Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W13: Topological Insulators: Bi2Se3, Pure and Chemically Doped |
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Sponsoring Units: DCMP Chair: James Analytis, University of California, Berkeley Room: 315 |
Thursday, March 21, 2013 2:30PM - 2:42PM |
W13.00001: Insulating behavior in ultra-low carrier density Bismuth Selenide single crystals Paul Syers, Johnpierre Paglione The topological insulator material Bi$_2$Se$_3$ is well known to suffer from a non-insulating bulk due to doping caused by selenium vacancies. We present results on the synthesis and characterization of pure undoped Bi$_2$Se$_3$ crystals that exhibit nonmetallic transport behavior over the entire measured temperature range, from room temperature down to at least 2 K. Measurements of longitudinal transport and Hall effect are used to characterize the transport temperature and magnetic field dependences, carrier sign and density, and sensitivity to air exposure. [Preview Abstract] |
Thursday, March 21, 2013 2:42PM - 2:54PM |
W13.00002: THz generation and the detection on the Dirac-cone surface states in topological insulator Bi$_2$Se$_3$ J.-Y. Lin, C.W. Luo A terahertz (THz) wave is generated from the (001) surface of cleaved Bi$_2$Se$_3$ and Cu-doped Bi$_2$Se$_3$ single crystals, using femtosecond pulses of 800 nm. The generated THz power is strongly dependent on the carrier concentration of the crystals, which can be explained by considering the absorption of both surface and bulk states altogether. In particular, the Dirac-cone surface states in Bi$_2$Se$_3$ significantly affect the THz emission efficiency. Therefore, the THz radiation from topological insulators can be used to ascertain the existence and characteristics of the Dirac-cone surface states. [Preview Abstract] |
Thursday, March 21, 2013 2:54PM - 3:06PM |
W13.00003: Coexistence of Bulk and Surface Shubnikov-de Haas Oscillations in Bi$_{2}$Se$_{3}$ Chi Zhang, Fanming Qu, Rui-Rui Du, Li Lu Topological insulator possesses insulating bulk state and spin-momentum interlocked conducting topological surface state. Among many materials, bismuth selenide (Bi$_{2}$Se$_{3})$ is an important candidate, which hosts a single Dirac cone in the surface energy spectrum. In electron transport measurements, 3-dimensional Shubnikov-de Haas (SdH) oscillations of bulk state were observed. Under a very high magnetic field, our rotating sample experimental results exhibit the coexistence of bulk and surface SdH oscillations: Hall bar shape device based on Bi$_{2}$Se$_{3}$ nano-plate was fabricated and studied at a dilution temperature with a tilted magnetic field up to 45 T. Three types of carrier, one of 3-dimensional and two of 2-dimensional, were identified by analyzing the angular dependence of SdH oscillations, which confirmed the coexistence of bulk carrier and band bending induced two-dimensional electron gas in transport experiment. The co-contributions to quantum oscillations indicated the independence of these states, without smearing out by scattering with each other, which may pave off the way for studying topological surface states with residual bulk carriers in Bi$_{2}$Se$_{3}$. The data analysis and experimental results are included in the presentation. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W13.00004: Bulk versus surface contributions to the Shubnikov-de Haas Effect E. Maniv, M. Petrushevsky, E. Lahoud, A. Ron, I. Neder, S. Wiedmann, V.K. Guduru, U. Zeitler, J.C. Maan, K. Chashka, A. Kanigel, Y. Dagan Among the bulk materials that are considered as experimental realizations of topological insulators Bi$_{2}$Se$_{3}$ is of particular interest due to its large bulk band gap and surface states with a single Dirac cone. It has been recently shown that Bi$_{2}$Se$_{3}$ can become superconducting when Cu$^{\, }$intercalation is introduced (Hor, Y. S.; Williams, A. J. et al. \textit{Phys. Rev. Lett. }\textbf{2010}, 104, 057001). We report on transport measurements of cleaved flakes $\sim $1$-$100 $\mu $m thick of Cu intercalated Bi$_{2}$Se$_{2}$. Clear Shubnikov-de Haas oscillations are observed. We study the temperature and angular dependence of these oscillations together with the Hall coefficient at low temperatures for various Cu concentrations. We discuss possible contributions from bulk and the protected surface states to the various transport channels. