Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D7: Focus Session: Synthesis & Characterization of Topological Insulators |
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Sponsoring Units: DMP DCMP Chair: Anthony Richardella, Pennylvania State University Room: 006B |
Monday, March 2, 2015 2:30PM - 2:42PM |
D7.00001: Single crystal growth and characterization of Na$_{3}$Bi and Bi$_{2}$Te$_{2}$Se topological materials Satya K. Kushwaha, Jason W. Krizan, R.J. Cava In recent years, the discoveries of topological insulators (TI) and three-dimensional (3D) Dirac semimetals (TDS) have been of significant interest in condensed matter science. To study these materials experimentally, it is of great importance to grow them in the form of high quality single crystals. Na$_{3}$Bi is recently discovered TDS and Bi$_{2}$Te$_{2}$Se$_{3}$ (BTS) is one of the interesting TI materials. Na$_{3}$Bi is extremely air sensitive and shows nontrivial crystallization behavior. BTS crystals usually grow with various point defects and typically exhibit metallic behavior. Here we will report the crystal growth of high quality Na$_{3}$Bi and insulating BTS single crystals. The characterization of their electronic properties by our collaborators in physics at Princeton and Brookhaven National Laboratory will be briefly described. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D7.00002: Intrinsic conduction through topological surface states of insulating Bi$_2$Te$_3$ epitaxial thin films Katharina Hoefer, Christoph Becker, Diana Rata, Jesse Swanson, Peter Thalmeier, Liu Hao Tjeng Topological insulators represent a novel state of matter with surface charge carriers having a massless Dirac dispersion and locked helical spin polarization. Many exciting experiments have been proposed by theory, yet, their execution have been hampered by the extrinsic conductivity associated with the unavoidable presence of defects in Bi$_2$Te$_3$ and Bi$_2$Se$_3$ bulk single crystals as well as impurities on their surfaces. Here we present the preparation of Bi$_2$Te$_3$ thin films that are insulating in the bulk and the four-point probe measurement of the conductivity of the Dirac states on surfaces that are intrinsically clean. The total amount of charge carriers in the experiment is of order 10$^{12}$ cm$^{-2}$ only and mobilities up to 4,600 cm$^2$/Vs have been observed. These values are achieved by carrying out the preparation, structural characterization, angle-resolved and x-ray photoemission analysis, and the temperature dependent four-point probe conductivity measurement all \textit{in-situ} under ultra-high-vacuum conditions. This experimental approach opens the way to prepare devices that can exploit the intrinsic topological properties of the Dirac surface states. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D7.00003: Stability investigation on the non-basal surfaces of topological insulator Bi$_{2}$Te$_{3}$: a first-principles study Na Wang, Yiyang Sun, Yuyang Zhang, Damien West, Wenhui Duan, Shengbai Zhang The basal (0001) surface of Bi$_{2}$Te$_{3}$ is the most stable surface and has been predominantly observed in experiment and studied in great details. The stability of other surfaces has been rarely discussed so far despite the fact that vicinal surfaces are always present at the crystal edges. In this work, we systematically study the thermodynamic stability of the vicinal surfaces of Bi$_{2}$Te$_{3}$ based on first-principles calculations. In particular, it is found that the (01$\bar{1}$5) surface has a surface energy of only about two times larger than that of the (0001) surface. Due to the particularly low surface energy, the sidewalls of free-standing Bi$_{2}$Te$_{3}$ crystals are predicted to be the (01$\bar{1}$5) surface, which is not perpendicular to the basal (0001) surface. The Wulff construction based on our calculated surface energies is consistent with experiment. However, in the presence of substrates, e.g., in molecular beam epitaxial growth, the shape of a crystal flake is further affected by the interface energy. In this case, the sidewalls could be (01$\bar{1}$0), (01$\bar{1}$ 10) or (01$\bar{1}$$\bar{4}$) depending on the binding strength between the flake and the substrate. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D7.00004: Measuring Surface and Bulk Conductance of a Topological Insulator Using Top and Bottom Contacts Yun Suk Eo, Steven Wolgast, Cagliyan Kurdak In most 3D topological insulators, the existence of bulk conduction makes electrical transport measurements of the surface conduction very challenging. Standard transport measurements that are performed only on one side of a bulk crystal are not capable of distinguishing surface conduction from bulk conduction. Recently, configurations that measure top and bottom surfaces together have enabled researchers to distinguish surface conduction from bulk conduction in the mixed-valence compound SmB$_{6}$.