Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J41: Topological Insulators: Experimental Tuning and Extreme Conditions |
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Sponsoring Units: DCMP Chair: Xiaohang Zhang, University of Maryland Room: Mile High Ballroom 3C |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J41.00001: In situ monitoring of resistivity and carrier concentration during molecular beam epitaxy of topological insulator Bi$_2$Se$_3$ J. Hellerstedt, J.H. Chen, D. Kim, W.G. Cullen, C.X. Zheng, M.S. Fuhrer Bismuth selenide (Bi$_2$Se$_3$) is a three-dimensional strong topological insulator of particular interest due to its relatively large bulk band gap (300 meV) and single set of topologically non-trivial surface states. However, there are outstanding problems in isolating the surface state from the bulk (trivial) conduction: this problem is frequently attributed to doping from selenium vacancies and atmospheric exposure. To address this question of doping, we have constructed a molecular beam epitaxy system with the additional capability of doing real time, in situ measurement of resistivity and Hall carrier density. Bismuth selenide has a micaceous crystal structure of quintuple layer units weakly bonded to one another making it well suited to this growth (van der Waals epitaxy) and measurement technique. Cooling to 15 K and controlled exposure to atmospheric dopants is additionally possible without breaking vacuum. We have been able to achieve direct measurement of mobilities on the order of 300 cm$^2/$Vs and carrier densities of 3$\times$10$^{13}$cm$^{-2}$ measured at growth temperatures of 200 to 300 $^\circ$C. The latest results of carrier density and mobility as a function of film thickness and growth parameters will be reported. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J41.00002: Ambipolar magnetotransport of topological insulator thin film in the extreme quantum limit Ross McDonald, Zuocheng Zhang, Zengwei Zhu, Xiao Feng, Yang Feng, Yunbo Ou, Minghua Guo, Kang Li, Ke He, Xucun Ma, Qikun Xue, Yayu Wang Topological insulators (TIs) are quantum materials with insulating bulk and topologically protected metallic surfaces. An outstanding challenge in the field of TIs is to reveal the intrinsic quantum transport properties of the topological surface states. Here, we investigate the transport properties of (Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ TI thin film in 60 T pulsed magnetic fields. A complex and systematic evolution of magnetoresistance (MR) is observed when the Fermi level is tuned across the Dirac point by gate voltage. In particular, an unusual negative MR prevails at the charge neutral point and gradually becomes positive at higher band filling. This intriguing phenomenon is related to the exotic property of surface states that is only shown in the extreme quantum limit. Our results approach the regime necessary to access the half quantum Hall effect in gated topological insulators. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J41.00003: Transport in Quantum Confined Sb(111) Shayne Cairns, Nolan Teasdale, Kaushini Wickramasinghe, Chomani Gaspe, Lin Lei, Tetsuya Mishima, Joel Keay, Michael Santos, Sheena Murphy Sb is a topological semi-metal with a negative bandgap of 180meV, however it is anticipated that in ultra-thin films, quantum confinement will open the bulk gap such that transport is dominated by topological surface states. We have studied the magneto-transport of ~1.5 nm to 3.2 nm films of Sb(111) grown via molecular beam epitaxy on nearly lattice matched epilayers of GaSb(111). SEM shows the Sb growth yielded smooth and continuous films that show significantly reduced bulk conduction at low temperatures. $\rho_{xx}$ displays a linear dependence at high magnetic fields that increases with decreasing film thickness in good agreement with calculations for a linearly dispersing systems with overlapping Landau levels.. At lower fields the films display positive magneto-resistance, well-described by the weak anti-localization (WAL) theory of Hikami, Larkin and Nagaoka for strong spin orbit coupling, yielding phase breaking lengths ~1$\mu$m at 300mK. Projects to investigate quantum interference in lithographically defined wires and to seek proximity-induced superconductivity are ongoing. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J41.00004: Evidence of the two surface states of (Bi$_{0.53}$Sb$_{0.47}$)$_{2}$Te$_{3}$ films grown by van der Waals Liang He, Xufeng Kou, Murong Lang, Kang Long Wang, Eun Sang Choi, Ying Jiang, Yong Wang, Faxian Xiu The discovery of topological insulators (TIs) has led to numerous exciting opportunities for studying topological states of quantum physics and for exploring spintronic applications due to the new physics arising from their robust metallic surface states. Here, we report the growth of high-quality topological insulator (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}}$)$_{2}$Te$_{3}$ thin films using a single van der Waals GaSe buffer layer by molecular beam epitaxy. Ultra-low surface carrier density of 1.3 $\times$ 10$^{12}$ cm$^{-2}$ and a high Hall mobility of 3100 cm$^{2}$/Vs have been achieved for (Bi$_{0.53}$Sb$_{0.47}$)$_{2}$Te$_{3}$. The high-quality films enable us to observe quantum oscillations associated with the top and bottom surface states and to manipulate the Dirac electrons and bulk holes' conduction properties. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J41.00005: Anomalous nuclear magnetic resonance spectra in powdered Bi$_2$Se$_3$ D.M. Nisson, A.P. Dioguardi, P. Klavins, X. Peng, D. Yu, N.J. Curro We present $^{209}$Bi NMR spectra and relaxation rate data on single crystal and powder samples of the topological insulator material Bi$_2$Se$_3$, including data on nanoscale powders with percentages of surface nuclei on the order of 2\%. Powder samples are measured as-prepared, annealed to relieve mechanical strains, and fixed in epoxy to prevent alignment of grains with the applied magnetic field of 9 T. Our results reveal anomalous behavior in both the angular dependence of the single crystal spectra and in the powder spectra. All powder spectra display features not accounted for by summation of spectra of single crystal orientations. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J41.00006: Two-carrier transport and multi-channel weak antilocalization in SnTe thin films grown by MBE Badih A. Assaf, Ferhat Katmis, Peng Wei, Biswarup Satpati, Zhan Zhang, Jagadeesh S. Moodera, Don Heiman SnTe has recently been identified as a four-fold degenerate topological system where each (001) surface accommodates four identically-chiral Dirac surface states [1]. The surface states were successfully studied by ARPES and STM [2,3]. We perform magnetotransport measurements at 2K and up to 5T on epitaxial SnTe thin films grown by MBE on BaF$_{\mathrm{2}}$ (001). We report the observation of a two-carrier contribution to the magnetotransport as well as a multi-channel weak antilocalization (WAL) correction to the low field magnetoresistance. Analysis of the WAL using the HLN model [4] yields 0.75\textless $\alpha $\textless 2.5 suggesting evidence of intervalley coupling on the surface of SnTe. Films grown under different growth conditions are discussed and compared. [1] T.H. Hsieh et al. \quad Nature Commun$.$ \textbf{3, }982 (2012). [2] S.X. Yu et al. Nature Commun$. $\textbf{3, }1192 (2012). Tanaka, Y. et al. Nature Phys$. $\textbf{8, }800 (2012). [3] Okada, Y. et al. Science 1239451 (2013). [4] Hikami et al. Prog. Theor. Phys. \textbf{63} 707 (1980). Supported by NSF-DMR-0907007, partly by NSF-DMR-1207469, ONR-N00014-13-1-0301, and MIT MRSEC through NSF-DMR-0819762 and partly by DOE under Contract DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J41.00007: Compensation of intrinsic charge carriers in topological insulators using high energy electron beams Lukas Zhao, Haiming Deng, Jeff Secor, Marcin Konczykowski, Andrzej Hruban, Lia Krusin-Elbaum One of the main challenges in probing charge transport of the topological Dirac surface states is a non-vanishing conductivity of the bulk. With the techniques employed thus far, reaching the charge neutrality point (CNP) has proved difficult. Here we demonstrate that we can reach CNP by compensating intrinsically \textit{p}-type topological insulators (TIs) by irradiation with high energy (2.5 MeV) electrons, and increase bulk resistivity by orders of magnitude. Irradiations, performed at liquid hydrogen, create Frenkel (vacancy-interstitial) pairs in the bulk, with donor-type vacancies that remain stable up to room temperature. The conversion of conductivity type (from \textit{p}- to \textit{n}-type) in Bi$_2$Te$_3$ and Sb$_2$Te$_3$ occurs at the resistivity maximum obtained for the beam fluence $\phi \cong 30-35~mC/cm^2$. The 2D character of longitudinal conductance $\sigma_{xx}$ near CNP is indicated by the appearance of weak anti-localization (WAL) cusp that scales with $H_\perp = H cos \theta$. The coherence length extracted from the fits to 2D Hikami-Larkin-Nagaoka theory of WAL is $\sim 210~\textrm{nm}$, comparable to that obtained in thin MBE films. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J41.00008: Quantum-interference effects in single- and poly-crystalline topological insulator Bi$_{2-x}$Te$_{3}$ Shao-Pin Chiu, Juhn-Jong Lin We have studied the carrier transport properties of both single- and poly-crystalline topological insulator (TI) Bi$_{2-x}$Te$_{3}$ samples. Single-crystalline microflakes were made by exfoliation from a single-crystalline Bi$_{2}$Te$_{3}$ bulk. Polycrystalline samples were made by flash evaporation of 5N purity Bi$_{2}$Te$_{3}$ sheets. In single-crystalline Bi$_{2}$Te$_{3}$ microflakes, temperature dependent resistances revealed two-dimensional (2D) electron-electron interaction effect. The extracted Coulomb screening parameter is negative, in accord with the situation of strong spin-orbit coupling in the TI materials. Positive magnetoresistances (MRs) originated from 2D weak-antilocalization (WAL) effect were measured in low magnetic fields, and satisfactorily described by a multichannel-conduction model. Especially, as T below 1 K and under high positive backgate voltages, signature of two coherent conduction channels was found. We discuss our results in terms of Dirac fermion states on the bottom surface, in addition to the bulk states. Polycrystalline Bi$_{2-x}$Te$_{3}$ thin films were patterned by electron-beam lithography. In low perpendicular magnetic fields, positive MRs due to the 2D WAL effect were observed. In parallel magnetic fields, Aharonov-Bohm oscillations were measured, suggesting the presence of metallic surface states. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J41.00009: Shubnikov-de Haas oscillations and electrical transport properties of Bi based topological insulators Yusuke Suzuki, Masashi Komatsu, Kasumi Tashima, Fumiya Kimizuka, Takashi Mochiku, Takanari Kashiwagi, Ryozo Yoshizaki, Kazuo Kadowaki The discovery of the anomalous superconductivity in Cu$_{x}$Bi$_{2}$Se$_{3}$ has attracted much attention because of the relation between superconducting state and topological surface state. In this study we present electronic transport properties in Cu$_{x}$Bi$_{2}$Se$_{3}$. We have grown various kinds of single crystals of topological insulators based on Bi related compounds and the transition metal doped compounds and have studied basic transport phenomenon in order to characterize them. The pronounced quantum oscillations in the magnetoresistance were observed in both doped and non-doped Cu$_{x}$Bi$_{2}$Se$_{3}$, which provide the precise information about their electronic structures. We will discuss the results as topologically interesting properties. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J41.00010: Robust ferromagnetism in V doped ultrathin three dimensional topological insulator Bi$_{2}$Te$_{3}$ films Cui-Zu Chang, Wei-Wei Zhao, Badih A. Assaf, Ferhat Katmis, Peng Wei, Don Heiman, Moses H. W. Chan, Jagadeesh S. Moodera Motivated by the discovery of quantum anomalous Hall effect in Cr doped (BiSb)2Te3 ferromagnetic topological insulator (TI) films,1 high quality vanadium (V) doped three dimensional (3D) TI Bi2Te3 films were successfully grown via molecular beam epitaxy on etched Si(111) and heat treated insulating SrTiO3 (111) substrates. Anomalous Hall effect measurements and magnetization studies found that a robust long range out-of-plane ferromagnetic order occurs in ultrathin Bi2-xVxTe3 films down to 5QLs. There was systematic dependence of ferromagnetism on the concentration of V. However, the ferromagnetic order was observed to be insensitive to the carrier type and density. Upon the application of a bottom gate electric field to reduce the carrier density, the anomalous Hall resistance increased, while the coercivity was unaffected. These observations are in contrast to that seen in conventional dilute magnetic semiconductors (DMSs). Our observation might lead to the carrier independent control of anomalous Hall voltage in ferromagnetic TIs and that could form the basis for magneto-electronics and spintronics applications. References:[1] Cui-Zu Chang et al. Science. 