Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session Y13: Topological Insulators: Thin Films and Interfaces |
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Sponsoring Units: DCMP Chair: Fazel Fallah Tafti, Universite de Sherbrooke Room: 315 |
Friday, March 22, 2013 8:00AM - 8:12AM |
Y13.00001: An Infrared Study of Bi2Se3 Thin Films Kirk Post, Brian Chapler, Liang He, Xufeng Kou, Alex Schafgans, Kang Wang, Dmitri Basov The experimental observation of surface states present in Bi$_2$Se$_3$ has been limited by self-doping via selenium vacancies. We have explored this issue by probing the electronic structure of Bi$_2$Se$_3$ using a combination of variable angle spectroscopic ellipsometry (VASE) and Fourier transform infrared spectroscopy (FTIR). Specifically, we have measured Bi$_2$Se$_3$ thin films grown on Si (111) substrates, ranging from 15 to 99 quintuple layers (QL) thick. These results show that both the carrier density and the energy gap are inversely related to the thickness. Surprisingly, the energy gap in all but the 15QL samples was smaller than the bulk band gap. Furthermore, the energy gap varied by over 100 meV between the 15QL and 99QL sample. The features that we observed are consistent with a modified picture of the band structure of Bi$_2$Se$_3$ that includes an impurity band below the conduction band and a Fermi level that is inversely related to the thickness. [Preview Abstract] |
Friday, March 22, 2013 8:12AM - 8:24AM |
Y13.00002: Impact of growth conditions on the MBE-grown topological insulator Bi$_2$Se$_3$ thin films Y. Liu, Y.Y. Li, S. Rajput, M. Weinert, L. Li Recently, molecular beam epitaxy (MBE) has been successfully applied to prepare atomically flat topological insulator thin films that exhibit helical Dirac states. In this work, we systematically investigate the effects of substrate temperature and Bi/Se flux ratio on the morphology and properties of Bi$_{2}$Se$_{3}$ thin films grown on graphene/SiC(0001) by MBE. Under optimal growth conditions, \textit{in situ} scanning tunneling microscopy indicates spiral growth [1], characterized by atomically smooth terraces 10 to 50 nm in width, separated by steps that are one quintuple-layer in height. \textit{Ex situ} Raman spectroscopy reveals two characteristic peaks at 130 and 171 cm$^{-1}$, corresponding to the in-plane E$_{\mathrm{g}}^{2}$ and out-of-plane A$_{\mathrm{1g}}^{2}$ vibrational modes, respectively. The close resemblance of the positions and line shapes of both these peaks to those of bulk Bi$_{2}$Se$_{3}$ attest to the high quality of the film. These results and the impact of growth spirals on the properties of the topologically protected Dirac surface states of Bi$_{2}$Se$_{3}$ will be presented at the meeting. \\[4pt] [1] Y. Liu et al. PRL \textbf{108}, 115501 (2012). [Preview Abstract] |
Friday, March 22, 2013 8:24AM - 8:36AM |
Y13.00003: Epitaxial Growth of Topological Insulators on Hexagonal Boron Nitride Christopher Gutierrez, Woo Chang Chung, Chockalingam Subbaiah, Matthew Brahlek, Seongshik Oh, Abhay Pasupathy Topological insulators (TIs) have attracted much attention for exhibiting exotic, topologically-protected surface states consisting of massless Dirac fermions. Investigations on thin film TIs have primarily relied on those either grown by MBE or by mechanical exfoliation onto suitable target substrates. Taking a cue from the graphene community, hexagonal boron nitride (hBN) has proven to be an excellent insulating substrate since it is atomically flat with no surface dangling bonds. In this talk I will report on recent transport and scanning probe measurements on epitaxial thin films of bismuth selenide TI grown by MBE on hBN/SiOx. [Preview Abstract] |
Friday, March 22, 2013 8:36AM - 8:48AM |
