Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B45: Topological Materials: Thin FilmFocus
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Sponsoring Units: DMP Chair: Vikram Deshpande, University of Utah Room: 392 |
Monday, March 13, 2017 11:15AM - 11:51AM |
B45.00001: TBD - Topological Materials: Synthesis and Characterization Invited Speaker: Sergey A. Medvedev |
Monday, March 13, 2017 11:51AM - 12:03PM |
B45.00002: Molecular beam epitaxy growth of SmB$_{6\pm\delta}$ thin films Jason Hoffman, Muhammad Saleem, James Day, Doug Bonn, Jennifer Hoffman SmB$_6$ has emerged as a leading candidate in the search for exotic topological states generated by strong interactions. The synthesis of epitaxial SmB$_6$ thin films presents new avenues to control surface termination, thickness, and strain in this system. In this work, we use molecular beam epitaxy (MBE) to deposit SmB$_{6\pm\delta}$ films on insulating (001)-oriented MgO substrates. We use ex-situ x-ray diffraction and magnetotransport measurements to assess the properties of the samples and compare them to previously reported values for single crystals. We also discuss the prospects of using rare-earth substitution to control the correlation strength and alter the topology of the bulk and surface electronic states. [Preview Abstract] |
Monday, March 13, 2017 12:03PM - 12:15PM |
B45.00003: High-Resolution Structural and Electronic Properties of Epitaxial Topological Crystalline Insulator Films Omur Dagdeviren, Chao Zhou, Ke Zou, Georg Simon, Stephen Albright, Subhasish Mandal, Mayra Morales-Acosta, Xiaodong Zhu, Sohrab Ismail-Beigi, Frederick Walker, Charles Ahn, Udo Schwarz, Eric Altman Revealing the local electronic properties of surfaces and their link to structural properties is an important problem for topological crystalline insulators (TCI) in which metallic surface states are protected by crystal symmetry. The microstructure and electronic properties of TCI SnTe film surfaces grown by molecular beam epitaxy were characterized using scanning probe microscopy. These results reveal the influence of various defects on the electronic properties: tilt boundaries leading to dislocation arrays that serve as periodic nucleation sites for pit growth; screw dislocations, and point defects. These features have varying length scale and display variations in the electronic structure of the surface, which are mapped with scanning tunneling microscopy images as standing waves superimposed on atomic scale images of the surface topography that consequently shape the wave patterns. Since the growth process results in symmetry breaking defects that patterns the topological states, we propose that the scanning probe tip can pattern the surface and electronic structure and enable the fabrication of topological devices on the SnTe surface. [Preview Abstract] |
Monday, March 13, 2017 12:15PM - 12:27PM |
B45.00004: Growth and characterization of epitaxial SmB6 thin films Seunghun Lee, Xiaohang Zhang, Drew Stasak, Iftekhar H. M. Jaim, Sheng Dai, Xiaoqing Pan, James Williams, Richard L. Greene, Ichiro Takeuchi Samarium hexaboride (SmB$_{\mathrm{6}})$ is a topological Kondo insulator, and it is one of the most promising candidates for exploring exotic quantum states based on the topological surface state. We have previously observed the superconducting proximity effect in the surface state of SmB$_{\mathrm{6}}$ at in-situ formed superconductor/SmB$_{\mathrm{6}}$ thin film bilayer interfaces [Phys. Rev. X. 6, 031031 (2016)]. Here we present structural and electrical characteristics of epitaxial SmB$_{\mathrm{6}}$ thin films prepared by co-sputtering of SmB$_{\mathrm{6}}$ and B targets. The stoichiometry in SmB$_{\mathrm{6}}$ thin film is carefully examined by WDS measurements. XRD phi-scan measurement reveals the epitaxial relation of SmB$_{\mathrm{6}}$ thin film and substrate. Epitaxial SmB$_{\mathrm{6}}$ thin films shows low-temperature resistance plateau attributed to the emergence of the surface state, consistent with many previous reports. We also discuss current research efforts with epitaxial SmB$_{\mathrm{6}}$ thin films to realize topological superconductivity. [Preview Abstract] |