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W13.00005: Modification of topological insulator transport properties by electron beam irradiation Zilong Jiang, Zhiyong Wang, Tao Lin, Jing Shi Topological insulators (TI) are predicted to present a variety of interesting surface transport phenomena. However, in TI devices, the metallic bulk conduction usually overwhelms the surface transport. In this work, we first fabricate TI devices based on our high bulk resistivity material ($\sim$5 $\Omega $.cm) Bi$_{\mathrm{x}}$Sb$_{\mathrm{2-x}}$Te$_{\mathrm{y}}$Se$_{\mathrm{3-y}}$ using ebeam lithography. Then we expose the devices with an electron beam to introduce disorders to localize the bulk carriers. The devices are $\sim$100-200 nm in thickness and the resistivity is weakly temperature dependent. Upon initial low-energy exposures, we find that the resistance of device decreases and reaches a saturation state as the dosage increases. We attribute this decrease in resistivity to an increased electron density in the devices. As we ramp up the energy of the electron beam, the resistance starts to increase, showing the effect of additional scattering. At low temperatures, the resistance rapidly increases in a diverging trend. At 4 K, the magnetoresistance starts to display oscillatory features that are likely the Shubnikov-de Haas oscillations from the surface states. We believe that the disorders introduced by the electron beam play an important role in modifying the transport of the bulk carriers. More detailed experimental results and discussions will be presented. [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W13.00006: Topological Spin-Polarized Electron Layer above the Surface of Ca-Terminated Bi$_2$Se$_3$ Xiaoxiong Wang, Guang Bian, Tom Miller, Taichang Chiang Spin-polarized gapless surface states on the boundary of topological insulators are of interest for spintronic applications. First-principles calculations show that adsorption of a Ca monolayer on films of the prototypical topological insulator, Bi$_2$Se$_3$, yields a substantial enhancement of the surface-state spin polarization, despite the low atomic mass of Ca and its weak spin-orbit coupling. Much of the topological surface electron distribution is transferred outside the Ca to form a polarized electron layer out in vacuum; this spatial separation from the substrate minimizes scattering by defects in Bi$_2$Se$_3$ and is very desirable for spin transport. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W13.00007: Strong single-ion anisotropy and anisotropic interactions of magnetic adatoms induced by topological surface states Zhenglu Li, Jihui Yang, Guohong Chen, M.-H. Whangbo, Hongjun Xiang, Xingao Gong The nature of the magnetism brought about by Fe adatoms on the surface of the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ was examined in terms of density functional calculations. The Fe adatoms exhibit strong easy-axis magnetic anisotropy in the dilute adsorption limit due to the topological surface states (TSS). The spin exchange J between the Fe adatoms follows a Ruderman-Kittel-Kasuya-Yosida behavior with substantial anisotropy, and the Dzyaloshinskii-Moriya interaction between them is quite strong with \textbar D/J\textbar\ $\approx $ 0.3 under the mediation by the TSS, and can be further raised to 0.6 by an external electric field. The apparent single-ion anisotropy of a Fe adatom is indispensable in determining the spin orientation. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W13.00008: The Fermi Surface of Highly Doped Bi$_{2}$Se$_{3}$ and the Implications for Superconductivity in CuBi$_{2}$Se$_{3}$ Elias Lahoud, Amit Kanigel, Muntaser Naamneh, Amit Ribak, Hanan Chaska, Michal Petrushevsky, Eran Maniv, Yoram Dagan The 3D Fermi-surface (FS) mapping of Bi$_{2}$Se$_{3}$ for different samples with carrier-density ranging from $10^{17}$ to $10^{20}$ cm$^{-3}$ was made using Angle Resolved Photoemission Spectroscopy. While in the low carrier density samples a closed FS was observed, in high carrier density superconducting Cu$_{x}$Bi$_{2}$Se$_{3}$ samples the FS was found to be open. The open FS puts constraints on the possible order-parameters in this system. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W13.00009: Scanning tunneling microscopy of defects and electronic fluctuations in Cu-doped Bi$_{2}$Se$_{3}$ Christopher Mann, Damien West, Ireneusz Miotkowski, Yong Chen, Shengbai Zhang, Chih-Kang Shih We report scanning tunneling microscopy and spectroscopy studies of the topological insulator Cu$_{x}$Bi$_{2}$Se$_{3}$. We have identified five different atomic-resolution signatures of Cu dopant-related point defects and correlated several of them to density functional theory simulations of the defects. Most interestingly, by investigating the dI/dV images of the known Bi$_{Se}$ antisite defects as a function of bias, we show that local electronic structure can vary substantially over a length scale of 30nm, with amplitudes as large as $\pm$50meV. The strong fluctuations appear to be caused by a variety of defects and may have consequences for the topological surface state, as revealed by quasiparticle scattering studies. Correlation of quasiparticle scattering with the various defects indicates that the surface state is robust to backscattering, though detailed analysis shows that some defects are more effective in producing stationary scattering states than others. [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W13.00010: Probing the pairing symmetry of candidate topological superconductor CuxBi2Se3 via point contact spectroscopy Xunchi Chen, Chao Huan, Yew San Hor, Carlos Sa de Melo, Zhigang Jiang We perform point contact spectroscopy measurements on the candidate topological superconducting material, Cu$_{\mathrm{0.25}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$, using a normal-metal gold tip or an s-wave superconductor niobium tip. For the Au- Cu$_{\mathrm{0.25}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ interface, we observe a marked zero-bias conductance peak in the point contact spectra on the superconducting area of the sample, indicative of unconventional superconducting pairing symmetry. The point contact spectra also exhibit a pronounced linear background, which we attribute to inelastic scattering at the tip-sample interface. We compare the background subtracted spectra to a single-band p-wave model. For the Nb- Cu$_{\mathrm{0.25}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ interface, we observe a two-gap-like feature in the spectra, corresponding to the superconducting gap of the niobium and the sample, respectively. In addition, we find that the spectra are highly dependent on the interface barrier strength, and exhibit non-monotonic temperature dependence at zero bias, possibly owing to the incompatibility of the pairing symmetries between the Nb tip and the sample. Our results signify the unconventional superconductivity in Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W13.00011: Quantum oscillations in topological superconductor candidate Cu$_{x}$Bi$_{2}$Se$_{3}$ Benjamin Lawson, Gang Li, Yew San Hor, Lu Li In Cu$_{x}$Bi$_{2}$Se$_{3}$, a candidate to be a 3-dimensional topological superconductor, it is of high interest to study how its bulk electronic structure differs from Bi$_{2}$Se$_{3}$, since the nature of the emergent bulk superconducting order puts constraints on the possible surface state. The de Hass-van Alphen effects is observed on a single crystals of Cu$_{0.25}$Bi$_{2}$Se$_{3}$ using sensitive torque magnetometry. Our results show that the Cu doping in Bi$_{2}$Se$_{3}$ increases the carrier density and the effective mass, without increasing the scattering rate or decreasing the mean free path. In addition, the Fermi velocity remains the same in copper doped compound as that in Bi$_{2}$Se$_{3}$. These results imply that the insertion of Cu does not change the overall band structure and that conduction electrons in Cu doped Bi$_{2}$Se$_{3}$ sit in the linear Dirac-like band. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W13.00012: Gapped Dirac surface states in In doped topological insulator Bi$_2$Se$_3$ Weida Wu, Quantong Shen, Xueyun Wang, Sang-Wook Cheong Topological insulators host helical Dirac surface states which linearly disperse through bulk band gap. The unusual helical surface states are protected by time reversal symmetry, and therefore believed to be robust against disorders that do not break time reversal symmetry. It has been debated whether massive Dirac surface states (i.e. a gap at the Dirac point) are experimentally observed in doped topological insulators [1-3]. Herein, we report the observation of a spectroscopic gap of topological surface states in Bi$_{2-x}$In$_x$Se$_3$ using low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS). The tunneling spectroscopic maps suggest that the interactions between In dopants effectively change the topological class of local band structure, resulting in a nanoscale mixture of topologically trivial and nontrivial states. This electronic inhomogeneity poses a nanoscale spatial confinement to the Dirac surface states so that the long wavelength surface states near the Dirac point are suppressed, i.e. a gap is opened at the Dirac point. \\[4pt] [1] Sato et al, Nat. Phys., 7, 840, (2011).