Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F28: Topological Insulator Thin FilmsFocus
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Sponsoring Units: DMP Chair: Matthew Brahlek, Pennsylvania State University Room: 327 |
Tuesday, March 15, 2016 11:15AM - 11:51AM |
F28.00001: Interfacing Topological Insulators with Ferromagnetism Invited Speaker: Anthony Richardella In topological insulators, the surface states arise from strong spin-orbit coupling while the degeneracy of the Dirac point is protected by time reversal symmetry. Introducing magnetism in proximity to the surface states breaks this symmetry, destroying the non-trivial Berry phase at the Dirac point and leads to a hedgehog spin texture near the newly opened magnetic gap.\footnote{S.-Y. Xu, M. Neupane, C. Liu, D. Zhang, A. Richardella, L. A. Wray,\ldots M. Zahid Hasan. Nature Physics, 8, 616 (2012)} This symmetry broken phase leads to a host of unusual physics, such as the quantum anomalous Hall (QAH) effect. In this talk, we discuss the growth by molecular beam epitaxy and characterization of such magnetically interfaced and magnetically doped topological insulators. Such materials often suffer from structural defects and interfacial layers, as well as from degradation during device fabrication.\footnote{A. Richardella, A. Kandala, J. S. Lee, N. Samarth. APL Materials, 3(8), 083303 (2015)} In particular, it is shown that Cr doped (Bi$_{1-x}$,Sb_x)$_2$Te$_3$ can exhibit perfect Hall quantization at low temperatures despite these defects. However, the magnetic ordering of this material was found to be quite unusual, displaying a super-paramagnetic like character, perhaps reflecting this disorder.\footnote{E. O. Lachman, A. F. Young, A. Richardella, J. Cuppens, H. R. Naren, Y. Anahory,\ldots E. Zeldov. Science Advances 1, e1500740 (2015)} Such observations highlight the surprising behavior of such broken symmetry phases in topological materials. This work was performed in collaboration with A. Kandala, M. Liu, W. Wang, N.P. Ong, C.-X. Liu, and N. Samarth, in addition to the authors of the references cited. [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:03PM |
F28.00002: Structure and transport of topological insulators on epitaxial graphene James Kally, Danielle Reifsnyder Hickey, Yu-Chuan Lin, Anthony Richardella, Joon Sue Lee, Joshua Robinson, K. Andre Mkhoyan, Nitin Samarth Recent advancements in spintronics have shown that a class of materials, topological insulators (TI), can be used as a spin-current generator or detector. Topological insulators have protected surface states with the electron’s spin locked to its momentum. To access these surface states, (Bi, Sb)$_{2}$Te$_{3}$ can be grown by molecular beam epitaxy to have the Fermi energy near the Dirac point so that transport occurs only through the spin-dependent surface states. Graphene is another 2D material of great interest for spintronics because of its very long spin diffusion length. This is an ideal material to act as a spin channel in devices. The van der Waals nature of the growth exhibited by 2D materials such as (Bi, Sb)$_{2}$Te$_{3}$ and graphene allows heterostructures to be formed despite the large lattice mismatch. We explore the structure and transport of (Bi, Sb)$_{2}$Te$_{3}$ grown on epitaxial graphene on 6H-SiC substrates for spintronic applications. [Preview Abstract] |
Tuesday, March 15, 2016 12:03PM - 12:15PM |
F28.00003: Band Bending Inversion in Bi2Se3 Nanostructures Louis Veyrat, Fabrice Iacovella, Joseph Dufouleur, Christian Nowka, Hannes Funke, Ming Yang, Walter Escoffier, Michel Goiran, Barbara Eichler, Oliver G. Schmidt, Bernd Büchner, Silke Hampel, Romain Giraud Shubnikov-de-Haas oscillations were studied under high magnetic field in Bi$_2$Se$_3$ nanostructures grown by Chemical Vapor Transport, for different bulk carrier densities ranging from $3\times10^{19}\text{cm}^{-3}$ to $6\times10^{17}\text{cm}^{-3}$. The contribution of topological surface states to electrical transport can be identified and separated from bulk carriers and massive two-dimensional electron gas. Band bending is investigated, and a crossover from upward to downward band bending is found at low bulk density, as a result of a competition between bulk and interface doping. These results highlight the need to control electrical doping both in the bulk and at interfaces in order to study only topological surface states[1]. [1] : Veyrat et al., Nano Lett., Article ASAP, DOI: 10.1021/acs.nanolett.5b03124 [Preview Abstract] |