Monday, March 13, 2017 12:27PM - 12:39PM |
B45.00005: TEM analysis of superconducting Pd doped Bi$_2$Se$_3$ Jerome T. Mlack, Gopinath Danda, Sarah Friedensen, Atikur Rahman, Natalia Drichko, Nina Markovic, Marija Drndic We investigate the material properties of Bi$_2$Se$_3$ doped with Pd via thermal annealing, which can be used to induce localized superconductivity$^1$. We utilize a transmission electron microscope and combine electron diffraction with energy-dispersive X-ray spectroscopy (EDS) mapping to better understand how the Pd extends into the Bi$_2$Se$_3$ as well as to reveal the atomic composition of both the Pd-doped and the intact Bi$_2$Se$_3$ regions. The results show a high concentration of Pd entering the nanostructures, exceeding atomic ratios of dopants (for example Cu) found to induce superconductivity in the more commonly measured Cu-doped Bi2Se3(Cu$_x$Bi$_2$Se$_3$). 1. J. T. Mlack et. al. "Patterning superconductivity in a topological insulator", arXiv:1610.08642 [cond-mat.mes-hall] [Preview Abstract] |
Monday, March 13, 2017 12:39PM - 12:51PM |
B45.00006: Scalable planar fabrication processes for chalcogenide-based topological insulators Peter Sharma, M. David Henry, Erica Douglas, Michael Wiwi, Ana Lima Sharma, Rupert Lewis, Joshua Sugar, Maryam Salehi, Nikesh Koirala, Seongshik Oh Surface currents in topological insulators are expected to have long spin diffusion lengths, which could lead to numerous applications. Experiments that show promising transport properties were conducted on exfoliated flakes from bulk material, thin films on substrates of limited dimensions, or bulk material, with limited yield. A planar thin film-based technology is needed to make topological insulator devices at scale and could also lead to new device designs. We address two problems related to fabricating chalcogenide-based topological insulator devices on 3'' wafers in the Sandia Microfabrication Facility using Bi2Te3 films. (2) Implantation damage and its subsequent mitigation through annealing is characterized. (2) The degradation in dielectric layers used to manipulate surface potential for elucidating topological surface state transport is characterized under different processing conditions. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. Funded by the Office of Naval Research (N0001416IP00098-0). [Preview Abstract] |
Monday, March 13, 2017 12:51PM - 1:03PM |
B45.00007: Molecular Beam Epitaxy of Remotely-Doped Sb Quantum-Well Structures Michael Santos, Kaushini Wickramasinghe, Shayne Cairns, Jeremy Massengale, Zhonghe Liu, Chomani Gaspe, Tetsuya Mishima, Joel Keay, Matthew Johnson, Sheena Murphy The Dirac point for the topological surface states in antimony (Sb) is at the $\Gamma$ point. Bulk Sb is a semimetal with neither the conduction band minimum nor the valence band maximum at the $\Gamma$ point. Our goal is to study the transport properties of the topological states by suppressing the bulk conductivity through quantum confinement and enhancing the surface conductivity through remote n-type doping at the $\Gamma$ point. A series of Sb quantum-well (QW) structures were grown by molecular beam epitaxy using GaSb barrier layers and GaSb substrates. Conductivity measurements on undoped Sb QWs, 0.7 to 6 nm thick, show a suppression of the bulk states, such that the surface conductivity is about 20$\%$ of the total conductivity for a 3.8 nm-thick QW. Interpretation of Hall-effect measurements, which nominally indicate p-type conduction for undoped QWs, are complicated by the presence of both electrons and holes. We have begun experiments to populate the topological electron states by doping the GaSb barrier with Te atoms, creating donor states at the $\Gamma$ point. We observed that the Hall coefficient decreases with the GaSb spacer thickness from 90 to 20 nm. We have carried out experiments at magnetic fields up to 18 T to separate the multiple carrier channels. [Preview Abstract] |