Y13.00004: Gate-tunable supercurrent in S-TI-S structures Vladimir Orlyanchik, Martin Stehno, Christopher Nugroho, Dale Van Harlingen, Matthew Brahlek, Namrata Bansal, Nikesh Koirala, Seongshik Oh Theoretical proposals for observation of the zero energy excitations (Majorana modes) involve coupling between the surface states of 3-D topological insulators (TI) and s-wave superconductors (SC). A prerequisite for such experiments is a highly tunable topological surface which is decoupled from bulk charge carriers and non-topological surface states. Here we report on measurements performed using high-quality MBE-grown thin films of Bi2Se3 patterned to create planar Josephson devices with Nb leads and a metallic top gate. We present the dependence of the conductance and proximity-induced supercurrent on the junction geometry, temperature, and the gate voltage. By analyzing the gate voltage dependence, we deduce that there are contributions to the supercurrent from two channels - topological surface states and a topologically-trivial surface accumulation layer. [Preview Abstract] |
Friday, March 22, 2013 8:48AM - 9:00AM |
Y13.00005: Terahertz dynamics of gated thin films of the topological insulator Bi$_{2}$Se$_{3}$ Andreas Stier, James Neilson, Liang Wu, Namrata Bansal, Matthew Brahlek, Sean Oh, N. Peter Armitage Topological insulators are a newly discovered class of materials, which in principle exhibit bulk insulating behavior and conducting surface channels with a Dirac like dispersion relation. Real materials, however, suffer from large residual bulk conductance due to donor defect sites. This places the chemical potential in the bulk bands. Ionic liquid gating techniques are capable of moving the chemical potential into the bulk band gap, making the exotic transport characteristics predicted for the surface states accessible. Here, we present terahertz time domain spectroscopy of gated thin films of the topological insulator Bi$_{2}$Se$_{3}$ utilizing an ionic liquid gel as a top gate. The evolution of the Drude like conductivity features as a function of gate bias show a sharp decrease in the scattering rate which we interpret as the chemical potential moving from the conduction band into the surface states. We also discuss efforts to optically observe potential axionic terms in the action governing Maxwell's equations for this material class, which are reflected in a discontinuous evolution of the Faraday rotation. [Preview Abstract] |
Friday, March 22, 2013 9:00AM - 9:12AM |
Y13.00006: Scanning tunneling spectroscopic (STS) studies of magnetically doped MBE-grown topological insulators (TIs) Hao Chu, Marcus Teague, Chien-Chang Chen, Nicholas Woodward, Nai-Chang Yeh, Xufeng Kou, Liang He, Murong Lang, Kang Long Wang We conduct STS studies on MBE-grown heterostructures of non-magnetic TI (Bi$_{2}$Se$_{3})$ with a range of thicknesses ($d =$ 1, 3, 5, 7 quintuple layers, QL) on top of 7-QL magnetically doped TI (Cr-doped Bi$_{2}$Se$_{3})$. For $d =$ 1 and 3-QL, a spatially homogeneous magnetism-induced surface gap (as large as about 150 meV for $d$ $=$ 1-QL) is observed at 77 K, whereas gapless Dirac spectra are found for $d =$ 5 and 7-QL, suggesting that the effective magnetic length for Cr-doped Bi$_{2}$Se$_{3}$ is approximately 4 $\sim$ 5-QL. These findings are further corroborated by ARPES and bulk electrical transport measurements. The magnetism-induced surface gap differs from those found in pure Bi$_{2}$Se$_{3}$ and (Bi$_{0.5}$Sb$_{0.5})_{2}$Te$_{3}$ films of thicknesses smaller than 6-QL, because the latter are due to overlaps of wave functions between the surface and interface layers, which lead to Rashba-like spin-orbit splitting and spin-preserving quasiparticle interference wave-vectors. In contrast, STS studies of TIs with magnetism-induced surface gap do not yield any quasiparticle interferences for energies within the bulk Bi$_{2}$Se$_{3}$ gap. Finally, comparative STS studies of pure and magnetically doped TIs in high magnetic fields will be discussed. [Preview Abstract] |
Friday, March 22, 2013 9:12AM - 9:24AM |