340, 167 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J41.00011: Linear magnetoresistance in disordered magnetically doped topological insulator Bi2Te3 Inna Korzhovska, Lukas Zhao, Zhiyi Chen, Milan Begliarbekov, Haiming Deng, Simone Raoux, Lia Krusin-Elbaum First-principle calculations predict that certain topological insulators (TIs) can turn ferromagnetic (ferro-TIs) when doped with magnetic ions such as Fe or Cr. These ferro-TIs support anomalous Hall effect (AHE) that becomes quantized in the thin film limit. In the absence of disorder, doping with vanadium (SP = 0.7 eV per V, comparable to Fe) is not expected to produce a ferro-TI due to the position of vanadium 3D bands. Here we show that disorder introduced by doping vanadium into Bi$_{2}$Te$_{3}$ thin films has three remarkable effects:(i) it shows unusual AHE as seen in the hysteretic behavior of Hall conductance that does not scale with magnetization M, (ii) it forms a donor band that turns conductivity type from p- to n- and turns R vs. T from metallic to semiconducting-like, and (iii) it results in a large region below 100 K that has negative linear magnetoresistance (MR) in high magnetic fields. A large positive linear MR was observed in silver chalcogenides Ag$_{2+\delta}$Se and Ag$_{2+\delta}$Te, consistent with the predicted quantum linear MR in disordered semimetals. We will discuss this mechanism in a TI, including the MR sign reversal arising from frustrated magnetic doping. * Supported in part by NSF-DMR-1122594, NSF-DMR-1312483-MWN, and DOD-W911NF-13-1-0159 [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J41.00012: Enhanced surface state of topological insulators by optimal magnetic doping Yan Ni, Nicholas Meyer, Xiaoyu Che, Zhen Zhang, Cajetan Nlebedim, Ravi Hadimani, David Jiles Topological insulators (TIs) attract attentions both for fundamental science and potential applications because of their bulk band inversion arising from the strong spin orbital coupling. In addition, magnetic impurities doped into TIs can lead to opening of energy gap and induce some interesting fundamental physical phenomena such as the quantum anomalous Hall effect and magnetoelectric effect. In this work, we investigate the manipulation of the Fermi level, the band structure, and related surface states of (Sb$_{\mathrm{x}}$Bi$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ by Cr doping. We will show the magnetic dopants in the TI films are necessary to sustain an insulating bulk while simultaneously keeping the Dirac point of the surface in the bulk gap of (Sb$_{\mathrm{x}}$Bi$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ thin film. It will be also shown that Cr doping in the films will both increase the magnetic response of the TI films by increasing the permeability and result in the opening a surface band gap. As a result, the materials can be made more suitable for spintronics and electronic devices such as magnetic sensors. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J41.00013: Electrically tuned magnetic order and magnetoresistance in a topological insulator Minghua Guo, Zuocheng Zhang, Xiao Feng, Kang Li, Jinsong Zhang, Yunbo Ou, Yang Feng, Lili Wang, Xi Chen, Ke He, Xucun Ma, Qikun Xue, Yayu Wang Topological insulators (TIs) possess spin-polarized, Dirac-like surface states protected by time reversal symmetry (TRS). Introducing magnetism into TI, which breaks the TRS, is expected to create exotic topological magnetoelectric effects. In particular, it may lead to highly unconventional magnetoresistance (MR) behavior that can find unique applications in magnetic sensing and data storage. In this talk, we present magneto transport studies of Cr doped (Bi,Sb)2Te3 ferromagnetic TI thin film fabricated into a field effect transistor device. We observe an unusually complex evolution of MR when the Fermi level is tuned across the Dirac point by gate voltage. The MR behavior cannot be explained by the simple localization picture, but is closely related to the gate-tuned ferromagnetic order. The underlying physics is the competition between the broken TRS and topological protection in magnetic TI. The simultaneous electrical control of magnetic order and magneto transport facilitates future TI-based spintronic devices. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J41.00014: Pressure evolution of electrical transport in the 3D topological insulator (Bi,Sb)2(Te,Se)3 Jason Jeffries, N.P. Butch, Y.K. Vohra, S.T. Weir The group V-VI compounds---like Bi$_{2}$Se$_{3}$, Sb$_{2}$Te$_{3}$, or Bi$_{2}$Te$_{3}$---have been widely studied in recent years for their bulk topological properties. The high-Z members of this series form with the same crystal structure, and are therefore amenable to isostructural substitution studies. It is possible to tune the Bi-Sb and Te-Se ratios such that the material exhibits insulating behavior, thus providing an excellent platform for understanding how a topological insulator evolves with applied pressure. We report our observations of the pressure-dependent electrical transport and compare that behavior with other binary V-VI compounds under pressure. [Preview Abstract] |
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