Monday, March 13, 2017 1:03PM - 1:15PM |
B45.00008: Growth of Bismuth onto Bismuth Selenide and Bismuth Telluride Haoshan Zhu, Weimin Zhou, Jory Yarmoff A topological insulator (TI) behaves as an insulator in the bulk, but conducts along the surface via topologically protected surface states. TI's have the potential for applications in areas such as spintronics and quantum computation. The surface termination of the prototypical TIs, Bismuth Selenide and Bismuth Telluride, has been an area of recent debate. Although some studies have demonstrated that the cleaved surface is terminated with Se or Te, as expected from the bulk crystal structure, there are other reports of a surface covered with an additional Bi bilayer. We are using low energy ion scattering (LEIS), in conjunction with other surface analysis tools, to investigate the surface composition and atomic structure of Bi films intentionally grown onto Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ substrates via molecular beam epitaxy (MBE). It is found that the first Bi bilayer grows commensurate with the underlying material, but contains triangular patches in which the substrate is revealed. Additional bilayers form an incommensurate film that has the lattice constant of bulk Bi. Exposure of these materials to molecular halogens shows a preference for bonding to the surface Bi atoms. [Preview Abstract] |
Monday, March 13, 2017 1:15PM - 1:27PM |
B45.00009: Quantum Hall effect in dual gated BiSbTeSe$_{2}$ topological insulator Su Kong Chong, Kyu Bum Han, Akira Nagaoka, Jared Harmer, Ryuichi Tsuchikawa, Taylor D. Sparks, Vikram V. Deshpande The discovery of topological insulators (TIs) has expanded the family of Dirac materials and enables the probing of exotic matter such as Majorana fermions and magnetic monopoles. Different from conventional 2D electron gas, 3D TIs exhibit a gapped insulating bulk and gapless topological surface states as a result of the strong spin-orbit coupling. BiSbTeSe$_{2}$ is also known to be a 3D TI with a large intrinsic bulk gap of about 0.3 eV and a single Dirac cone surface state. The highly bulk insulating BiSbTeSe$_{2}$ permits surface dominated conduction, which is an ideal system for the study of quantum Hall effect (QHE). Due to the spin-momentum locking, the Dirac fermions at the topological surface states have a degeneracy of one. In the QH regime, the Hall conductance is quantized to $(n+1/2)e^{2}/h$, where n is an integer and the factor of half is related to Berry curvature. In this work, we study the QHE 3D TI using a dual gated BiSbTeSe$_{2}$ device. By tuning the chemical potentials on top and bottom surfaces, integer QHE with Landau filling factors, $\nu =$0, $\pm $1, and $\pm $2 are observed. [Preview Abstract] |
Monday, March 13, 2017 1:27PM - 1:39PM |
B45.00010: Disorder-induced dimensionality transition and non-local transport in Sb$_2$Te$_3$ thin films Inna Korzhovska, Shihua Zhao, Lukas Zhao, Zhiyi Chen, Lia Krusin-Elbaum, Simone Raoux, Ghidewon Arefe We examine the effects of disorder on local and non-local charge transport in thin (20-50 nm) films of topological insulator (TI) Sb$_2$Te$_3$, where a very large range of structural disorder is obtained by a suitable annealing protocol. The films were patterned in H- and $\Pi$- shaped structures of various sizes, and the response of each structure was then recorded as a function of disorder. Under strong disorder, the non-local response is found to be strong. Remarkably, we find that highly disordered films also exhibit a nontrivial magnetic response. With decreasing disorder W the non-local signal is progressively reduced and disappears concurrently with the magnetic response. This occurs at the level of disorder W$_t$ at which magnetoconductance (MC) changes its character from a 3D positive MC state to a 2D WAL (weak antilocalization) negative MC state, characteristic of TIs. The 3D positive MC suggests that for $W > W_t$, MC is controlled by disorder-driven spin correlations (rather than orbital effects). We explore the connection of non-local transport to helical edges and discuss our findings in the context of recently proposed spin-memory effects in disordered systems. [Preview Abstract] |