Y13.00007: Thickness-Independent Transport Channels in Topological Insulator Bi2Se3 Thin Films Namrata Bansal, Yong-Seung Kim, Matthew Brahlek, Eliav Edrey, Nikesh Koirala, Seongshik Oh With high quality Bi2Se3 thin films grown on Al2O3(0001), we report thickness-independent transport properties over wide thickness ranges. Low temperature conductance remained nominally constant as the sample thickness changed from 256 to $\sim $8QL (where QL refers to quintuple layer, 1QL$\approx $1nm). Two surface channels with very different behaviors were identified. The sheet carrier density of one channel remained constant at $\sim $3x10$^{13}$cm$^{\mathrm{-2}}$ down to 2QL, while the other, which exhibited quantum oscillations, remained constant at $\sim $8x10$^{12}$cm$^{-2}$ only down to $\sim $8QL. The weak antilocalization effect also exhibited similar thickness independence. These two channels are most consistent with the topological surface states and the surface accumulation layers, respectively. We will also discuss surface signatures present in Bi2Se3 thin films grown on Si(111) and amorphous SiO2. [Preview Abstract] |
Friday, March 22, 2013 9:24AM - 9:36AM |
Y13.00008: An abrupt change in transport dynamics across the topological phase transition in the (Bi$_{1-x}$In$_x$)$_2$Se$_3$ and ultra-thin Bi2$_2$Se$_3$ systems Liang Wu, Rolando Valdes Aguilar, Andreas V. Stier, Lucas S. Bilbro, Yuval Lubashevsky, N. Peter Armitage, Matthew Brahlek, Namrata Bansal, Sean Oh We have utilized time-domain terahertz (THz) spectroscopy to investigate the low frequency optical conductivity in (Bi$_{1-x}$In$_x$)$_2$Se$_3$ through its topological phase transition from the pure ($x=0$) compound to the topologically trivial strongly insulating material ($x>0.25$). The thickness independent Drude peak shows only minor broadening at low In substitutions. However, above $x\sim0.05$ we observe a sudden collapse in the transport lifetime. This substitution level closely coincides with a maximum in the mid-infrared (MIR) absorption coefficient which can be identified with the substitution level where the band gap closes, the band structure inverts, and hence the topological class changes. We therefore associate the collapse in the transport lifetime with the loss of topological protection of surface states as the system enters the topologically trivial phase. Topological phase transition driven by reducing film thickness is also investigated. Similar collapse in the transport lifetime is observed in the ultra-thin limit. [Preview Abstract] |
Friday, March 22, 2013 9:36AM - 9:48AM |
Y13.00009: Fabrication and transport measurements of stacked double layer topological insulator devices Tai-Lung Wu, Jiuning Hu, Jifa Tian, Ireneusz Mitkowski, Yong P. Chen A double-layer structure, consisting of two separated two-dimensional electron systems close in proximity, has been an interesting system to study novel ground states and transport properties driven by electron-electron interaction, e.g. Coulomb drag, exciton condensation, and counterflow superfluidity. Recently, topological insulators (TI), such as $Bi_{2}Se_{3}$ and $Bi_{2}Te_{3}$, have attracted much attention due to their exotic topologically protected spin-helical and Dirac-particle surface states. Motivated by a recently proposed ``topological exciton condensate'' that may be formed in two interacting TI surfaces, we have fabricated stacking double-layer TI structures and studied their electrical transport properties. Using a polyvinyl alcohol (PVA) based support film and micro-manipulator, double layer TI structures ($Bi_{2}Se_{3}$/boron nitride/$Bi_{2}Se_{3}$) were fabricated with exfoliated $Bi_{2}Se_{3}$ separated by thin boron nitride flakes ($\sim$ 20 nm). We will present results from transport measurements including mutual-gated electrical field effect, Coulomb drag ,and counterflow conductivity. [Preview Abstract] |
Friday, March 22, 2013 9:48AM - 10:00AM |