Tuesday, March 15, 2016 12:15PM - 12:27PM |
F28.00004: High-mobility surface states and conductance fluctuations in Bismuth Telluro-Sulfide topological insulator devices Tanuj Trivedi, Sushant Sonde, Hema C. P. Movva, Sanjay K. Banerjee Since the experimental discovery of three-dimensional topological insulators (TI), (Bi,Sb)$_2$(Se,Te)$_3$ binary compounds are the principal material systems to explore TI physics. However, transport experiments studying surface-states are complicated by parallel bulk conductivity contribution, which is expected to improve for ternary and quaternary chalcogenide compounds of Bi and Sb. A promising alternative is the Sulfur-based tetradymite with an ideal formula of Bi$_2$Te$_2$S, which has received little attention. We present van der Waals epitaxial growth and magnetotransport in Bismuth Telluro-Sulfide (BTS) crystalline nanosheets. Gating-enhanced Weak-antilocalization (WAL) and Universal Conductance Fluctuations (UCF) are observed in BTS devices. Empirical modeling of the data shows the existence of two-dimensional surface transport. A three-channel Hall conductivity model is proposed, which is utilized in conjunction with an extended-WAL analysis, showing the presence of a high-mobility surface component and indication for separation of transport channels in BTS devices. Our growth and comprehensive transport experiments demonstrate BTS as a promising candidate TI material. [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 12:39PM |
F28.00005: In Situ Thin Film Growth and Characterization of Topological Dirac Semimetal Na$_3$Bi Jack Hellerstedt, Mark Edmonds, Chang Liu, Navneeth Ramakrishnan, Shaffique Adam, Michael Fuhrer The alkali pnictide Na$_3$Bi is a three-dimensional Dirac semimetal possessing Dirac-like dispersions in k$_x$, k$_y$ and k$_z$, that has attracted recent interest as a condensed matter system for realizing the chiral anomaly [1]. The high reactivity of sodium makes conventional synthesis and characterization extremely difficult: we circumvent this issue by combining thin film growth with low temperature STM and magnetotransport in one comprehensive UHV system. We have successfully grown Na$_3$Bi on $\alpha$-Al$_2$O$_3$ (0001) substrates, achieving low temperature mobilites in excess of 3,500 cm$^2$/Vs and carrier densities as low as 5 $\times$ 10 $^{12}$ cm$^{-2}$. Perpendicular magnetoresistance up to 1T shows quadratic behavior with weak anti-localization at low field. Quantitative analysis of this data suggests that our samples are in a charge inhomogeneous regime reminiscent of charge puddling in graphene [2]. [1] Xiong et. al Science (2015) doi:10.1126/science.aac6089 [2] Ramakrishnan et. al arXiv:1501.03815 (2015) [Preview Abstract] |
Tuesday, March 15, 2016 12:39PM - 12:51PM |
F28.00006: Epitaxial Cd$_{3}$As$_{2}$ Thin Films Synthesized by Molecular Beam Epitaxy Timo Schumann, Manik Goyal, Susanne Stemmer Cd$_{3}$As$_{2}$ is a three-dimensional (3D) Dirac semimetal, i.e. it possesses Dirac cones in a 3D bulk state where the band dispersion relation is linear near the Fermi energy. Cd$_{3}$As$_{2}$ is has raised considerable interest due to its high electron mobilities in bulk crystals and for novel quantum phenomena, such as chiral anomalies. However, few studies have been performed using thin films of Cd$_{3}$As$_{2}$. In this presentation, we report on the synthesis of Cd$_{3}$As$_{2}$ thin films by molecular beam epitaxy (MBE). Single phase, epitaxial films were grown on undoped GaSb(111)B substrates with the (112) facet of Cd$_{3}$As$_{2}$ parallel to the GaSb(111) surface. We report on the structural quality and orientation variants in the films. Electrical transport properties indicate electron mobilities exceeding 6000 cm$^{2}$V$^{-1}$s$^{-1}$. We discuss the impact of the MBE growth parameters and substrate preparation on the structural and electrical properties of the films. [Preview Abstract] |
Tuesday, March 15, 2016 12:51PM - 1:03PM |
