Symposium Organizers
Yi Cui Stanford University
Hailin Peng Peking University
Claudia Felser Johannes Gutenberg University Mainz
Shuichi Murakami Tokyo Institute of Technology
Monday PM, November 28, 2011
Room 301 (Hynes)
9:30 AM - *L1.1
Topological Materials.
Shoucheng Zhang 1
1 Department of Physics, Applied Physics, Stanford University, Stanford, California, United States
Show AbstractRecently, a new class of topological materials has been theoretically predicted and experimentally realized. The topological insulators have an insulating gap in the bulk, but have topologically protected edge or surface states due to the time reversal symmetry. In two dimensions the edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. I shall review the theoretical prediction[1] of the QSH state in HgTe/CdTe semiconductor quantum wells, and its recent experimental observation[2]. The edge states of the QSH state supports fractionally charged excitations[3]. The QSH effect can be generalized to three dimensions as the topological magneto-electric effect (TME) of the topological insulators[4]. Topological insulators Bi2Te3, Bi2Se3 have been discovered theoretically and experimentally to have surface states consisting of a single Dirac cone[5,6,7].In this tutorial, I shall introduce the models and materials of topological insulators, and discuss their fascinating physical properties. Students can prepare before the tutorial lecture by reading the review papers on the subject[8,9,10].[1] A. Bernevig, T. Hughes and S. C. Zhang, Science, 314, 1757,(2006)[2] M. Koenig et al, Science 318, 766, (2007)[3] J. Maciejko, Chaoxing Liu, Yuval Oreg, Xiao-Liang Qi, CongjunWu, and Shou-Cheng Zhang, , Phys. Rev. Lett. {\bf 102}, 256803(2009).[4] Xiao-Liang Qi, Taylor Hughes and Shou-Cheng Zhang, Phys. Rev B.78, 195424 (2008)[5] Haijun Zhang, Chao-Xing Liu, Xiao-Liang Qi, Xi Dai, Zhong Fang,and Shou-Cheng Zhang, Nature Physics 5, 438 (2009).[6] Y. Xia, L. Wray, D. Qian, D. Hsieh, A. Pal, H. Lin, A. Bansil,D. Grauer, Y. Hor, R. Cava, et al., Nat. Phys. 5, 398 (2009).[7] Y. L. Chen, J. G. Analytis, J.-H. Chu, Z. K. Liu, S.-K. Mo, X.L. Qi, H. J. Zhang, D. H. Lu, X. Dai, Z. Fang, et al., Science 325,178 (2009).[8] Xiao-Liang Qi and Shou-Cheng Zhang, ``The quantum spin Hall effect and topological insulators",Physics Today, 63, 33, (2010).[9] Xiao-Liang Qi and Shou-Cheng Zhang, ``Topological insulators and superconductors", arXiv:1008.2026, to appear in Review of Modern Physics.[10] Z. Hasan and C. Kane, ``Colloquium: Topological insulators",Rev. Mod. Phys. 82, 3045 (2010).
10:15 AM - L1.2
Penetration Depth of Edge/Surface States of Topological Insulators.
Shuichi Murakami 1 , Masaki Wada 1
1 Department of Physics, Tokyo Institute of Technology, Tokyo, 0, Japan
Show AbstractIn topological insulators, topologically protected edge/surface states exists irrespective of the details of the edge/surface of the system. The penetration depth describes how deep these edge/surface states penetrate into the bulk, and depends on materials. In my talk, we study how the penetration depth is determined by materials. We find that the inverse of the penetration depth scales with the momentum-space width of the edge-state dispersion, and we discuss several examples including the HgTe quantum well. As another example of well-localized edge states, we take the Bi(111) ultrathin film. We find that its edge states extend almost over the whole Brillouin zone. Correspondingly, it is proposed to have well-localized edge states in contrast to the HgTe quantum well. We also discuss how the penetration depth behaves as we increase the external magnetic field. Under the magnetic field, edge/surface states becomes gapped, thereby the edge/surface states come close to the bulk bands in energy. Thus the penetration depth is expected to become longer. Detailed calculation using the model of Bi2Se3 shows that it is indeed true. By considering the effect of the Zeeman field and the orbital magnetic field, we show how the penetration depth becomes longer, and how the behavior of edge/surface wavefunctions change. Reference: M. Wada, S. Murakami, F. Freimuth, and G. Bihlmayer, Phys. Rev. B 83, 121310(R) (2011)
10:30 AM - L1.3
Theoretical Simulation of Topological Surface State in Topologically Nontrivial-Trivial Heterostructure.
Qianfan Zhang 1 , Zhiyong Zhang 1 , Yi Cui 1
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractThe most exciting property for three-dimensional topological insulator (TI) is the gapless topological state on surface. Due to exciting topological properties, like linear-dispersion and spin-momentum locking, this state has been viewed with huge potentials of transformational applications in a number of technical fields including spintronics, quantum computing, superconductivity. Recently, enlightened by this topological surface state, many studies have focused on proximity effect due to interface correspondence between topologically-nontrivial and topologically-trivial materials. For instance, one-dimensional fermionic excitations, which are topologically protected and not scattered by disorder, was suggested when introducing dislocation line in topological insulator; image magnetic monopole effect is proposed on the interface between TI and ferromagnetic material; Majorana Fermions is predicted on the interface between TI and s-wave superconductor. A large number of works, based on consideration of model Hamiltonians with variable parameters, have predicted proximity effect induced exotic topological properties. However, as far as we know, no results have been presented so far to unequivocally illustrate the effects of general interface interactions. Furthermore, the real space characteristics of topological surface state are critical to the interface coupling, and conversely, the proximity effect can also make influence on the spatial properties of topological state. On the computational simulation aspect, although some works studied spatial distribution of topological surface state on TI’s surface, such analysis has rarely been performed in interface system.Using accurate ab-initio density functional theory approach, we theoretically studied real space properties of topological state in trivial material coated Bi2Te3 family compound and simulated different nontrival-trivial heterojunction interface. We found that for different kinds of coating material, the real space properties of topological state are quite different. Therefore, despite the gapless topological state can be protected by time-reversal symmetry in momentum space, the feature in real space can not be protected. Our study changes the traditional viewpoint on real space properties of topological surface state, and at the same time, shows a real example on TI proximity effect.
10:45 AM - L1.4
Vertical Transport in Topological Insulator Thin Films.