Monday, March 13, 2017 1:39PM - 1:51PM |
B45.00011: Nontrivial spin correlations in disordered Sb$_2$Te$_3$ films Shihua Zhao, Inna Korzhovska, Lukas Zhao, Zhiyi Chen, Haiming Deng, Lia Krusin-Elbaum, Marcin Konczykowski, Simone Raoux Disorder plays a fundamental role in low-dimensional electronic systems; it can lead to electron localization and quantum phase transitions. Two-dimensional (2D) metallic Dirac surface states of 3D topological insulators (TIs) are expected to be robust against nonmagnetic disorder, but the range of this robustness is still unclear. Here we show that under \textit{strong nonmagnetic disorder}, the 3D topological material Sb$_2$Te$_3$ develops nontrivial spin correlations that break time reversal symmetry, which may set the limits to the topological state. To measure magnetization $M(T)$, thin (20-50 nm) films were exfoliated atop custom-designed on-chip micro-Hall sensors. The disorder $W$ was tuned from amorphous to crystalline state by thermal annealing, and tracked by Raman spectroscopy and TEM. $M$ onsets sharply at $T \sim 200 $ K, and for large $W$ at low $T$ is surprisingly large ($m \simeq 0.001g\mu_B$, \textit{g}-factor $\sim 50$). It is history and time dependent, and is associated with the 3D character of measured negative magnetoresistance in the Anderson localization (hopping) regime. $M$ vanishes at the 3D-2D transition into a topological state, which occurs at a threshold disorder level $W = W_t$. [Preview Abstract] |
Monday, March 13, 2017 1:51PM - 2:03PM |
B45.00012: Engineering the electronic properties of topological insulator heterostructures Demet Usanmaz, Pinku Nath, Jose J. Plata, Marco Fornari, Marco Buongiorno Nardelli, Stefano Curtarolo Topological insulators (TIs) have been the subject of extensive research due to their exotic properties, such as behaving as an insulator in the bulk and having topologically protected metallic states at the boundaries. The emergence of metallic states relies on the spin-orbit induced band inversion in bulk materials and is protected by time-reversal symmetry or crystal symmetry. These characteristics of TIs make them potential candidates for various applications from spintronics to quantum computers. Here, we investigate TI heterostructures by varying the thickness of the layers to define the transition from 3D to 2D behavior. This approach gives a better insight into the topological interface states, which is essential for the design of new materials with tunable electronic properties. [Preview Abstract] |
Monday, March 13, 2017 2:03PM - 2:15PM |
B45.00013: Synthesis and surface-state transport of ultrathin topological crystalline insulator SnTe films Ke Zou, Stephen Albright, Omur Dagdeviren, M.D. Morales-Acosta, Georg Simon, Chao Zhou, Subhasish Mandal, Sohrab Ismail-Beigi, Udo Schwarz, Eric Altman, Frederick Walker, Charles Ahn SnTe is a topological crystalline insulator that exhibits crystal symmetry protected surface states that may impact the development of novel electronic devices. With a practical implementation of these materials in view, we introduce a new way to synthesize ultrathin films of SnTe ($\sim$ 10 unit cell thick) for transport measurements on SrTiO$_{3}$ (001) substrates using molecular beam epitaxy. Commonly observed bulk conduction by Sn vacancies is greatly suppressed in these ultrathin films. We observe that the surface states near the SnTe/vacuum interface are depleted due to band bending. Importantly, we find that the surface-state carriers are buried and protected from depletion at the SnTe/SrTiO$_{3}$ interface and dominate the measured conductance at thicknesses smaller than 40 unit cells, and that these carriers have a high density and mobility. [Preview Abstract] |
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