Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y35: Topological Insulators: Applications |
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Sponsoring Units: DCMP Chair: Xiaoliang Qi, Stanford University Room: C140 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y35.00001: Physical Vapor Deposition Growth of Topological Insulator Nanostructures Loren Alegria, Anasua Chatterjee, Zhong Zhang, Michael Pretko, James Ting, Shivang Patel, Jason Petta Nanostructures consisting of strong topological insulators are of interest for the fabrication of devices in which surface state transport is dominant. We report Bi$_2$Se$_3$ nanoribbon and nanoplatelet growth using a multi-zone furnace.\footnote{D. Kong \textit {et al.}, Nano Lett. \textbf{10}, 329 (2010).} Nanoribbons are grown by the vapor-liquid-solid method, using Au nanoparticles or Au thin films ($\sim$5 nm) as catalysts, while nanoplatelets are grown on bare silicon. We systematically vary the growth parameters, including the temperatures of the powdered Bi$_2$Se$_3$ precursor and growth substrate, the growth pressure and duration, the rate of the Argon carrier gas flow, size of the gold catalyst, and the quantity of Bi$_2$Se$_3$ source material. Typical nanoribbon growth occurs at 450$^\circ$C and 350 Torr, with the precursor held at 530$^\circ$C in an Argon carrier gas flow rate rate of 140 sccm. Typical platelet growth occurs at lower pressures and temperatures. High resolution transmission electron microscopy, diffraction, and energy dispersive x-ray analysis are used to characterize the synthesized structures. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y35.00002: Topological insulators for high performance terahertz to infrared applications Xiao Zhang, Jing Wang, Shou-Cheng Zhang Topological insulators in the Bi2Se3 family have an energy gap in the bulk and a gapless surface state consisting of a single Dirac cone. Low frequency optical absorption due to the surface state is universally determined by the fine structure constant. When the thickness of these three dimensional topological insulators is reduced, they become quasi-two dimensional insulators with enhanced absorbance. The two dimensional insulators can be topologically trivial or non-trivial depending on the thickness, and we predict that the optical absorption is larger for topological non-trivial case compared with the trivial case. Since the three dimensional topological insulator surface state is intrinsically gapless, we propose its potential application in wide bandwidth, high performance photo-detection covering a broad spectrum ranging from terahertz to infrared. The performance of photodetection can be dramatically enhanced when the thickness is reduced to several quintuple layers, with a widely tunable band gap depending on the thickness. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y35.00003: Two Dimensional Transport Induced Linear Magneto-resistance in Topological Insulator Bi$_{2}$Se$_{3}$ Nanoribbons Dong Liang, Hao Tang, Richard Qiu, Xuan Gao Bulk Bi$_{2}$Se$_{3}$ has been proposed and confirmed as a type of three-dimensional (3D) topological insulators (TI's) with a single Dirac cone for the surface state. Although the existence of topological surface state in Bi$_{2}$Se$_{3}$ has been established by surface sensitive techniques (ARPES, STM), the transport properties of two dimensional (2D) surface state in 3D TI's has been plagued by the dominating conductivity from bulk carriers. Here, we report the study of a novel linear magneto-resistance (MR) under perpendicular magnetic fields in Bi$_{2}$Se$_{3}$ nanoribbons, and show that this linear MR is purely due to 2D transport by angular dependence experiments. The 2D magneto-transport induced linear MR in Bi$_{2}$Se$_{3}$ nanoribbons is in agreement with the recently discovered linear MR from topological surface state in bulk Bi$_{2}$Te$_{3}$, and the MR of other gapless semiconductors and graphene. We further show that the linear MR of Bi$_{2}$Se$_{3}$ nanoribbons persists up to room temperature, underscoring the potential of exploiting TI's for room temperature magneto-electronic applications. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y35.00004: Quantum impurity spin in Majorana edge modes Ryuichi Shindou, Akira Furusaki, Naoto Nagaosa We show that Majorana edge modes of two-dimensional spin-triplet topological superconductors (superfluids) have Ising-like spin density whose direction is determined by the d-vector characterizing the spin-triplet pairing symmetry in the bulk. Thus, when a quantum impurity spin is introduced at the edge of the spin-triplet topological superconductors (superfluids), the exchange coupling between this impurity spin and the Majorana modes becomes Ising-type. Observing this, we argue that, under the external magnetic fields, this quantum impurity spin exhibits anisotropic dissipative quantum dynamics due to the `background' massless Majorana edge modes. We also discuss how the magnetic response of this impurity spin can serve as a local probe for spin-triplet superconducting order parameter in the bulk. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y35.00005: Surface state transport in Bi2Se3 nanodevices Hadar Steinberg, Valla Fatemi, Pablo Jarillo-Herrero We report on electronic transport measurements on thin ($<$100 nm) Bi2Se3 devices and show that the density of the surface states can be modulated via the electric field effect by using a top-gate with a high-k dielectric insulator. The conductance dependence on geometry, gate voltage, and temperature all indicate that transport is governed by parallel surface and bulk contributions. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y35.00006: Thermoelectric transport of edge/surface states of topological insulators Shuichi Murakami, Ryuji Takahashi In my talk we theoretically study thermoelectric properties of topological insulators (TI) [1], where novel properties of edge/surface states are expected to appear. As compared to the number of bulk states, the edge/surface states are very few; we therefore consider a narrow ribbon for 2D and a thin slab for 3D TI to make the edge/surface-state transport larger. By considering edge/surface and bulk transport together, we calculate the charge and heat conductivity, and Seebeck coefficient. We find that in 2D TI the bulk and edge transport compete each other in the thermoelectric transport. By lowering temperature, the thermoelectric figure of merit ZT has a minimum, corresponding to the bulk-to-edge crossover, and then increases again at low temperature where the edge state dominates. The crossover is estimated to be at around 5K-10K for 10nm-width ribbon. We also discuss surface state transport for 3D TI as well.\\[4pt] [1] R. Takahashi and S. Murakami, Phys. Rev. B81, 161302 (R) (2010).\\[0pt] [2] S. Murakami, R. Takahashi, O. A. Tretiakov, Ar. Abanov, J. Sinova, arXiv:1010.2304. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y35.00007: Mobility and thermopower of surface and bulklike charges in Bi and Sb nanowires T.E. Huber, A. Adeyeye, A. Nikolaeva, L. Konopko, R.C. Johnson, M.J. Graf Topological insulators (TI) surface charges are predicted to have high mobilities and other properties. Bi and Sb, that are classified as TI trivial and true, respectively, are interesting candidates but are not very good bulk insulators. However, in very thin nanowires, quantum confinement opens a gap for the bulk states that is not expected to change the material's TI character. We studied the electronic transport of 18-nm to and 200-nm diameter nanowires in arrays, fabricated by Bi injection in porous alumina, via coupled measurements of resistance and thermopower (4-300 K). Surface carriers and holes Landau level spectra were analyzed to extract densities. The nanowires low temperature thermopower (T$<$100 K) is -1 T microvolt/(K$^2$) consistent in sign and magnitude with surface electrons. Coexistence of bulklike holes with surface electrons, consistent with the carrier's hybridization that is expected in Bi, is observed. Results for Sb will be presented also. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y35.00008: Raman spectroscopy of exfoliated Bi2Se3 Luke Sandilands, Frank Zhao, Christianne Beekman, John Bashucky, Daniel Kwok, Nara Lee, Sang-Wook Cheong, Kenneth Burch The study of topological insulators is often frustrated by the presence of a residual bulk conductivity arising from defects which makes isolating the surface contribution to a given measurement difficult. Nanoscale topological insulators are therefore an appealing alternative to bulk crystals, as a small volume should emphasize surface contributions and allow the suppression of the residual bulk carriers by gating. To this end we have produced, via mechanical exfoliation, nanocrystals as thin as a 2 nm of the topological insulator Bi2Se3 on Mica substrates. Exfoliated crystals of a variety of thicknesses have been characterized by optical, Raman, and atomic force microscopies. We observe an emergent mode at 158 cm-1 which is attributed to the breaking of inversion symmetry at the Bi2Se3 surfaces. The utility of this emergent mode for determining nanocrystal thickness is discussed. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y35.00009: Mechanical Exfoliation and Electron Transport of Topological Insulator Nanoribbons Seung Sae Hong, Worasom Kundhikanjana, Judy Cha, Keji Lai, Desheng Kong, Zhi-Xun Shen, Yi Cui Bismuth selenide (Bi$_{2}$Se$_{3}$), a stoichiometric material of a single Dirac-cone band structure, is one of the most promising candidates to realize the topologically non-trivial surface state protected by time reversal symmetry. Especially, many exotic physical phenomena are predicted to emerge in low dimensional nanostructures of Bi$_{2}$Se$_{3}$, such as the crossover between 3D to 2D topological insulator. Due to the weak Van der Waals interaction between adjacent quintuple layers (QLs), Bi$_{2}$Se$_{3}$ can be exfoliated down to a few QLs. We will present the mechanical exfoliation of topological insulator nanoribbons by an atomic force microscope (AFM) tip, which enables ultra-thin topological insulator down to a single QL. Electron transport measurement on low dimensional topological insulator will be also discussed, as well as the conductivity mapping experiment using a microwave scanning probe technique. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y35.00010: Topological surface states along antiwires Alexander Punnoose, Wei Liu Surface states along amorphous columnar defects in three-dimensional topological insulators are considered. The response of these states to a magnetic field and their contribution to thermoelectric transport will be discussed. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y35.00011: Bias dependence of h/e and h/2e Aharonov-Bohm oscillations in topological insulators Pramey Upadhyaya, Faxian Xiu, Yabin Fan, Igor Ovchinnikov, Kang Wang Recently Aharonov-Bohm (AB) oscillations were observed in Bi$_{2}$Se$_{3}$ nanoribbons by Peng \textit{et al}. [1] as a direct evidence for the existence of surface states in topological insulator. However, the resistance showed only h/e oscillations with a minimum in resistance at zero flux while the ballistic and diffusive theory predicts either h/e oscillations with a maximum in resistance at zero flux or h/2e oscillations with a minimum in resistance at zero flux respectively [2]. A possible explanation of the results of Peng \textit{et al.} was given in the theory of disordered topological insulators proposed by Bardarson \textit{et al }.[2] and Zhang \textit{et al.} [3] where they attributed the results of Peng \textit{et al.} to presence of weak disorder. Furthermore authors of [2] and [3] studied dependence of h/e and h/2e oscillations on disorder strength and doping using their proposed theory. In this work we look at the effect of doping by studying bias dependence of AB oscillations using a gated device and observe both h/e and h/2e oscillations whose relative strength depends on the applied bias and compare the proposed theory of ref. [2] and [3] with the experimental results. [1] H. Peng, \textit{et al}. Nature Mater. 9, 225 (2010).[2] J. Bardarson, \textit{et al}, Phys. Rev. Lett. 105, 156803 (2010).[3] Y. Zhang and A. Vishwanathan, Phys. Rev. Lett. 105, 206601 (2010). [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y35.00012: Spin polarization and transport of surface states in the topological insulators Bi$_2$Se$_3$ and Bi$_2$Te$_3$ from first principles Oleg Yazyev, Joel Moore, Steven Louie We investigate the band dispersion and the spin texture of topologically protected surface states in the reference bulk topological insulators Bi$_2$Se$_3$ and Bi$_2$Te$_3$ by using a first-principles approach. Exceptionally strong spin-orbit interaction in these materials entangles the electronic states across broad energy ranges thus reducing the spin-polarization of the topologically protected surface states to $\sim$50\% in both cases. This reduction is absent in simple phenomenological models but of important implications to essentially any application of bulk topological insulators in spintronics and likely to some other phenomena. We further propose a way of controlling the magnitude of spin polarization associated with a charge current in thin films of topological insulators by means of an external electric field. The proposed dual-gate device configuration provides new possibilities for electrical control of spin. [Preview Abstract] |
Friday, March 25, 2011 10:24AM - 10:36AM |
Y35.00013: Manipulating surface states in topological insulator nanostructures Faxian Xiu, Liang He, Kang L. Wang Topological insulators show unique properties resulting from massless, Dirac-like surface states that are protected by time-reversal symmetry. Theory predicts that the surface states exhibit quantum spin Hall effect that allows for spins to transport without scattering. However, to date, the direct manipulation of these states with external means remains a significant challenge owing to the predominance of bulk carriers. Here we show the first experimental evidence of surface-state modulation through the observation of voltage-controlled quantum oscillations in Bi2Te3 nanostructures. The surface conduction can be dramatically enhanced with external gate bias. Up to 51 percent of the total conductance is attributed to the surface states. The ability to manipulate the surface states mark an important milestone in the development of TI materials and may further open up exciting and novel applications in nanoelectronics and spintronics. [Preview Abstract] |
Friday, March 25, 2011 10:36AM - 10:48AM |
Y35.00014: Aharonov-Bohm oscillations in disordered topological insulator nanowires Jens H. Bardarson, P.W. Brouwer, J.E. Moore A direct signature of electron transport at the metallic surface of a topological insulator is the Aharonov-Bohm oscillation observed in a recent study of Bi$_2$Se$_3$ nanowires [Peng {\it et~al.}, Nature Mater.\ 2010] where conductance was found to oscillate as a function of magnetic flux $\phi$ through the wire, with a period of $h/e$ and {\it maximum} conductance at zero flux. This seemingly agrees neither with diffusive theory (period of $h/2e$) nor with ballistic theory, which in the simplest form predicts a period of $h/e$ but a {\it minimum} at zero flux due to a nontrivial Berry phase. We show how the magneto-conductance depends on doping and disorder strength, provide a possible explanation for the experiments, and discuss further experiments that could verify the theory. [Preview Abstract] |
Friday, March 25, 2011 10:48AM - 11:00AM |
Y35.00015: Terahertz conductivity of Bi$_2$Se$_3$ topological insulator thin films Rolando Vald\'es Aguilar, L.S. Bilbro, N. Bansal, Y-S. Kim, S. Oh, C. Chang, Y. Zhang, K. He, X. Ma, X. Zhu, Q-K. Xue, N.P. Armitage We report a study of high quality MBE grown Bi$_2$Se$_3$ topological insulator thin films. We have measured the ac conductivity in the terahertz region using time domain terahertz spectroscopy. By measuring films with different thickness we can set limits on the value of the bulk and surface conductivites. We will also report on measurements of the Faraday rotation using polarized light at these frequencies. [Preview Abstract] |
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