Byounghak Lee 1 2 , Rafi Bistrizer 2 , Allan MacDonald 2
1 Physics, Texas State University, San Marcos, Texas, United States, 2 Physics, The University of Texas at Austin, Austin, Texas, United States
Show AbstractWe present a theory of inter-surface transport in topological insulator thin film. We demonstrate that current leakage is strongly suppressed by spin-orbit coupling in thin film dielectrics based on the V2VI3 family of topological insulators. Our conclusions are based on ab initio electronic structure calculations that allow us to quantitatively estimate the dependence of currents through the film bulk on its thickness and on bias voltage. A key observation in our analysis is that top to bottom surface transport in V2VI3 topological insulators occurs only between conduction and valence band surface states. Intraband tunneling is completely suppressed by spin-orbit interactions. Resonant tunneling between surfaces is present only when external fields that break inversion symmetry are applied. More generally tunneling is strongest when the Fermi level lies in the conduction band on one surface and in the valence band on the other surface. We discuss manipulation of vertical transport by dual gates and by in-plane magnetic fields and compare with other 2-dimension to 2-dimension tunneling systems.
11:30 AM - **L1.5
Topological Insulator Surface States for Magnetoelectric, Spintronic, and Thermoelectric Effects.
Joel Moore 1 2
1 Department of Physics, UC Berkeley, Berkeley, California, United States, 2 Materials Sciences Division, LBNL, Berkeley, California, United States
Show AbstractMuch of condensed matter physics is concerned with understanding how different kinds of order emerge from interactions between a large number of simple constituents. In ordered phases such as crystals, magnets, and superfluids, the order is understood through "symmetry breaking": in a crystal, for example, the continuous symmetries of space under rotations and translations are not reflected in the ground state. A major discovery of the 1980s was that electrons confined to two dimensions and in a strong magnetic field exhibit a completely different, "topological" type of order that underlies the quantum Hall effect.In the past few years, we have learned that topological order also occurs in some three-dimensional materials, dubbed "topological insulators", in zero magnetic field. Spin-orbit coupling, an intrinsic property of all solids, drives the formation of the topological state. Topological insulators have metallic surface states with several unusual properties that are connected to basic problems in magnetoelectricity, spintronics, and thermoelectricity. We present theories of these types of effects in topological insulators including the actively studied materials Bi2Se3 and Bi2Te3. Applications include: the creation of new types of spin torque devices; fast magnetoelectric materials driven by purely orbital motion of electrons; and improved thermoelectric figure of merit at low temperatures.
12:00 PM - L1.6
False-Positive and False-Negative Assignments of Topological Insulators in Density-Functional Theory and Hybrids.
Julien Vidal 1 , Xiuwen Zhang 2 , Jun-Wei Luo 1 , Alex Zunger 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 Department of Physics , Colorado School of Mines, Golden, Colorado, United States
Show AbstractThe massless dispersed surface bands crossing inside the forbidden gap of bulk bands form helical Dirac fermions in topological insulators. This novel property originates from a band inversion between, for example, s-like conduction band and p-like valence band in the bulk band structure. Looking for such band inversion in bulk band structures of materials is the main method to find new topological insulators. Therefore, this search approach is very sensitive to the level of theory used to predict the band structure. Density-functional Theory (DFT) approaches have recently been used to judge the topological order of various materials despite its well-known band gap underestimation. Use of the more accurate quasi-particle GW approach reveals cases where DFT identifications are false-positive, possibly misguiding experimental searches of materials that are topological insulators in DFT but not expected to be topological insulators in reality. We also present cases of false-positive due to the incorrect choice of crystal structures and address the relevancy of such choice of crystal structure with respect to the ground state one and thermodynamical instability with respect to binary competing phases. We conclude that current ab initio calculations must consider both the correct ground state crystal structure and the correct Hamiltonian in order to remain predictive in the context of topological insulators. Research supported by the U.S. Department of Energy,Office of Basic Sciences, Division of Materials Sciences and Engineering, under Grant No. DE-AC36-08GO28308 to NREL.
12:15 PM - L1.7
Topological Insulator Application: All-Electrical Switching of Conductance.
Michael Povolotskyi 1 , Tillmann Kubis 1 , Gerhard Klimeck 1
1 , Purdue University, West Lafayette, Indiana, United States
Show AbstractBismuth and Tellurium compound materials such as HgTe,Bi2Se3 and Bi2Te3 are known to exhibit electronic band structures with unusually pronounced spin-orbit interactions. Recent theoretical and experimental work on nanodevices based on these materials has unveiled specific properties that make them most interesting for charge and spin applications. In particular, devices without an inversion center have been shown to behave like topological insulators: electrons in the volume of the devices tend to form an insulating band structure, while surface states of the same devices lie within the band gap and support a high conductivity. The high surface conductivity originates from the pronounced spin polarization of the surface states: the orientation of the spin polarization direction is perpendicular to the in-plane momentum, that efficiently suppresses elastic backscattering.In this work, these effects are studied in terms of self-consistent Schrödinger-Poisson calculations based on a sp3d5s* atomistic tight binding representation with spin-orbit interaction that is proven to be suitablefor nanoscale device modeling. Thereby, several of the predictions of the approximate models (such as k.p approximation) are assessed and specified. These predictions include the penetration depth and the spin orientation of the surface states, the width dependence of the coupling between the surface states and the probability for elastic backscattering of surface electrons. As a result, a novel device concept is proposed that exploits the coupling between the spin polarized states. The device in based on a 111-grown Bi2Te3 quantum well. The well is considered to be periodic in the transverse directions. The quantum well is assumed to consist of integer number of quintuple layers while all well surfaces are terminated with Te atoms. Consequently, these Bi2Te3 quantum wells are symmetric with respect to inversion if no external perturbation applies. The electric field applied parallel to the growth direction destroys the symmetry and causes spin polarization of the conducting states. We demonstrate that this device allows an all electric switching between ballistic surface conductance due to topologically protected spin states and resistive charge transport in the case of inversion symmetry. The feasibility of this concept is analyzed with respect to quantum well thickness and strength of the required switching field. We estimate that an external electric field of about 0.3MV/cm applied to the 6nm quantum well can suppress the backscattering rate of electron lying at the Fermi surface by four orders of magnitude.
Monday PM, November 28, 2011
Room 301 (Hynes)
2:30 PM - **L2.1
STM Studies of Topological Insulators Grown by MBE.
