Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q42: Focus Session: Topological Materials, Synthesis and Characterization |
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Sponsoring Units: DMP Chair: Ale Lanzara, Lawrence Berkeley National Laboratory Room: Mile High Ballroom 4A |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q42.00001: High quality topological insulator Bi2Se3 thin film growth on amorphous SiO2 using MBE Nikesh Koirala, Namrata Bansal, Matthew Brahlek, Seongshik Oh We report on the growth of Bi2Se3 thin films on amorphous SiO2 by molecular beam epitaxy method. These films show high c-axis crystalline order and sharp interface with SiO2 substrate. ARPES measurement shows topologically protected surface states in these films. Comparison among Bi2Se3 thin films grown on Al2O3(0001), Si(111) and amorphous SiO2 shows that films grown on amorphous SiO2 are much better than those grown on Si(111) and roughly comparable to those grown on Al2O3 in terms of their electrical transport properties. This suggests that chemical inertness of substrate is more important than lattice matching in determining the transport properties of Bi2se3 films. Additionally, our study on a thinner film shows significant modulation of longitudinal resistivity and sign reversal of hall coefficient with application of gate voltage indicating tunablility of chemical potential through Dirac point. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q42.00002: High-quality Bi$_{2}$Se$_{3}$ MBE grown films from a cracked selenium source Brian Mulcahy, Gustaf Olson, Victor Chua, Edmond Chow, James N. Eckstein Over the past several years, the topological insulator Bi$_{2}$Se$_{3}$ has become a favorite among experimentalists due to its relatively large bulk band gap and simple electronic structure near the Fermi energy. However, the energy cost for an anti-site defect or selenium vacancy is not large compared to growth temperature. These defects dope the material and lead to bulk conductance which masks the role of the metallic surface states in transport. We report on Bi$_{2}$Se$_{3}$ thin films grown using a thermally cracked selenium source by MBE. Cracked selenium molecules are more reactive and this reduces the defect density and the bulk carrier density. Transport measurements show an asymptotic sheet resistance at T=0 more than an order of magnitude larger than what is obtained from films grown with a standard selenium source. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q42.00003: Magnetotransport measurements on Mn-doped Bi$_{2}$Se$_{3}$ Thin Films Sercan Babakiray, Trent Johnson, Pavel Borisov, David Lederman The intrinsic n type conductivity of bulk topological insulator Bi2Se3 was compensated with Mn dopant to increase the resistivity. In addition, the magnetic character of Mn ions causes a gap opening of the corresponding Dirac cone surface states. We investigated the effect of the Mn on crystal structure as well as the transport and magnetic properties of Bi2-xMnxSe3 thin films grown by molecular beam epitaxy on Al2O3 (0001) substrates. Characteristic features in the form of the Kondo effect and weak anti-localization were observed at different Mn concentrations up to temperatures of 50 K accompanied by enhanced resistance and reduced carrier mobility. The phase coherence length of the two-dimensional sheet conductance decreased with increasing Mn-concentration, however the protected surface states were still present up to x$=$0.063. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q42.00004: Transfer of large-area MBE grown BiSefilms to arbitrary substrates Namrata Bansal, Myung R. Cho, Matthew Brahlek, Nikesh Koirala, Yoichi Horibe, Jing Chen, Weida Wu, Yun D. Park, Seongshik Oh Mechanical exfoliation of bulk crystals has been widely used to exfoliate thin topological insulator (TI) flakes to fabricate devices such as field-effect transistors. However, such a process produces only micro-sized flakes that are highly irregular in shape and thickness. In this work, we developed a process to transfer the entire area of TI Bi2Se3 thin films grown epitaxially on Al2O3 and SiO2 to arbitrary substrates. Ultrathin films of 4 quintuple layers (1 QL $=$ 1 nm) with 1 cm x cm lateral size have been successfully transferred with no defects or cracks, as observed by optical microscopy. Transport measurements on the transferred films show that this process yields films with lower carrier concentrations and comparable or higher mobilities than before the transfer. Atomic force microscopy and transmission electron microscopy further confirm the pristine morphology and crystallinity of the transferred films. Furthermore, utilizing this process, we show that as the Fermi level is tuned into the proximity gap at the Dirac point of an ultrathin film, the film makes a clear metal-insulator transition with more than four orders of resistance change. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q42.00005: Growth and characterization of molecular beam epitaxial Bi2Se3 films and heterostructures Zhiyi Chen, Thor Garcia, Joel Jesus, Lukas Zhao, Haiming Deng, Jeff Secor, Milan Begliarbekov, Lia Krusin-Elbaum, Maria Tamargo Significant bulk conduction in the bulk of topological insulators (TIs) has been a major challenge in the studies of the their spin-helical Dirac surface conduction channels, a problem particularly severe in charge transport. Growth of high quality low-carrier concentration TI films is crucial not only for the fundamental study of TIs, but also for manufacture of heterostructures and devices. Here we report our results on synthesis and characterization of high-quality Bi$_2$Se$_3$ films grown using molecular beam epitaxy (MBE). A superior surface topography (smoothness) of the MBE Bi$_2$Se$_3$ films was obtained by a suitable choice of buffer layers used. A precise control of layer thickness was achieved and layers with good uniformity and surface quality were obtained. Hall measurements showed the films to be \textit{n}-type, with sheet carrier concentrations typically in the $6\sim9\times10^{12} \textrm{cm}^{-2}$ range. Using optimal growth conditions for the best quality Bi$_2$Se$_3$ films, magnetically doped Bi$_2$Se$_3$ and heterostructures such as Bi$_2$Se$_3$/ZnSe were also grown and characterized in transport and with optical measurements. Novel magnetically ordered insulating state induced by magnetic doping, and exotic effects at the interfaces will be presented. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q42.00006: Toward Insulating Behavior in Bi$_2$Se$_3$ Paul Syers, Johnpierre Paglione Research in the area of Topological Insulators has made great progress with Bismuth Selenide in recent years. However, achieving true insulating behavior in bulk samples of Bi$_2$Se$_3$ has proven elusive due to the difficulty in controlling the stoichiometry of this compound during synthesis. Here we report on progress with the synthesis and characterization of high purity, undoped Bi$_2$Se$_3$ crystals with the lowest carrier densities and highest resistivities reported to date. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q42.00007: High-pressure phases of Bi$_2$Se$_3$ C. Stephen Hellberg, Jason R. Jeffries, Igor I. Mazin, Steve M. Young, Nicholas P. Butch, Kevin Kirshenbaum, Paul S. Syers, Johnpierre Paglione We present x-ray diffraction measurements and density functional calculations of Bi$_2$Se$_3$ at pressures up to 80 GPa. Four phases are observed as the pressure is increased. The diffraction pattern of each phase agrees well with diffraction patterns computed from structures determined by density functional theory. The electronic structure of the high-pressure phases will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q42.00008: Epitaxial growth of Bi$_2$Te$_3$ on the Ferromagnetic Insulator Cr$_2$Ge$_2$Te$_6$ L.D. Alegria, H. Ji, N. Yao, R.J. Cava, J.R. Petta We report the experimental realization of a new topological insulator-ferromagnetic insulator (TI-FI) material system: the Cr$_2$Ge$_2$Te$_6$-Bi$_2$Te$_3$ heterostructure. The layered chalcogenide FI Cr$_2$Ge$_2$Te$_6$ exhibits a high Curie temperature of 61 K and a resistivity greater than $10^3$ $\Omega$-cm below 77 K, which suit it well for the study of magnetic proximity effects in TI-FI heterostructures. We fabricate heterostructures by growing single crystalline Cr$_2$Ge$_2$Te$_6$ substrates and depositing Bi$_2$Te$_3$ thin films using metalorganic chemical vapor deposition. Cross-sectional transmission electron microscopy reveals a sharp interface along the (0 0 1) planes of the two crystals, with the structures uniformly oriented as Cr$_2$Ge$_2$Te$_6$[1$\bar 1$0]//Bi$_2$Te$_3$[0$\bar 1$0]. The coupling between the Cr$_2$Ge$_2$Te$_6$ and Bi$_2$Te$_3$ layers is studied via the anomalous hall effect. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q42.00009: Bottom up synthesis of topological crystalline insulator SnTe nanostructures with controlled facets and morphologies Zhen Li, Shuai Shao, Nan Li, Kyle McCall, Jian Wang, Shixiong Zhang Tin Telluride (SnTe) has recently been demonstrated to be a topological crystalline insulator with unique metallic surface states protected by crystalline mirror symmetry. The topological surface properties have been predicted to depend on the surface orientation. By combining synthesis experiments and density functional theory (DFT) calculations, we demonstrate the growth of single crystalline nanostructures of SnTe with controlled facets and morphologies. In particular, by tailoring the growth temperature, we obtained two types of single crystalline nanowires: smooth nanowires dominated by \textbraceleft 100\textbraceright facets at high temperatures, and zigzag nanowires composed of both \textbraceleft 100\textbraceright and \textbraceleft 111\textbraceright surfaces at low temperatures. No \textbraceleft 110\textbraceright facet was observed in any of our nanostructures. The experiment results agree well with our DFT calculations of surface energies. Our device fabrication and preliminary electrical characterizations suggest that both types of nanowires are suitable for transport studies. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q42.00010: Searching for ideal Bi-system topological insulator, Pb-system topological crystalline insulator and their topological superconductor Invited Speaker: Genda Gu The discovery of 3D topological insulator materials and topological superconductor open up a new research field in the condensed matter physics. In order to search for the ideal topological insulator, topological crystalline insulator and topological superconductor, we have grown a large number of the single crystals of Bi-system (Bi-Sb-Te-Se) topological insulator Pb-system (Pb-Sn-In-Te-Se) topological crystalline insulator and their topological superconductor. We have measured the physical properties on these single crystals by various techniques. We have studied the effect of crystal growth condition, impurity and composition on the bulk electrical conductivity of these single crystals. We try to find out which composition and crystal growth condition is the best for the ideal topological insulator, topological crystalline insulator and topological superconductor. We have got the bulk topological superconductor with T$_{\mathrm{c}}=$5K. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q42.00011: Charging Dirac states at grain boundaries in the 3D topological insulator Bi$_{2}$Se$_{3}$ Y. Liu, Y.Y. Li, S. Rajput, D. Gilks, V.K. Lazarov, M. Weinert, L. Li Using scanning tunneling microscopy and transmission electron microscopy, we demonstrate the existence of antiphase boundaries between neighboring grains shifted by a fraction of a quintuple layer in the MBE-grown 3D topological insulator Bi$_{2}$Se$_{3}$ (0001) films [1,2]. Scanning tunneling spectroscopy and first-principles calculations further reveal that these boundaries provide electrostatic fields that locally charge the Dirac states, modulating the carrier density, and shifting the Dirac point by up to 120 meV. This intrinsic electric field effect, demonstrated here near interfaces between Bi$_{2}$Se$_{3}$ grains, provides direct experimental evidence at the atomic scale that the Dirac states are indeed robust against extended structural defects and tunable by electric field. \\[4pt] [1] Y. Liu et al. PRL \textbf{108}, 115501 (2012).\\[0pt] [2] Y. Liu et al. PRL \textbf{110}, 186804 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q42.00012: Quantum Transport of Surface State Dirac Fermions of a 3D Topological Insulator Yang Xu, Ireneusz Miotkowski, Jiuning Hu, Tai-lung Wu, Yong P. Chen A three-dimentional (3D) strong topological insulator (TI) has a fully insulating gap in the bulk and topological surface states of gapless Dirac fermions. However it is a great challenge to eliminate bulk conduction and reveal the transport signatures of the Dirac fermion from surface states in real 3D TI materials. By Bridgman method, we have successfully grown high-quality single crystal of 3D TI $BiSbTeSe_2$ with very low bulk carrier density (p type, less than $1.5\times$$10^{15}$ $cm^{-3}$) and high surface mobility (above 1000 $cm^{2}/Vs$ at low temperature). The insulating bulk and dominated surface conduction are confirmed by transport measurements of samples with various thicknesses (20 nm to 52 $\mu$m). In high magnetic fields (up to 31 T), we studied quantum oscillations and quantum Hall transport from topological surface states in exfoliated flake devices on $SiO_2/Si$ substrates, where the density of the bottom surface can be tuned by a back gate voltage. Our experiements reveal an intrinsic 3D TI material and paves the way for further application of topological quantum electronics. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q42.00013: Electronic structure study of Topological Insulator Bi$_{2}$Se$_{3}$ on graphene substrates Xueping Jiang, Bhanu Mahanti, Saroj Nayak In recent experiments, graphene has been found to be a suitable substrate for Topological Insulators (TIs) due to better lattice match and smaller defect density. Recent studies have also shown that the presence of a substrate can modulate the electronic properties of TIs. One system that has not been well studied is Bi2Se3 supported by graphene. In this work, density functional theory has been used to study the electronic structure of such a heterostructure. Six quintuple layers (QLs) and one QL were considered for Bi2Se3. When six QLs of Bi2Se3 are supported by graphene on either one or both sides, the Dirac cone of the TI is shifted below the Fermi energy and a band gap is opened in graphene. In addition, the influence of the graphene substrate on the topological surfaces states is negligible. The four-fold degeneracy around the Dirac point for Bi2Se3 is maintained when the TI is supported on both sides by graphene, but is split when graphene is deposited on only one side. For one QL Bi2Se3 the electronic structure near the band gap was strongly perturbed due to the interaction with graphene orbitals. These results will be compared with other works of TIs on substrates. [Preview Abstract] |
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