\\[0pt] [2] Xu, et al., arXiv:1206.0278v1 (2012).\\[0pt] [3] Souma et al, PRL, 109, 186804 (2012). [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W13.00013: Topological phase transition in the (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ system investigated via STM Wenwen Zhou, Yoshinori Okada, Zhensong Ren, Daniel Walkup, Stephen Wilson, Vidya Madhavan Transport and photoemission measurements on (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ have shown that the system transforms from a pure (x$=$0) topological insulator (TI) into a topologically trivial material (x \textgreater\ 0.07) through a topological phase transition. Indium (In) substitution for heavier Bismuth is expected to have a large effect on the electronic properties of TIs and is a very sensitive way to tune spin-orbit coupling while maintaining the same lattice structure. In this talk we present scanning tunneling microscopy measurements of the surface state and electronic structure of (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ single crystals over a wide range of In concentrations. We identify the local density signature of the In impurities and use these local measurements to determine the actual doping levels. Using spectroscopy and Fourier transform maps we then trace the evolution of the topological insulator into the trivial phase, thereby providing insights into the nanoscale evolution of this process. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W13.00014: Topological-Metal to Band-Insulator Transition in (Bi$_{1-x}$In$_x)_2$Se$_3$ Thin Films Matthew Brahlek, Namrata Bansal, Nikesh Koirala, Su-Yang Xu, Madhab Neupane, Chang Liu, M. Zahid Hasan, Seongshik Oh By combining transport and photoemission measurements on (Bi$_{1-x}$In$_x)_2$Se$_3$ thin films, we report that this system transforms from a topologically nontrivial metal into a topologically trivial band insulator through three quantum phase transitions. At $x\approx $3{\%}--7{\%}, there is a transition from a topologically nontrivial metal to a trivial metal. At $x\approx $15{\%}, the metal becomes a variable-range-hopping insulator. Finally, above $x\approx $25{\%}, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically tunable physics and devices with seamless gating or tunneling insulators. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W13.00015: Theoretical study of topological phase transitions in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ Jianpeng Liu, David Vanderbilt We use first-principles calculations to study the phase transition from a topological to a normal insulator with concentration $x$ in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ in the Bi$_2$Se$_3$ crystal structure. The spin-orbital coupling (SOC) strength is similar in In and Sb, which have similar atomic numbers, so that if the topological transitions in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ are purely driven by the decrease of SOC strength, we would expect to see similar critical concentrations $x_{\rm c}$ in the two systems. However, based on our preliminary calculations, $x_{\rm c}$ is much lower in (Bi$_{1-x}$In$_{x})_2$Se$_3$ than in (Bi$_{1-x}$Sb$_{x})_2$Se$_3$, indicating that different mechanisms control the behavior in the two cases. Specifically, in (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ we find that the phase transition is mostly dominated by the decrease of SOC. However, for (Bi$_{1-x}$In$_{x})_2$Se$_3$, the In $5s$ orbitals also play an important role, both in the phase-transition behavior and in determining the indirect bulk band gap. Finally, we discuss the accuracy of the energy-level position of the In $5s$ orbitals in (Bi$_{1-x}$In$_{x})_2$Se$_3$ as predicted by density-functional theory and more advanced methods. [Preview Abstract] |
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