Xi Chen 1 , Ke He 2 , Xucun Ma 2 , Xue Qi-Kun 1 2
1 Physics, Tsinghua University, Beijing China, 2 Institute of Physics, Chinese Academy of Sciences, Beijng China
Show AbstractI will summarize our recent activities of using scanning tunneling microscope (STM) to study topological insulators grown by molecular beam epitaxy (MBE). The Landau quantization in three dimensional topological insulators was directly observed in the tunneling spectra. In particular, we discovered the zeroth Landau level, which is predicted to give rise to the half-quantized Hall effect for the topological surface states. The existence of the discrete Landau levels and the suppression of Landau levels by surface impurities strongly support the 2D nature of the topological states. In addition, we studied the quantum interference pattern formed by the topological surface states near the step edges and impurities in Bi2Se3 and Bi2Te3. The decay behavior of the standing waves is in good agreement with the Dirac cone structure of the topological surface states. The forbidden of backscattering is explicitly demonstrated for the topological surface states. We show that the combination of MBE and high energy resolution scanning tunneling spectroscopy provides a powerful way to probing the novel physics in the topological insulators.
3:00 PM - **L2.2
Manipulating the Dirac Fermions of the 3D Topological Insulators Studied by STM and Synchrotron Radiation ARPES.
Akio Kimura 1
1 Graduate School of Science, Hiroshima University, Higashi-Hiroshima Japan
Show Abstract Three-dimensional topological insulators, which harbor massless helical Dirac Fermions in a bulk energy gap, provide a fertile ground to realize new phenomena in condensed matter physics [1]. The topological insulator phase has been predicted and confirmed to exist in a number of binary compounds. Among them, Bi2Se3 has been regarded as the most promising candidate owing to the single Dirac cone residing in a wide band gap (0.3eV). Therefore, a significant effort has been applied towards the spintronic applications using the quantum topological transport, but the surface contribution to conduction has been hardly observed even at low carrier density, which pose the question of what limits the surface electron conduction in Bi2Se3. We have recently shown by scanning tunneling microscopy and differential tunneling conductance measurements that a strong surface state – bulk continuum scattering occurs near the Dirac node at the surface of Bi2Se3 [2]. This implies the necessity of modifying and engineering the topological states to isolate the surface conduction. As a first step, we have tried to search for the new topological insulators possessing more ideal Dirac Fermions isolated from the bulk continuum by the angle resolved photoemission spectroscopy (ARPES) with tunable synchrotron radiation. We have actually found that TlBiSe2 possesses the more ideal surface Dirac cone than Bi2Se3, where both of the lower and the upper Dirac cones are well separated from the bulk valence band [3]. Besides, some of the Pb based ternary chalcogenides are found to have the Dirac cone situated well below the topmost layer. We also demonstrate by using the ARPES combined with the scanning tunneling microscopy (STM) that the surface Dirac cone of the 3D topological insulator can be modified through adsorption and intercalation of guest atoms. Especially, the deposition of Ag atoms on the surface of Bi2Se3 promotes the detachment of the topmost quintuple layer units from the substrate. By making use of the tunable photon energy as well as the bulk sensitivity of the low-energy synchrotron radiation, we probe that the topological surface state is relocated beneath the detached quintuple layers, accompanied by the appearance of trivial two-dimensional states [4]. These novel findings open a pathway to the engineering of Dirac Fermions shielded from the ambient contamination and may facilitate the realization of fault-tolerant quantum devices. These works have been done in collaboration with M. Ye, K. Kuroda, S. Kim, S. V. Eremeev, E. E. Krasovskii, E. V. Chulkov, M. Nakatake, M. Arita, K. Miyamoto, T. Okuda, K. Shimada, H. Namatame, Y. Ueda, M. Taniguchi.References[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).[2] Sunghun Kim et al., arXiv: 1106.2681.[3] Kenta Kuroda et al., Phys. Rev. Lett. 105, 146801 (2010).[4] Mao Ye et al., submitted.
3:30 PM - L2.3
Two-Dimensional Electron Gases on the Surface of the Topological Insulator Bi2Se3.
Marco Bianchi 1 , Richard Hatch 1 , Jianli Mi 2 , Bo Iversen 2 , Philip King 3 , Philip Hofmann 1
1 Physics and Astronomy and Interdisciplinary Nanoscience Center, Aarhus University, Aarhus C Denmark, 2 Chemistry and Center for Materials Crystallography and Interdisciplinary Nanoscience Center, Aarhus University, Aarhus C Denmark, 3 Physics and Astronomy, University of St. Andrews, St. Andrews United Kingdom
Show AbstractExposing the (111) surface of the topological insulator Bi2Se3 to carbon monoxide results in shifts of the features observed in angle-resolved photoemission to higher binding energies. The behavior is very similar to an often-reported "aging" effect of the surface and it is concluded that this aging is most likely due to the adsorption of rest gas molecules. Initially, the downward bending of the conduction band leads to the formation of a two-dimensional electron gas (2DEG) near the surface. For higher CO coverages, this 2DEG undergoes a strong Rashba splitting and the valence band states also become quantized. The spectral changes are similar to those recently reported in connection with the adsorption of the magnetic adatom Fe [1] and the valence band quantization leads to features that closely resemble those of a band gap opening at the Dirac point. [1] L. A. Wray et al., Nature Physics 7, 32 (2011).
3:45 PM - L2.4
Electronic Structure of Topologically Nontrivial Sb Films: A Reconciliation of Spin-Polarized Surface States and Unpolarized Quantum-Well States.
Guang Bian 1 , Thomas Miller 1 , Tai Chiang 1
1 Physics, Univ. of Illinois, Urbana, Illinois, United States
Show AbstractTopological insulators support protected spin-polarized currents on their surfaces, which make them candidates for potential spintronics applications. In a centrosymmetric crystal, the bulk bands are spin-degenerate. Yet the polarized surface states have dispersions that carry them, in parts of the Brillouin zone, into these unpolarized bulk conduction and valence bands. We present experimental and theoretical work on thin films of Sb to elucidate this passage from spin-polarized surface states to unpolarized bulk states. Thin films are likely to be relevant for device fabrication, and the use of a thin film in the experiments converts the normal bulk-band continuum into a spectrum of discrete quantum-well subbands which are more amenable to observation by angle-resolved photoemission. Comparison of experiment with theoretical calculations provides a reconciliation of the behavior of the surface and bulk states in parts of the zone where they must intermix.
4:30 PM - **L2.5
Theoretical Design of Topological Insulators.
