In$_{\mathrm{x}}$Bi$_{\mathrm{1-x}})_{\mathrm{2}}$Se$_{\mathrm{3\thinspace }}$by scanning tunneling spectroscopy (STS) in the range 0 $\le $ x ~3{\%}. We carefully examine the low-lying Landau levels of the topological surface states in attempt to extract the parameters of the surface-state Hamiltonian as a function of doping. Close examination of the data oblige us to index the Landau levels in a manner different to precedent on pristine Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, and fits to the Landau level spectra yield large g-factors on the order of 40, which decrease with increasing x. The Landau levels of pristine Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ are also reexamined, yielding high g-factors roughly consistent with results obtained from magnetic oscillations, and suggesting a decrease in the surface-state Zeeman coupling with increasing In as the topological phase transition is approached.   

Nonequilibrium spin texture within a thin layer below the surface of current-carrying topological insulator Bi2Se3: A first-principles quantum transport study

Using extension of nonequilibrium Green function combined with density functional theory (NEGF+DFT) formalism to situations involving noncollinear spins and spin-orbit coupling, we investigate microscopic details (on the 1 ° A scale) of nonequilibrium spin density S(r) driven by unpolarized charge current injection into a ballistic thin film of Bi 2 Se 3 as prototypical topological insulator (TI) material. We find large nonzero component of S(r) in the direction transverse to current flow on the metallic surfaces of TI, as well as within few bulk atomic layers near the surfaces because of penetration of evanescent wavefunctions from the metallic surfaces into the bulk. In addition, an order of magnitude smaller components emerge in the perpendicular (within surfaces and nearly bulk regions of TI) and longitudinal (within bulk region of TI near its surface) directions, thereby creating a complex nonequilibrium spin texture. We also demonstrate how DFT calcula- tions 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 .   

Tunneling spectroscopy of a magnetic adatoms on topological insulator surfaces

In this communication, we address the question of how the presence of a magnetic impurity on a topological insulator (TI) surface manifests in the inelastic electron tunneling spectroscopy (IETS) when such a system is probed by a STM. For this purpose, we consider a single magnetic adatom with arbitrary spin, whose dynamics is governed by the local magnetic anisotropy. The spin is exchange-coupled to two-dimensional helical surface electrons, corresponding to the surface of a three-dimensional TI like Bi2Se3, with its characteristic hexagonally warped Dirac cone band structure. Employing an effective exchange-tunneling model, we calculate the non-linear differential conductance from a spin-polarized STM tip to the helical substrate, valid in the perturbative regime of weak exchange-tunneling and including the nonequilibrium pumping of the adatom spin states. The interplay between the magnetic anisotropy and the spin-momentum locked surface electrons is shown to give a number of specific imprints in the IETS, which could be investigated by spin-resolved scanning tunneling spectroscopy.\\ M. Misiorny, M. Bjerngaard and J. Paaske, manuscript in preparation   

Laser-driven parametric instability and generation of entangled photon-plasmon states in graphene and topological insulators

Massless Dirac electrons in graphene and on the surface of topological insulators such as Bi$_2$Se$_3$ demonstrate strong nonlinear optical response and support tightly confined surface plasmon modes. Although both systems constitute an isotropic medium for low-energy in-plane electron excitations, their second-order nonlinear susceptibility becomes non-zero when its spatial dispersion is taken into account. In this case the anisotropy is induced by in-plane wave vectors of obliquely incident or in-plane propagating electromagnetic waves. In this work we show that a strong (0.1-1 MW/cm$^2$) near-infrared or mid-infrared laser beam obliquely incident on graphene can experience a parametric instability with respect to decay into lower-frequency (idler) photons and THz surface plasmons. The parametric gain leads to efficient generation of THz plasmons. Furthermore, the parametric decay process gives rise to quantum entanglement of idler photon and surface plasmon states. This enables diagnostics and control of surface plasmons by detecting idler photons. A similar parametric process can be implemented in topological insulator thin films.   