F28.00007: Asymmetric topological interfaces and charge transfer in epitaxial Bi2Se3/II-VI superlattices Zhiyi Chen, Lukasf Zhao, Inna Korzhovska, Thor Garcia, Maria Tamargo, Lia Krusin-Elbaum, Kyungwha Park Access to charge transport through Dirac surface states in topological insulators (TIs) can be challenging due to their intermixing with the bulk or with non-topological subsurface two-dimensional electron gas (2DEG) quantum well states. Formed by bending of bulk electronic bands near the surface, 2DEG states arise via charge transfer to the topological surfaces, so the choice of layers abutting these surfaces is critical. Here we report molecular beam epitaxial growth of Bi$_2$Se$_3$/Zn$_{x}$Cd$_{1-x}$Se superlattices that support only one topological surface channel per TI layer. The topological nature of conducting channels is evidenced by $\pi$-Berry phase and by the two-dimensional weak antilocalization. Both density functional theory calculations and transport measurements suggest that a single topological Dirac cone per TI layer arises from the asymmetry between the Se-terminated and Zn-terminated interfaces of Zn$_{x}$Cd$_{1-x}$Se with Bi$_2$Se$_3$. Our findings suggest that topological transport could be controlled by adjusting charge transfer from non-topological spacers in hybrid structures. \\$^\star$Supported by NSF-DMR-1420634, NSF-DMR-1312483, DOD-W911NF-13-1-0159, NSF DMR-1206354 and computer resources from SDSC under DMR060009N and VT ARC [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F28.00008: Study of the circular photo-galvanic effect in electrically gated (Bi,Sb)$_2$Te$_3$ thin films Yu Pan, Timothy Pillsbury, Anthony Richardella, Thomas Flanagan, Nitin Samarth Illumination with circularly polarized light is known to produce a helicity dependent photocurrent in topological insulators such as Bi$_2$Se$_3$ [Nature Nanotech. 7, 96 (2012)]. Symmetry considerations suggest that this “circular photo-galvanic effect” (CPGE) arises purely from the surface. However, whether or not the CPGE is directly related to optical excitations from the helical surface states is still under debate. To clarify the origin of the CPGE, we first compare the helicity dependent photocurrent in intrinsic (Bi,Sb)$_2$Te$_3$ to Cr doped (Bi,Sb)$_2$Te$_3$ thin films in which the Dirac surface states are perturbed by magnetic coupling. Secondly, we discuss the tunable CPGE in electrically gated (Bi,Sb)$_2$Te$_3$ thin films excited by optical excitations at different wavelengths. The dependence on the chemical potential and the photon energy of the excitation unveils the origin of the CPGE. Funded by ONR. [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F28.00009: Low Temperature Quantum Transport Properties of Bi$_2$Se$_3$ Topological Insulator Thin Films David Lederman, Sercan Babakiray, Pavel Borisov, Amit KC, Yuri Glinka Bi$_2$Se$_3$ thin films with nominal thickness values of 12, 16, 20 and 25 quintuple layers (QLs) were grown by molecular beam epitaxy (MBE) on Al$_2$O$_3$ substrates. The magnetoconductance (MC) was analyzed using the two-dimensional Altshuler-Aronov (AA) and Hikami-Larkin-Nagaoka (HLN) mechanisms. Using a simple model where the channels for the bulk and surface states are independent from each other, and assuming that all channels undergo WAL, it was possible to determine the phase coherence length ($L_\phi$) of the carriers of the surface and bulk contributions independently from the MC with the field perpendicular to the surface. The value of $L_\phi$ for the surface states was independent of thickness, as expected, while $L_\phi$ for the bulk channel was strongly dependent on film thickness. WAL was also measured with the field applied parallel to the surface, and from the MC data in this configuration, it was possible to obtain values for $L_\phi$ that were similar to the perpendicular configuration for all samples except for the thinnest sample (12 QL), which may be a result of interactions between the metallic surfaces states on opposite sides of the film. We will discuss these results in view of other results from the literature. [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F28.00010: High quality topological insulator thin films grown by molecular beam epitaxy using MoS2 monolayer as buffer layer K. H. Chen, H. Y. Lin, C. Y. Wang, S. R. Yang, J. Kwo, C. K. Cheng, M. Hong, X. Q. Zhang, Y. H. Lee Topological insulators (TIs), a new state of quantum matter, display a rich variety of physical phenomena. High quality TI films of Bi2Se3 and Bi2Te3 have been obtained by van der Waals epitaxy on various substrates. To further reduce the high defect density level common in these films, we have extended the investigation to utilize 2D layered materials of hexagonal symmetry as substrates, such as large area CVD-grown MoS2 monolayer. Streaky RHEED patterns