Naoto Nagaosa 1
1 Department of Applied Physics, University of Tokyo, Tokyo Japan
Show AbstractRecently, the geometrical properties of electrons in solids attract much attention both from the scientific and technological viewpoints. Especially the topological insulator (TI) is a new state of matter realized by the relativistic spin-orbit interaction in the nonmagnetic system. Up to now, several candidate materials have been proposed and actually confirmed experimentally to be TI’s. However, all of these materials are based on the s- and p-orbitals, and also centrosymmetric systems. In this talk, I will discuss the possibility of TI in d-electron systems and noncentrosymmetric systems. In the former case, we expect a variety of phenomena related to the magnetism, ferroelectricity, and superconductivity due to the many-body interactions, and the interplay between the topology and correlation can be studied in depth. Especially, the possible TI at the hetero-structures of transition metal oxides are studied in details. For the latter category, some novel effects such as the magneto-electric effect, spin Galvanic effect, and spin Hall effect are expected even without the TI, and hence the additional features added by the TI are of interests. I will discuss some candidate materials including their quantum critical behavior and spintronics functions.
5:00 PM - L2.6
Direct Evidence of Three-Dimensional Topological Insulator Phase in Pb Based Ternary Chalcogenide.
Kenta Kuroda 1 , Mao Ye 1 , Hirokazu Miyahara 1 , Yoshifumi Ueda 2 , Koji Miyamoto 3 , Taichi Okuda 3 , Masasi Arita 3 , Kenya Shimada 3 , Hirofumi Namatame 3 , Masaki Taniguchi 1 3 , Akio Kimura 1
1 , Hiroshima University, Higashi Hiroshima, Hiroshima Japan, 2 , Kure National Technology of College, Kure, Hiroshima Japan, 3 , Hiroshima Synchrotron Radiation Center, Higashi Hiorshima, Hiroshima Japan
Show Abstract A topological insulator accompanies the novel surface Dirac Fermions within its bulk energy gap. More importantly, their spin orientations are locked with electron momenta, which results in forming the spin helical texture in the momentum space. In striking contrast to the graphene also showing the Dirac cone but being described with pseudo spins, a lot of new exotic physical phenomena are expected in the topological insulator [1]. Up to now, a number of topological insulators have been studied, such as Bi2Te3, Bi2Se3 and some thallium-based ternary compounds. Among them, Bi2Se3 is a one of the most promising candidates as an ideal topological insulator for its potential of applications. However, according to recent macroscopic experiments, even in Bi2Se3, the transport properties are dominated by the bulk charge carriers. Therefore, to search more ideal topological insulator is an urgent task. Recently, some of the Pb-based ternary chalcogenides have been predicted as a new family of topological insulators [2, 3, 4]. Among them, PbBi2Te4 is composed of the seven-layer (7L) blocks formed by the atomic layer sequence, Te-Bi-Te-Pb-Te-Bi-Te. The theoretical analysis with an inclusion of spin-orbit coupling indicates that this compound is a strong topological insulator [2]. We have studied the electronic states of the PbBi2Te4 by angle resolved photoemission spectroscopy (ARPES) with tunable photon energy at the helical undulator beamline BL9A and the linear undulator beamline BL1 in Hiroshima Synchrotron Radiation Center. The ARPES measurement has revealed the presence of Dirac cone in PbBi2Te4, whose signatures are pronounced at lower excitation energy below 10eV. This result indicates that the Dirac cone can be located deep inside of the crystal rather than at the surface. Besides, the size of the constant energy contours acquired by the ARPES, corresponding to the carrier density of the Dirac Fermions in the bulk energy gap region, is turned out to be larger than those of the other reported materials. These results suggest a pathway to realizing the quantum topological transport.[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).[2] T. V. Menshchikova et al, JETP Letters 93, 15 (2011).[3] H.Jin et al,Phys. Rev. B 83, 041202(R) (2011).[4] S.V. Eremeev et al., JETP Lett. 92, 161 (2010).
5:15 PM - L2.7
Epitaxial Growth of High-Quality Bi2Te3 and Sb2Te3 Thin Films on GaAs (111).
Zhaoquan Zeng 1 3 , Dongsheng Fan 2 3 , Chen Li 1 3 , Yusuke Hirono 1 3 , Timothy Morgan 1 3 , Jian Wang 5 6 , Joon Sue Lee 5 , Zhiming Wang 1 3 4 , Shui-Qing Yu 2 3 , Aqiang Guo 1 3 , Gregory Salamo 1 3
1 Department of Physics, University of Arkansas, Fayetteville, Arkansas, United States, 3 Arkansas Institute for Nanoscale Materials Science and Engineering, University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas, United States, 5 The Center for Nanoscale Science and Department of Physics, The Pennsylvania State University, University Park, Pennsylvania, United States, 6 International Center for Quantum Materials and State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing China, 4 State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, Sichuang, China
Show AbstractRecently, three-dimensional topological insulator (TI), as a new state of matter, causes wide attention. To date, most studies were on the electronic properties of TI surface states. Very recently, Zhang et al. predicted the potential application of TIs for the high performance photodetector in the terahertz to infrared frequency range. Thus, it becomes a very important question how to integrate topological insulator photodetector devices with present semiconductor device fabrication technologies. One solution is to grow topological insulator on GaAs substrates widely used in optoelectric industry. In this presentation, we systematically investigate the growth behaviors of both Bi2Te3 and Sb2Te3 thin films on GaAs (111) substrates by solid source molecular beam epitaxy. The crystal quality and the epitaxial relationship between the film and substrate were determined by in-situ reflection high energy electron diffraction and ex-situ x-ray diffraction. Atomic force microscopy was used to characterize the surface morphology, and the surface state transport measurement is discussed. These results indicate that high quality topological insulator films were achieved on GaAs (111) substrates. These endeavors provide more routes for the device applications of topological insulator.
Symposium Organizers
Yi Cui Stanford University
Hailin Peng Peking University
Claudia Felser Johannes Gutenberg University Mainz
Shuichi Murakami Tokyo Institute of Technology
Tuesday AM, November 29, 2011
Room 301 (Hynes)
9:30 AM - *L3.1
Topological Surface States in Topological Insulators and Superconductors: Discovery and the New Frontiers.