\footnote{Wolgast et al., Phys. Rev. B \textbf{88}, 180405(2013)}$^,$\footnote{D. Kim et al., Scientific Reports \textbf{3}, 3150 (2013)} Inspired by these measurements, we analyze different types of top/bottom transport geometries such as a Corbino disc patterned on the top surface/a metal disc patterned on the bottom surface, a four prong measurement on the top/entirely metalized on the bottom, etc. by numerical simulations. Although each configuration has certain limitations, the surface and bulk conductance can be found respectively from our results. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D7.00005: Anomalous magneto-transport properties of epitaxial Bi$_{2}$Te$_{3}$ films grown by chemical vapor deposition (CVD) Suyoun Lee, Hyunwoo Jin, Kwang-Chon Kim, Seong Keun Kim, Byung-ki Cheong, Jinsang Kim We investigated the magneto-transport properties of Bi$_{2}$Te$_{3}$ films grown by using the chemical vapor deposition (CVD) technique. From the structural analysis using the x-ray diffractometer (XRD) and transmission electron microscope (TEM), the films were found to be highly c-axis oriented with good crystalline quality. The films were found to remain metallic down to 1.8 K with a very high carrier mobility around 30,000 cm$^{2}$/Vs despite rather high carrier concentration of about 10$^{18\, }$ (cm$^{-3})$. We investigated the behavior of the magneto-resistance (MR) under various orientations of the magnetic field ($B)$. Interestingly, it was found that the MR showed a clear oscillation signal under the in-plane $B$. In addition, the observed oscillation was quite periodic in 1/$B$ implying that the Landau quantization plays a role. As the orientation of $B$ was rotated toward the direction perpendicular to the plane, the oscillation signal disappeared and the resistance showed a very sharp decrease at low magnetic fields, which was consistent with the weak antilocalization (WAL) behavior. Those new findings are believed to be related to the nature of Bi$_{2}$Te$_{3}$ films as a topological insulator (TI) and to unveil the unexplored aspects of TIs waiting for an explanation. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D7.00006: Study of long-time aging effect on low carrier MBE-grown Bi2Se3 with various capping layers Maryam Salehi, Matthew Brahlek, Nikesh Koirala, Seongshik Oh Although there have been a lot of advances in the growth engineering and characterization of the topological insulators (TI) since their discovery, one of the remaining challenges is to protect them against degrading due to aging over time. Stabilizing the properties of TIs under the ambient conditions is of great interest, and indeed is a crucial step towards building stable TI-integrated electronic devices. One of the immediate and effective solutions to the aging problem is to find an efficient, and compatible capping layer. In this work we focus on how properties of Bi$_{2}$Se$_{3}$ as a prototypical TI can be stabilized in air. We were able to achieve high quality Bi$_{2}$Se$_{3}$ thin films with low carriers concentration and high mobility using Molecular Beam Epitaxy (MBE). Aging study of samples with such low carriers will be more reliable compared to other MBE grown samples with higher carriers concentration. In this work, a long-term study (over 200 days) of aging effect on the transport properties of thin films with no capping layer, and ones with various in-situ (in the MBE chamber) and ex-situ deposited capping layers will be presented. Finally, by comparing the results between different capping layers, the most effective capping layer will be reported. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D7.00007: Investigation of the transport properties of Bi2Se3 films grown on various substrates H.Y. Lin, C.Y. Wang, K.H.M. Chen, Y.H. Lin, K.H. Chen, B.Y. Yang, M. Hong, J. Kwo Topological insulators, a new state of quantum matter, displayed a variety of physical phenomena. We have obtained high quality TI films of Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Sb$_{2}$Te$_{3}$ grown on various substrates with streaky RHEED patterns and large domains 1-2um in size. However, the Fermi level of Bi$_{2}$Se$_{3}$ tends to locate in the bulk conduction band due to the high density of intrinsic defects in TIs. To fine tune the Fermi level to be within the band gap, Bi$_{2}$Se$_{3}$ films were grown on amorphous oxide layers such as SiO$_{2}$, Y$_{2}$O$_{3}$, and Al$_{2}$O$_{3}$ $\sim$20 nm thick deposited on GaAs and Si substrates in a back gate structure for the electrical field effect. Compare to Bi$_{2}$Se$_{3}$ thin films on crystalline substrates such as sapphire, samples grown on amorphous oxides such as Al$_{2}$O$_{3}$ showed lower carrier concentration for the film thickness less than 10 QL, and the resistivity showed an insulating behavior at T below 50K. Other transport properties such as mobility, WAL effects are underway. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D7.00008: Demonstration of large field effect in topological insulator films via a high-$\kappa $ back gate C.Y. Wang, H.Y. Lin, Y.H. Lin, K.H. Chen, B.Y. Yang, K.H.M. Chen, Z.J. Peng, S.F. Lee, M. Hong, J. Kwo In topological insulators (TI) the spins are locked to opposite momentum direction when the Fermi level passes through Dirac point of its helical surface states, and the electrical field effect is a very promising way to modulate TI spins for spintronic devices. We have fabricated the back gate structure by growing TI films on high-$\kappa $ oxide layers including amorphous oxides of Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ and Y$_{\mathrm{2}}$O$_{\mathrm{3}}$ deposited on conducting substrates for applying the field effect with smaller operating voltage compared to SiO$_{\mathrm{2}}$, and keeping the top surface entirely open for subsequent fabrications of FM/TI, SC/TI structures intended for various studies. For Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ grown on these amorphous oxide thin films, streaky RHEED patterns indicated the film is highly crystalline. Weak antilocalization effect was observed to verify the time-reversal symmetry protected transport property. Very large field effect was demonstrated; for example, in the 6QL samples we are able to modulate as much as 2E13 cm$^{\mathrm{-2\thinspace }}$holes in applying negative gate bias. Field effect of Sb doped Bi$_{\mathrm{2}}$Te$_{\mathrm{3}}$ to realize sign reversal of carrier concentration will also be presented. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D7.00009: Shubnikov-de Haas Oscillations from topological surface states of metallic Bi$_{2}$Se$_{2.1}$Te$_{0.9}$ Keshav Shrestha, Vera Marinova, Bernd Lorenz, Paul C.W. Chu We have studied the quantum oscillations in the conductivity of metallic, p-type Bi$_{2}$Se$_{2.1}$Te$_{0.9}$. The dependence of the oscillations on the angle of the magnetic field with the surface as well as the Berry phase determined from the Landau level fan plot indicate that the observed oscillations arise from surface carriers with the characteristic Dirac dispersion. Several quantities characterizing the surface conduction are calculated employing on the Lifshitz-Kosevich theory. The low value of the Fermi energy with respect to the Dirac point is consistent with the metallic character of the bulk hole carriers. We conclude that, due to the peculiar shape of the valence band, the Shubnikov-de Haas oscillations of the bulk carriers are shifted to higher magnetic fields which allows for the detection of the quantum oscillations from the topological surface states at lower field. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D7.00010: Synthesis and low-temperature transport measurement of pure and In-doped SnTe nanoplate Jie Shen, YuJun Xie, Judy J. Cha SnTe is a topological crystalline insulator that possesses different surface states on the \textbraceleft 100\textbraceright , \textbraceleft 110\textbraceright , and \textbraceleft 111\textbraceright surfaces. Thus, to access surface states selectively, it is critical to control the morphology of SnTe. Moreover, indium doping in SnTe induces superconductivity, making it a candidate for a topological superconductor. Here, we grow pure and In-doped SnTe nanoplates, whose top and bottom surfaces are either (100) or (111), via vapor-liquid-solid and vapor-solid growth mechanisms. For pure SnTe nanoplate, we observe a structural phase transition from rock salt to rhombohedral structure in samples with low carrier density and electron-electron interactions in samples with high carrier density. In addition,by studying nanoplates with indium-doping concentrations ranging from 0{\%} to 10{\%}, we show that nanoplates become more diffusive in bulk and such that the surface states appear at higher concentrations of indium. This is supported by a three-dimensional weak antilocalization in low magnetic fields and a two-dimensional(2D) linear magnetoresistance(LMR) in high magnetic fields. This 2D LMR comes from the Dirac-dispersive surface state, in agreement with Abrikosov's quantum limit model. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D7.00011: Defect induced negative magnetoresistance and surface state immunity in topological insulator BiSbTeSe$_{2}$ Karan Banerjee, Jaesung Son, Praveen Deorani, Peng Ren, Lan Wang, Hyunsoo Yang The absence of backscattering due to time reversal symmetry is one of the hallmark features of a topological insulator. However, the introduction of defects can result in diminishing the transport properties of topological insulators. In this work, we introduce defects into the topological insulator BiSbTeSe$_{2}$ by subjecting it to ion milling and study the effect of disorder on the transport properties. We find that a negative contribution arises in the magnetoresistance of BiSbTeSe$_{2}$ at low temperatures. However, the surface state remains remarkably robust to the introduction of disorder. We demonstrate that the negative magnetoresistance originates from an increase in the density of defect states created by the introduction of disorder. We also find the bulk contribution to remain negligible even after subjecting to ion milling. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D7.00012: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 4:54PM - 5:06PM |
D7.00013: Surface conduction in encapsulated topological gated structures Yury Deshko, Inna Korzhovska, Lukas Zhao, Ghidewon Arefe, Marcin Konczykowski, Lia Krusin-Elbaum In three-dimensional (3D) topological insulators (TIs), the surface Dirac fermions intermix with the conducting bulk, thereby complicating access to the low-energy surface charge transport or magnetic response. The subsurface 2D states of bulk origin are vulnerable to bandbending due to surface adatoms, a band modification thought to be responsible for the `ageing' effect. To minimize this effect, we have developed an inert environment mechanical exfoliation technique to fabricate transistor-like gated structures in which prototypical binary TIs as well as ultra-low bulk carrier density ternaries (such as Bi$_2$Te$_2$Se) were encapsulated by thin h-BN layers, with electrical contacts made using exfoliated graphene. The effects of electrostatic tuning by the gate bias voltage on surface conductivity as a function of thickness of the TI layers and the variation with disorder will be presented. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D7.00014: Van der Waals epitaxy of Bismuth Telluro-Sulfide nanosheets and magnetotransport in devices Tanuj Trivedi, Sushant Sonde, Sanjay K. Banerjee Growing interest in probing topological surface states from transport experiments has led to the recent development of ternary and quaternary compounds of Bismuth chalcogenides. The search for complex 3D topological insulator compounds is motivated by the need for reduced bulk conduction and easier access to the Dirac point of the surface states, as compared to in the binary phases Bi$_2$X$_3$ (X=Te,Se). To this end, we have grown nanosheets of Bismuth Telluro-Sulfide directly on different substrates with van der Waals epitaxy. The growth method utilizes novel solid-state, non-elemental precursors of Bismuth, Tellurium and Sulfur. Nanosheet growth is observed on different substrates, such as amorphous SiO$_2$, hexagonal-BN and mica, and grow layer-by-layer in varying thicknesses (3 nm to $>$100 nm) and sizes (up to few microns). Stoichiometric analysis of the nanosheets is close to previously reported crystal growth of tetradymites, and the crystalline nature is confirmed with Raman and XRD measurements. We have fabricated devices on as-grown nanosheets of varying thicknesses, with nonmagnetic metal contacts. Preliminary magneto-transport experiments are promising, motivating further in-depth transport analysis for probing topological surface states. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D7.00015: Molecular Beam Epitaxy of Ultra-Thin Sb Films for Surface Transport Studies Kaushini Wickramasinghe, Chomani Gaspe, Shayne Cairns, Nolan Teasdale, Tetsuya Mishima, Joel Keay, Matthew Johnson, Sheena Murphy, Michael Santos Our growth study of ultra-thin Sb films is motivated by theoretical studies that predict a topoelectronic phase transition as a function of Sb film thickness due to quantum confinement and surface coupling effects. In thick films, transport measurements will be dominated by bulk conduction because the band structure of bulk elemental Sb is semi-metallic. Our goal is to enable transport measurements of topological surface states by suppressing the bulk conductivity through quantum confinement in thin Sb layers. Good control over the growth conditions allowed us to vary the thickness of the Sb films ranges from 0.7 nm to 6 nm with good repeatability. Electrical transport measurements indicate that surface states are responsible for about 25{\%} of the conductivity at 20K in a 3.7 nm thick Sb layer where the bulk conductivity is suppressed by about a factor of 2. We will discuss the structural properties of the Sb films using different electron microscopy techniques. [Preview Abstract] |
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