Y13.00010: Angle-resolved photoemission spectroscopy study of the magnetic doped topological insulator ultra-thin film Bi$_{2}$Fe$_{x}$Se$_{3}$ Yi Zhang, Bo Zhou, Yulin Chen, Sung-Kwan Mo, Zahid Hussain, Zhi-Xun Shen Topological insulator is a new type of quantum matter with gapped bulk states coexisting with a gapless surface state (SS) that is protected by time reversal symmetry and robust against non-magnetic impurities. Researches have shown that there exist two routes to open a gap in the SS: doping with magnetic impurities and the coupling of SS on opposite surfaces in ultra-thin films. In order to study the mixing of these two types of gap-opening, we prepared ultra-thin Bi$_{2}$Fe$_{x}$Se$_{3}$ films, grown by molecular beam epitaxy, with different Fe concentration and thickness. Size of the gap and its development with Fe concentration and film thickness as well as its momentum dependence have been systematically characterized by in-situ angle resolved photoemission spectroscopy using synchrotron light source. [Preview Abstract] |
Friday, March 22, 2013 10:00AM - 10:12AM |
Y13.00011: Tunneling tuned spin modulations in ultrathin topological insulator films Madhab Neupane, S.-Y. Xu, N. Alidoust, I. Belopolski, Chang Liu, D.M. Zhang, A. Richardella, J. Sanchez-Barriga, D. Marchenko, A. Varykhalov, O. Rader, M. Leandersson, T. Balasubramanian, L.A. Wray, T.-R. Chang, H.-T. Jeng, H. Lin, A. Bansil, N. Samarth, M.Z. Hasan Understanding the spin behavior of boundary modes in ultrathin topological insulator films is critically essential for the design and fabrication of functional nano-devices. We report tunneling-dependent evolution of spin configuration in topological insulator thin films across the metal-to-insulator transition. We observe that for a given film thickness of the prototype topological insulator Bi$_2$Se$_3$ ultrathin films, the spin polarization is large for larger wave-vectors or for momenta far from the center of the surface Brillouin zone. In addition, the polarization is observed to decrease significantly with enhanced tunneling realized systematically in thin insulating films. We present theoretical model calculations that qualitatively capture the delicate relationship between quantum tunneling and Fermi surface spin polarization. [Preview Abstract] |
Friday, March 22, 2013 10:12AM - 10:24AM |
Y13.00012: Study of proximity effect in superconductor - topological insulator heterostructures by scanning SQUID microscope Ilya Sochnikov, Andrew J. Bestwick, James R. Williams, Thomas M. Lippman, Andrew S. Bleich, James G. Analytis, Ian R. Fisher, David Goldhaber-Gordon, John R. Kirtley, Kathryn A. Moler A proximity induced superconducting state in topological insulators is potentially an enabling condition for exotic forms of superconductivity that may support Majorana fermions in some geometries. Initial studies of induced superconductivity in topological insulators have relied on transport measurements. We present a different contactless characterization approach based on a scanning SQUID microscope. We characterized Al superconducting rings with Josephson junctions made of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, long Al/Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Al Josephson junctions, and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/Al dots. We observe both induced proximity and inverse proximity effects in these heterostructures. Each of the structures provides unique information about the proximity effect, such as the critical current, the magnetic field penetration depth, and the critical temperatures of the induced superconducting state. These measured parameters allow the determination of limits on contributions from the surface and the bulk to the proximity effects in the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. [Preview Abstract] |
Friday, March 22, 2013 10:24AM - 10:36AM |