M. Zahid Hasan 1
1 Department of Physics, Princeton University, Princeton, New Jersey, United States
Show AbstractThree dimensional topological insulators (originally called ``Topological Insulators”) are a new phase of matter which realizes a non-quantum-Hall-like topological state in bulk matter and unlike the quantum Hall liquids can be turned into superconductors [1-3]. In this talk, I will briefly review the basic theory, predictions and experimental discovery of topological insulators at ALS-LBNL. I will then discuss experimental results that demonstrate the properties of topological insulators such as spin-momentum helical locking, non-trivial Berry’s phases, mirror Chern number, absence of backscattering or no U-turn, protection by time-reversal symmetry and the existence of room temperature topological order. I will also report the possible exotic roles of superconductivity and magnetism in doped topological insulators and their potential applications. [1] M.Z.Hasan and C.L. Kane; Rev. of Mod. Phys. 82, 3045 (2010).[2] M.Z.Hasan and J.E.Moore; Ann. Rev. of Cond.Mat.Phys. (2011).[3] M.Z.Hasan, D. Hsieh, Y. Xia, L.A. Wray, S.-Y. Xu and C.L. Kane; A new experimental approach for the exploration of topological quantum phenomena : Topological Insulators and Superconductors arXiv:1105.0396 (Review Article) (2011)
10:15 AM - L3.2
Thin Film Electronic Properties of Ternary Topological Insulator.
Jiwon Chang 1 , Leonard Register 1 , Sanjay Banerjee 1 , Bhagawan Sahu 1
1 , The University of Texas at Austin, Austin, Texas, United States
Show AbstractWe study thin film electronic properties of two topological insulators (TIs) based on ternary compounds of Tl (thallium) and Bi (bismuth). We consider TlBiX2 (X=Se, Te) and Bi2X2Y (X, Y=Se, Te) compounds which provide better Dirac cones, compared to the model binary compounds Bi2X3 (X=Se, Te). With this property in combination with a structurally perfect bulk crystal, the latter ternary compound has been found to have improved surface electronic transport in recent experiments. Here we discuss the nature of surface states, their locations in the Brillouin Zone and their interactions within the bulk region. TIs can have intrinsic size limits which can protect the metallic nature of the surface bands, the surface states can spread inside the bulk region, and atomic rearrangements in thin layers can have profound effects on the Dirac cone itself. We address these issues using a density functional based electronic structure method and compare our results with available experimental results as well as with the results of binary Bi-based TIs. The critical thin film thicknesses required to maintain the Dirac cone and the size of the induced gaps for different topological insulators with various thicknesses are presented. And with the help of layer-projected surface charge densities, the extent of the spread of surface states into the bulk region is shown. Our studies predict that the Dirac cone remains protected in thinner layers of ternary Bi-based TIs Bi2X2Y (X, Y=Se, Te) than in binary Bi-based TIs Bi2X3 (X=Se, Te) or than in Tl-based TIs TlBiX2 (X=Se, Te), since surface states in ternary Bi-based TIs are more localized to the surfaces than others, which may be advantageous for certain applications. However, we also suggest that in the atomically relaxed structure, the Dirac cone of all but Bi2Se3 lie in or near the bulk valance band. However, the Dirac cone of Bi2Se2Te remains outside of the bulk valance band in the absence of relaxation, as might occur with dielectrics replacing the air gap. Our studies will have implications for the understanding of topological surface states in ternary TIs and their intended applications.
10:30 AM - L3.3
Crystal Growth and Physical Property of Bi-Sb-Te-Se Topological Insulator Materials.
Genda Gu 1 , Alina Yang 1 , Zhijun Xu 1 , J. Tranquada 1 , J. Zhao 1 , Z. Pan 1 , W. Si 1 , T. Valla 1
1 , Brookhaven National laboratory, Upton, New York, United States
Show AbstractThe discovery of 3D topological insulator materials opens up a new research field in the condensed matter physics. In order to exploit the novel surface properties of these topological insulators, it is crucial to achieve a bulk-insulating state in these topological insulator crystals. Unfortunately, all available topological insulator crystals are not bulk-insulating. We have grown a number of Bi-Se, Bi-Te, Sb-Te-Se, Bi-Sb-Se, Bi-Sb-Te-Se and Bi-Sb-Te-Se-S topological insulator single crystals by using 5N and 6N pure elements. We have measured the physical properties on these single crystals. We have studied the effect of growth condition and impurity on the bulk electrical conductivity of these single crystals. We try to answer two questions if it is possible to grow the bulk-insulating topological insulator single crystals and Which maximum resistivity of these topological insulator single crystals we can grow.
10:45 AM - L3.4
Engineering of the Interfacial Topological States in the Guest-Atom-Intercalated 3D Topological Insulator.
Mao Ye 1 , S. Eremeev 2 3 , K. Kuroda 1 , M. Nakatake 4 , E. Krasovskii 5 6 7 , E. Chulkov 5 6 , M. Arita 4 , K. Miyamoto 4 , T. Okuda 4 , K. Shimada 4 , H. Namatame 4 , M. Taniguchi 1 4 , Y. Ueda 8 , A. Kimura 1
1 Graduate School of Science, Hiroshima University, Higashi-Hiroshima Japan, 2 , Institute of Strength Physics and Materials Science, Tomsk Academic City, Tomsk, Russian Federation, 3 , Tomsk State University, Tomsk Russian Federation, 4 Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima Japan, 5 , Departamento de Física de Materiales UPV/EHU and Centro de Física de Materiales CFM and Centro Mixto CSIC-UPV/EHU, SanSebastián/Donostia Spain, 6 , Donostia International Physics Center (DIPC), SanSebastián/Donostia Spain, 7 , IKERBASQUE, Basque Foundation for Science, Bilbao Spain, 8 , Kure National College of Technology, Kure Japan
Show AbstractA novel class of quantum materials, called topological insulators (TIs), has provoked much research interest. A number of materials that hold nontrivial spin-polarized metallic surface states have been intensively studied, such as Bi1-xSbx, Bi2Te3, Bi2Se3, and thallium-based TIs , among which Bi2Se3 is one of the most promising candidates for potential applications in ultra-low power consumption quantum devices that can work stably at room temperature due to a sufficiently large bulk energy gap. Owing to the time-reversal symmetry, topological surface states are protected from backscattering in the presence of a weak perturbation, which is important for the realization of dissipationless spin transport in novel quantum devices. However, such surface state is not protected from scattering by arbitrary angles, so the shielding of this state from impurities is an important problem. In this work we employ state-of-the-art methods of scanning tunneling microscopy, angle resolved photoemission spectroscopy and first principles calculations to study the evolution of the surface electronic structure of Bi2Se3 under silver atom intercalation. We demonstrate that silver atoms deposited on the surface of Bi2Se3 are intercalated between the quintuple layer (QL) units of the crystal, causing their detachment from the substrate. This leads to a relocation (in the real space) of the topological surface state beneath the detached quintuple layers, accompanied by the appearance of trivial two-dimensional (2D) states. These novel findings open a pathway to the engineering of Dirac fermions shielded from the ambient contamination and may facilitate the realization of fault-tolerant quantum devices.