Electronic structures of topological insulators with non-conventional terminations

Until now, most works on topological insulators focus on the natural cleaving surfaces, \emph{i.e.}, conventional terminations. However, researches on the non-conventional surfaces of TIs are hindered due to the difficulties in preparation of those surfaces and the existence of large number of dangling bonds on those surfaces. What is more, due to the complications in the surface lattice structures, DFT calculations on the non-conventional surfaces are not favorable. In this work, by adopting the tight binding method based Green's Function, we systematically studied the surface states of non-conventional terminations of topological insulator Bi2Te3 and Bi2Se3. By using MBE, we manage to prepare topological insulator Bi2Te3 thin films with fractional quintuple layer (FQL) termination. Scanning tunneling microscopy (STM) reveals that the as-grown Bi2Te3 thin films may not necessarily terminate at the Van der Waals gap between two adjacent quintuple layers. The electronic structures of the FQL surfaces are studied in combination with quasi-particle interference (QPI) by scanning tunneling spectroscopy (STS). Our results suggest that the topological nature of SSs be preserved on non-conventional terminations. The robustness of the topological SSs is also demonstrated.   

Role of spin-orbit scattering in quasiparticle interference

Quasiparticle interference measured by scanning tunneling spectroscopy is profoundly affected by spin textures in momentum space. In this spin effect, used to study spin-polarized electronic states with scanning tunneling spectroscopy, spin of electrons are usually supposed to be preserved unless magnetic impurities are doped. We report that electron spin is indeed not preserved but rotated by nonmagnetic impurities in the process of quasiparticle interference of a spin-polarized two-dimensional electron gas formed on the surface of a polar semiconductor BiTeI. The results imply that spin-orbit scattering plays a significant role in quasiparticle interference of materials where spin-orbit interaction is strong.   

Considering a Topological Insulator as a Viscous Electronic Fluid

Certain topological insulators' protected surface states may be better treated as hydrodynamic fluids than as collections of quasiparticles. We will present data showing that Bi$_{\mathrm{0.5}}$Sb$_{\mathrm{1.5}}$Se$_{\mathrm{1.6}}$Te$_{\mathrm{1.4}}$ natively exists in the hydrodynamic regime at room temperature. A calculation of the viscosity finds that Bi$_{\mathrm{0.5}}$Sb$_{\mathrm{1.5}}$Se$_{\mathrm{1.6}}$Te$_{\mathrm{1.4}}$ is surprisingly comparable to that of standard fluids such as water and helium, when normalized to the entropy of each system. This finite viscosity implies an unexpected method for current dissipation via turbulence.   

Modification of the electronic band structure of the topological insulator Bi$_{2}$Te$_{3}$ by the adsorption of the organic molecule Manganese Phthalocyanine

Topological insulators (TIs) have a spin-textured surface state protected by time-reversal symmetry within a bulk insulating gap. Typical approaches to breaking time-reversal symmetry have been to introduce dilute magnetic impurities into a solid-solution synthesis. Organic molecules offer another route for magnetic-doping of TIs. It has been shown that a coupling may exist, along with a new hybrid-interface state, between the magnetic molecule Manganese Phthalocyanine (MnPc) and the TI Bi$_{2}$Te$_{3}$. We report the modification of the electronic band structure by the adsorption of MnPc molecules as measured by ultraviolet photoelectron spectroscopy. We show a new state emerging below the Fermi level at less than a monolayer coverage of MnPc molecules. We also observe an $n$-doping effect as charge is transferred from the molecule to the TI substrate in agreement with recent work. We suggest that this interface system may have important implications for understanding the role of local time reversal symmetry breaking in TI's and in controlling spin injection into these novel materials.   

\textbf{Electrical Detection of Spin-to-Charge Conversion in a Topological Insulator Bi}$_{\mathrm{\mathbf{2}}}$\textbf{Te}$_{\mathrm{\mathbf{3}}}$.

Spin-momentum locking in topological insulators (TIs) dictates that an unpolarized charge current creates a net spin polarization. We recently demonstrated the first electrical detection of this spontaneous polarization in a transport geometry, using a ferromagnetic (FM) / tunnel barrier contact, where the projection of the TI surface state spin on the magnetization of detector is measured as a voltage [1]. Alternatively, if spins are injected into the TI surface state system, it is distinctively associated with a unique carrier momentum, and hence should generated a charge accumulation, similar to that of inverse spin Hall effect. Here we experimentally demonstrate both effects in the same device fabricated in Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$: the electrical detection of the spin accumulation generated by an unpolarized current flowing through the surface states, and that of the charge accumulation generated by spins injected into the surface states system. This reverse measurement is an independent confirmation of spin-momentum locking in the TI surface states, and offers additional avenue for spin manipulation. It further demonstrates the robustness and versatility of electrical access to the TI surface state spin system, an important step towards its utilization in TI-based spintronics devices.