were observed during growth. Normal scans of x-ray diffraction indicated that the c-axis of films grown on both Al2O3(0001) and MoS2/Al2O3(0001) were fully strain relaxed with a FWHM varying from 0.01° to 0.03°, suggesting a very high degree of crystallinity. Using AFM, we found that size of triangular shaped domains were substantially bigger (\textasciitilde 1.5um) than those without MoS2 (\textasciitilde 0.6um). Furthermore, reduction by \textasciitilde 16{\%} in carrier concentration and a mobility as high as 5700cm2/Vs were observed in our 50nm film with MoS2. Other thickness dependent transport properties such as WAL are underway, along with ARPES study of the electronic structures. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F28.00011: Visualizing virgin magnetic domains of V-doped Sb$_2$Te$_3$ thin films Wenbo Wang, Cui-Zu Chang, Jagadeesh S. Moodera, Weida Wu Quantum anomalous Hall effect (QAHE) was experimentally realized in a ferromagnetic topological insulator Cr-doped (Bi,Sb)$_2$Te$_3$ thin film for the first time[1]. Recently, a more robust QAHE has been observed in V-doped (Bi,Sb)$_2$Te$_3$ thin film, which has a much larger coercive field and higher Curie temperature at the same doping level[2]. However, a mysterious self-magnetization phenomenon was observed in the V-doped Sb$_2$Te$_3$, where net magnetization spontaneously develops after zero field cooling. In this talk, we utilize cryogenic magnetic force microscopy (MFM) technique to study the domain states of V doped Sb$_2$Te$_3$ film under various cooling fields. A zero net magnetization state with equally distributed up and down domains was observed after zero-field cooling. In addition, a small external magnetic field ($\sim$5 Oe) is able to significantly polarize the magnetization of the film. Our MFM results are qualitatively consistent with in-situ magnetoresistance measurements. [1] C.-Z. Chang et al., Science 340, 167 (2013). [2] C.-Z. Chang et al., Nature Materials 14, 473–477(2015). [Preview Abstract] |
Tuesday, March 15, 2016 1:51PM - 2:03PM |
F28.00012: Controlling the 2DEG states evolution at a metal/Bi$_2$X$_3$ (X=Se,Te) interface Han-Jin Noh, Jinwon Jeong, En-Jin Cho, Joonbum Park, Jun Sung Kim, Ilyou Kim, Byeong-Gyu Park, Hyeong-Do Kim We have demonstrated that the evolution of the two-dimensional electron gas (2DEG) system at an interface of metal and the model topological insulator (TI) Bi$_2$X$_3$ (X=Se, Te) can be controlled by choosing an appropriate kind of metal elements and by applying a low temperature evaporation procedure. In particular, we find that only topological surface states (TSSs) can exist at a Mn/Bi$_2$X$_3$ interface, which would be useful for implementing a TI-based device with surface current channels only. The existence of the TSSs alone at the interface was confirmed by angle-resolved photoemission spectroscopy (ARPES). Based on the ARPES and core-level x-ray photoemission spectroscopy measurements, we propose a cation intercalation model to explain our findings. [Preview Abstract] |
Tuesday, March 15, 2016 2:03PM - 2:15PM |
F28.00013: Characterization of high-quality Bi$_{2}$Se$_{3}$ films grown using a selenium cracker source Theresa Ginley, Stephanie Law Topological insulators, including Bi$_{2}$Se$_{3}$, are becoming increasingly prevalent in research due to their unique electronic properties—these materials exhibit an insulating bulk but conducting surfaces with electron spin-momentum locking. Using Molecular Beam Epitaxy (MBE) it is possible to grow high-quality thin films of Bi$_{2}$Se$_{3}$. Yet these films have not lived up to their potential, in part due to significant bulk conductivity arising from material defects like selenium vacancies. Current MBE growth methods for Bi$_{2}$Se$_{3}$ use standard selenium sources that evaporate large selenium molecules which must then be cracked into smaller molecules to be incorporated into the film. This process is inefficient and requires very high fluxes of selenium for good quality growths. However, using a selenium cracking source results in the evaporation of monomers and dimers, facilitating incorporation into the film. We will present electrical, structural, and optical measurements demonstrating that the use of a cracker source allows films to be grown using much lower selenium:bismuth flux ratios with good mobility and low carrier density. [Preview Abstract] |
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