11:30 AM - **L3.5
Electronic Structure of Topological Insulators.
Zhi-xun Shen 1
1 Stanford University , Department of Applied Physics, Physics, SSRL, Stanford, California, United States
Show Abstract[email protected] class of new quantum matter, so-called topological insulators, has unique properties. It has a symmetry protected surface state in the absence of time reversal symmetry breaking, leading to dissipation less edge currents. The strong spin-orbit coupling provides an interesting way to manipulate spin through orbital current. This new class of materials provides a platform to study novel physics such as Majorana fermion, Axion physics, image magnetic monopole from a source charge, etc.. In addition, it also provides opportunities for possible applications ranging from spintronics to thermoelectric materials. In this talk, I will report angle-resolved photoemission spectroscopy (ARPES) data focusing on the following: i) realization of large gap topological insulator with a single dirac cone; ii) creation of massive Dirac fermion on the surface of topological insulator with broken time reversal symmetry; iii) observation of single Dirac cone topological surface state in a candidate topological superconductor.Referencesi) Y.L. Chen et al.; Science 325, 178 (2009); ii) Y.L. Chen et al.; Science 329, 659 (2010); iii) Y.L. Chen et al.; Phys. Rev. Lett., 105, 266401 (2011)
12:00 PM - L3.6
The Third Generation of the Dirac Cone as a Proof of Stacked 2D Electron Systems in Iron Pnictides.
Koichi Kusakabe 1
1 Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan
Show AbstractThe confirmation of the Dirac cone in the SDW state of Ba(FeAs)2[1] is attracting much attention, because this curious electronic state may have relevance for understanding of the high-temperature superconductivity. The transport experiment by Huynh et al.[2] has revealed existence of the electron- and hole-Dirac cones. Specialty of the iron pnictides has been discussed by two- or five-band models for the two-dimensional (2D) electron system of the FeAs plane, where formation of the Dirac cone is caused by the nodal SDW order.[3] This concepts suggests controlling methods of the Dirac cone by perturbation via the SDW, which is not possible for graphene and α-(BEDT-TTF)2I3. As for the DFT-based band structure calculations, the finding was motivated researchers to improve description of the metallic anti-ferromagnetic state.[4] Therefore, construction of an electronic structure calculation method is demanded for the application to the SDW-generated Dirac cone states. For the theoretically stable consideration, we start from an iron chalchogenide. The planer structure allows us to consider relevant charge-charge fluctuations based on the fluctuation reference method.[5] The exchange channels derived for the inter-plane magnetic interaction determined for a double layers should be the super-exchange counterpart of the pair-hopping channel for the layered superconductivity.[6] Thus a possible proof of 2D nature of the third generation Dirac cone in iron pnictides induced by the SDW also promotes understanding of the high-temperature superconductivity. Keyword: Nano space, iron pnictide, Dirac cone, SDW, superconductivity[1] P. Richard, et al., Phys. Rev. Lett. 104, 137001 (2010).[2] K. K. Huynh, Y. Tanabe, and K. Tanigaki, Phys. Rev. Lett. 106, 217004 (2011).[3] Y. Ran, et al., Phys. Rev. B 79, 014505 (2009).[4] J. G. Analytis et al, Phys. Rev. B 80, 064507 (2009).[5] K. Kusakabe, et al., J. Phys.: Condens. Matter 19, 445009 (2007).[6] K. Kusakabe, J. Phys. Soc. Jpn. 78, 114716 (2009).
12:15 PM - L3.7
Effects of Magnetic Doping on Weak Antilocalization in Bi2Se3 Topological Insulator Nanoribbons.
Judy Cha 1 , Desheng Kong 1 , Seung Sae Hong 2 , Kristie Koski 1 , Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Department of Applied Physics, Stanford University, Stanford, California, United States
Show AbstractTopological insulators exhibit odd numbers of surface states with Dirac dispersion, residing inside bulk band gap. Due to time reversal symmetry, the surface states are predicted to be robust against non-magnetic impurities. Angle-resolved photoemission spectroscopy (ARPES) experiments clearly show presence of topological surface states in Bi2Se3 and Bi2Te3 heavily doped with various charge dopants [1]. In contrast, magnetic impurities can break time reversal symmetry, thus opening a band gap in the surface bands [2]. This was confirmed by ARPES experiments in Fe-doped Bi2Se3 bulk crystals [3].
In transport measurements, the most straightforward signature of magnetic doping would be a true insulating behavior due to a surface gap opening. However, both Bi2Se3 and Bi2Te3 possess residual bulk carriers, indicating that the Fermi level is not inside the bulk band gap. Hence, an observation of a true insulating behavior due to magnetic doping is difficult. Another signature of the magnetic doping is a cross-over between weak localization and anti-localization. Recently, magnetically doped Bi2Se3 and Bi2Te3 thin films grown by molecular beam epitaxy show weak localization, instead of anti-localization observed in undoped films [4].
Here, we magnetically dope Bi2Se3 nanoribbons grown by vapor-liquid-solid mechanism [5]. First, we observe Kondo effect in Fe-doped Bi2Se3 nanoribbons, establishing that Fe impurities are magnetically active [6]. Then, we show the cross-over between weak anti-localization and localization in Fe-doped Bi2Se3 nanoribbons. We will also discuss the effect of gating and temperature on the evolution of the weak anti-localization.
[1] D. Hsieh, et al., Nature 460, 1101 (2009); Y. L. Chen, et al., Science 325, 178 (2009).[2] Q. Liu, et al., Phys. Rev. Lett. 102, 156603 (2009).[3] Y. L. Chen, et al., Science 329, 659 (2010); L. A. Wray, et al., Nat. Phys. 7, 32 (2011).[4] H.-T. He, et al., Phys. Rev. Lett. 106, 166805 (2011); M.-H. Liu, et al., arXiv:1103.3353v1 (2011).[5] D. Kong, et al., Nano Lett. 10, 329 (2010).[6] J. J. Cha, et al., Nano Lett. 10, 1076 (2010).
L4
Session Chairs
Alex Balandin
Hailin Peng
Tuesday PM, November 29, 2011
Room 301 (Hynes)
2:30 PM - **L4.1
Topological Insulators: Non-Magnetic vs. Magnetic.