Y13.00013: Local Magnetic Imaging of Proximity Effect-Induced Superconductivity at the Bi$_{2}$Se$_{3}$-Nb Interface Philip Kratz, John Kirtley, Ilya Sochnikov, Phillip Wu, Eric Spanton, Kristie Koski, Yi Cui, Robert Hammond, Malcolm Beasley, Kathryn Moler The interface between a topological insulator (TI) and an s-wave superconductor (SC) is predicted to host Majorana bound states analogous to vortices in a spinless p$_{\mathrm{x}}+$ip$_{\mathrm{y}}$ superconductor. For 3D TIs coupled to s-wave superconductors, the winding of the superconducting vortices can counteract the TI pi-Berry's phase, resulting in zero-energy Majorana fermion excitations at the interface. Transport measurements of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ superconducting junctions have shown Josephson junction effects[1,3,4] and established the existence of a supercurrent that is tunable with gate voltage [2], but the relative contributions of the bulk and bound states to the supercurrent is not well-understood. We report on measurements of the local superfluid density at the interface between Bi$_{2}$Se$_{3}$ nanoplatelets and Nb using a scanning SQUID microscope and quartz tuning fork sensor for simultaneous AFM characterization. We demonstrate that the local penetration depth measurements have increased accuracy and provide an experimentally tractable method for studying proximity effect-induced superconductivity at the SC-TI interface, which is a precursor for observation of the elusive Majorana fermion in Bi$_{2}$Se$_{3}$ and other 3D TIs.\\[4pt] [1] arXiv:1209.5830 (2012). [2] Nat. Comm. \textbf{2} (2011). [3] Nat. Mat. \textbf{11}, 421 (2012). [4] Phys. Rev. Lett. \textbf{109}, 056803 (2012). [5] Phys. Rev. B \textbf{84}, 165120 (2011). [Preview Abstract] |
Friday, March 22, 2013 10:36AM - 10:48AM |
Y13.00014: Dirac cone shift and potential fluctuations in a passivated In$_2$Se$_3$/Bi$_2$Se$_3$ topological interface state Gregory S. Jenkins, A.B. Sushkov, D.C. Schmadel, M.-H. Kim, H.D. Drew, G. Koblmueller, M. Bichler, N. Bansal, M. Brahlek, S. Oh The topological interface state of Bi$_2$Se$_3$ capped with In$_2$Se$_3$ is measured by gated THz cyclotron resonance. An observed shift of 70 meV in the position of the Dirac point towards mid-gap due to the physical properties of the trivial insulator In$_2$Se$_3$ on Bi$_2$Se$_3$ opens new possibilities in tailoring Dirac cone properties in topological insulators. Modulating and sweeping a semi-transparant gate while probing at terahertz frequencies in magnetic field enables characterization of the burried In$_2$Se$_3$/Bi$_2$Se$_3$ topological interface state, even in the presence of significant bulk conductivity. Near the Dirac point, the mobility is 3500 cm$^2$/V$\cdot$s with potential fluctuations of 60 meV. The scattering rate shows a precipitous drop with Fermi energy indicating decoupling of the surface states from bulk states. At Fermi energies above the conduction band edge, a plateau is observed in the real part of the Faraday angle that is 80 times flatter than the step size expected from a single Landau Level, quantized in units of the fine structure constant. [Preview Abstract] |
Friday, March 22, 2013 10:48AM - 11:00AM |
Y13.00015: Massive Dirac surface states in topological insulator/magnetic insulator heterostructures Weidong Luo, Xiao-Liang Qi We study the behavior of topological surface states in heterostructures formed by a topological insulator (TI) and a magnetic insulator (MI). Several MIs with compatible magnetic structure and relatively good lattice matching with TIs are identified, and the best candidate material is found to be MnSe, an anti-ferromagnetic insulator. We perform first-principles calculation in Bi$_2$Se$_3$/MnSe superlattices and obtain the surface state bandstructure. The magnetic exchange coupling with MnSe induces a gap of 54 meV at the surface states. In addition we tune the distance between Mn ions and TI surface to study the distance dependence of the exchange coupling. Finally, we study the band bending effect at the Bi$_2$Se$_3$/MnSe interface, and propose possible solutions to avoid band bending. [Preview Abstract] |
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