Zhong Fang 1
1 , Institute of Physics, Beijing China
Show AbstractTopological insulator is a new state of quantum matter, characterized by topological invariants. Exotic quantum phenomena, such as Majorana Fermions, magneto-electric effect, and quantum anomalous Hall effect, have been expected from topological insulators, while their experimental realizations remain challenging, due to the lack of suitable samples or requirement of extreme conditions. Within recent couple of years, more and more topological insulators were discovered, yet lots of new compounds still wait to be explored. In this talk, I will start from our earlier predictions for Bi2Se3 family compounds, and discuss the characterization of topological nature from the Wannier representation and Willson loop method [1]. I then move to the recent study for the topological aspect and quantum magnetoresistance of Ag2Te [2]. The possible realization of quantized Anomalous Hall effect and Majorana fermions after breaking time reversal symmetry will be discussed from the view point of materials design [3,4].References:[1] R. Yu, et.al., arXiv:1101.2011 (2011).[2] W. Zhang, et.al., Phys. Rev. Lett. 106, 156808 (2011).[3] R. Yu, et.al., Science 329, 61 (2010).[4] H. M. Weng, et.al., arXiv: 1103.1930 (2011).
3:00 PM - **L4.2
Materials-Oriented Research of Topological Insulators and Superconductors.
Yoichi Ando 1
1 ISIR, Osaka Univeristy, Osaka Japan
Show AbstractA topological state of matter is characterized by a topological feature of the quantum-mechanical wavefunction in the Hilbert space. In 3D topological insulators (TIs), a non-trivial Z2 topology of the bulk valence band leads to the emergence of Dirac fermions on the surface. Similarly, in 3D topological superconductors (TSCs), a non-trivial winding number of the superconducting wavefunction leads to the appearance of Majorana fermions on the surface. The Dirac or Majorana fermions in those topological states of matter are not only of intellectual interest, but they are also expected to play a key role in future information devices. However, there have been significant materials problems that have hindered experimental studies of those intriguing states of matter: In the case of TIs, most of the materials identified as TIs are poor insulators in the bulk, making it difficult to probe the peculiar surface transport properties; in the case of TSCs, no concrete example has yet been discovered. In this talk, I will present our materials efforts to address these issues. For TIs, we discovered that the chalcogen-ordered tetradymite compound Bi2Te2Se presents a high bulk resistivity, allowing one to observe clear surface quantum oscillations [1]; more recently, we discovered that the bulk-insulating nature can be further improved in the solid-solution system Bi2-xSbxTe3-ySey [2]. For TSCs, a prime candidate has been the electron-doped topological insulator CuxBi2Se3 which superconduct below 3.8 K, but the difficulty in synthesizing this material has kept it from experimental scrutiny. Recently, we developed a new synthesis technique for this material to obtain single-crystal samples with a high shielding fraction [3], and we are now elucidating the nature of its superconducting state with various experiments. This work was supported by JSPS (NEXT Program), MEXT (Innovative Area "Topological Quantum Phenomena"), and AFOSR-AOARD.REFERENCES: [1] Z. Ren et al., Phys. Rev. B 82, 241306(R) (2010).[2] A. A. Taskin et al., Phys. Rev. Lett. 107, 016801 (2011).[3] M. Kriener et al., Phys. Rev. Lett. 106, 127004 (2011).
3:30 PM - L4.3
Electronic Transport of Surface State in Doped Bi2Se3 Topological Insulator Nanoribbons.
Seung Sae Hong 1 , Judy Cha 2 , Desheng Kong 2 , Yi Cui 2
1 Department of Applied Physics, Stanford university, Stanford, California, United States, 2 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractTopological insulator is a new state of matter, which accommodates gapless surface states crossing bulk band gap. The surface state, induced by strong spin-orbit coupling, is robust against impurities or defects, unless the perturbation breaks time reversal symmetry. The unique properties of surface states in topological insulator promote a lot of interests, especially due to the possibility of spintronics applications, as well as the candidate to detect Majorana Fermion. Direct observation of the surface states is often hindered due to materials’ imperfections – bulk impurities and environmental doping – so that large background signal of additional carriers diminishes clear signature of the surface states.In this talk, we report electronic transport of antimony (Sb) doped Bi2Se3 nanoribbons. By introducing Sb dopants during vapor-liquid-solid (VLS) growth of nanoribbons, we are able to decrease bulk electron contribution significantly. Combined with an appropriate surface protective layer, Sb doped nanoribbons show large gating response even in very thick (300nm) samples, which implies extremely low bulk/impurity band contribution. We will also discuss magnetotransport of the doped nanoribbons, measured at different gate voltages and temperatures.
3:45 PM - L4.4
High Density Intercalation of Metal Atoms in Topological Insulator Nanoribbons.
Kristie Koski 1 , Judy Cha 1 , Desheng Kong 1 , Seung Sae Hong 1 , Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractIntercalation in layered chalcogenide materials can have profound effects on the material behavior and properties. For example, in topological insulator materials, intercalation of palladium into Bi2Te3 and intercalation of copper into Bi2Se3 can produce superconductivity (1,2). We have developed a synthetic approach to intercalate metal atoms into vapor-liquid-solid (VLS) grown chalcogenide topological insulator nanoribbons. We show that extreme densities, up to 60 atomic percent, of metal atoms can be intercalated without precipitate formation or destruction of the crystal structure. Intercalated atoms essentially form two-dimensional, atomically thin metal sheets resulting in many new and surprising behaviors in topological insulator nanoribbon materials. (1) Y.S. Hor et al, Phys. Rev. Lett. 104, 057001 (2010)(2) Y.S. Hor et al, J. Phys. Chem. Sol. 72, 572 (2011)
4:30 PM - **L4.5
Micro-Raman and Low-Frequency Noise Spectroscopy of Mechanically Exfoliated Thin Films of Topological Insulators of Bi2Te3 Family.
Alexander Balandin 1
1 Department of Electrical Engineering, University of California - Riverside, Riverside, California, United States
Show AbstractWe used “graphene-like” mechanical exfoliation from single crystal films bismuth telluride (Bi2Te3) and bismuth selenide (Bi2Se3) to obtain few-quintuple layer (FQL) films of topological insulators [1-2]. Following the analogy with graphene we proposed micro-Raman spectroscopy was as nanometrology tool for identification of FQL topological insulator films and determining their quality. The micro-Raman investigation was carried out for Bi2Te3 and Bi2Se3 films with the thickness ranging from a few-nm to bulk limit [3]. It was found that the optical phonon mode A1u, which is not-Raman active in bulk Bi2Te3, appears in the atomically-thin films due to the crystal-symmetry breaking at the film-substrate interfaces. The intensity ratios of the out-of-plane A1u and A1g modes to the in-plane Eg mode grow with the decreasing film thickness. The calibrated evolution of the Raman peaks with the changing film thickness can be used for FQL characterization. Low-frequency noise measurements have been used to investigate the carrier and trap dynamics in various materials and devices. We fabricated four-terminal devices with the topological insulator channels and measured their noise response. The resistance dependence on the film thickness indicated that the surface transport represents a substantial component in the channel current [4]. The current fluctuations have the noise spectral density SI ∞1/f (f is the frequency) for the frequency range of f <10 kHz. The noise density SI follows the quadratic dependence on drain-source current and changes from ~10-22 to 10-18 A2/Hz as the current increases from ~10-7 to 10-5 A. The thermal and thermoelectric properties of the exfoliated topological insulator films have also been investigated [5]. The work at UCR was supported, in part, by SRC – DARPA through FCRP Functional Engineered Nano Architectonics (FENA) center. [1] D. Teweldebrhan, V. Goyal, M. Rahman and A. A. Balandin, Appl. Phys. Lett., 96, 053107 (2010).[2] D. Teweldebrhan, V. Goyal and A. A. Balandin, Nano Lett., 10, 1209 (2010).[3] K. M. F. Shahil, M. Z. Hossain, D. Teweldebrhan and A. A. Balandin, Appl. Phys. Lett., 96, 153103 (2010).[4] M.Z. Hossain, S.L. Rumyantsev, K.M.F. Shahil, D. Teweldebrhan, M. Shur and A.A. Balandin, ACS Nano, 5, 2657 (2011).[5] V. Goyal, D. Teweldebrhan and A.A. Balandin, Appl. Phys. Lett., 97, 133117 (2010).
5:00 PM - L4.6
Raman Spectroscopy of Topological Insulator Bi2Se3, Bi2Te3 and Sb2Te3 Nanoplatelets.
Jun Zhang 1 2 , Zeping Peng 1 , Mildred Dresselhaus 5 , Qihua Xiong 3 4
1 School of Physical and Mathematical Sciences, Nanyang Technological Uni, Singapore Singapore, 2 School of Electrical and Electronic Engineering, Nanyang Technological University, singapore Singapore, 5 School of Physical Science and Technology, Southwest University, Chongqing China, 3 Department of Physics, Masachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Electrical Engineering and Computer Science, Masachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe report systematic studies on Raman spectroscopy of few quintuple layer topological insulator bismuth selenide (Bi2Se3), bismuth telluride (Bi2Te3) and antimony telluride (Sb2Te3) nanoplatelets (NPs), synthesized by a solution-based polyol method. As-grown NPs exhibit excellent crystalline quality, hexagonal or truncated trigonal morphology, flat surfaces down to a few quintuple layers. For Bi2Se3 nanoplatelets,[1] both Stokes and anti-Stokes Raman spectroscopy for the first time resolve all four optical phonon modes from individual NPs down to 4 nm, where out-of-plane vibrational mode shows a few wavenumbers redshift as the thickness decreases below ~ 15 nm. This thickness-dependent redshift is tentatively explained by a “phonon softening” as the decreasing of van der Waals forces between adjacent layers. Quantitatively, we found that 2D phonon confinement model proposed by Faucet and Campbell cannot explain the redshift values and lineshape of mode, which can be described better by a Breit-Wigner-Fano resonance lineshape. Considerable broadening (~ 17 cm-1 for 6 quintuple layers) especially for in-plane vibrational mode was identified suggesting that the layer-to-layer stacking affects the intra-layer bonding, therefore a significant decreasing of phonon lifetime of in-plane vibrational modes is resulted probably due to an enhanced electron-phonon coupling in few quintuple layer regime. Raman modes of Bi2Se3, Bi2Te3 and Sb2Te3 show significant laser power dependence. With an increasing laser power, several high-frequency (>200 cm-1) Raman peaks appears which might be due to laser introduced variation of composition. We didn’t observe additional IR active mode activated due to symmetry breaking as suggested by A. A. Balandin and co-authors. [2] [1]. Jun Zhang, Zeping Peng, Ajay Soni, Yanyuan Zhao, Yi Xiong, Bo Peng, Jianbo Wang, Mildred S. Dresselhaus, and Qihua Xiong, Nano Letters 2011, 11 (6), 2407–2414[2]. D. Teweldebrhan, V. Goyal, A. A. Balandin, Nano Letters 2010, 10, (4), 1209-1218.
Symposium Organizers
Yi Cui Stanford University
Hailin Peng Peking University
Claudia Felser Johannes Gutenberg University Mainz
Shuichi Murakami Tokyo Institute of Technology
Wednesday AM, November 30, 2011
Room 301 (Hynes)
9:30 AM - *L5.1
Dirac Fermions in HgTe Quantum Wells.
Laurens Molenkamp 1
1 Department of Physics, University of Wuerzburg, Wuerzburg, Bavaria, Germany
Show AbstractHgTe quantum wells have a linear band dispersion at low energies and thus mimic the Dirac Hamiltonian.Changing the well width tunes the band gap (i.e., the Dirac mass) from positive, through zero, to negative.Wells with a negative Dirac mass are 2-dimensional topological insulators and exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating.Our transport data provide very direct evidence for the existence of this third quantum Hall effect.Wells with a thickness of 6.3 nm are zero gap Dirac systems, similar to graphene. However, zero gap HgTe wells possess only a single Dirac valley, which avoids inter-valley scattering.
10:15 AM - **L5.2
Ambipolar Field Effect in Ultrathin Nanoplates of Topological Insulators.
Desheng Kong 1 , Yulin Chen 2 3 4 , Judy Cha 1 , Qianfan Zhang 1 , James Analytis 2 4 , Keji Lai 2 3 , Zhongkai Liu 2 3 4 , Seung Sae Hong 2 , Kristie Koski 1 , Sung-Kwan Mo 5 , Zahid Hussain 5 , Ian Fisher 2 4 , Zhi-Xun Shen 2 3 4 , Yi Cui 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Applied Physics, Stanford University, Stanford, California, United States, 3 Physics, Stanford University, Stanford, California, United States, 4 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California, United States, 5 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractTopological insulators are a state of matter with exotic surface electronic states, which are attractive to fundamental physics studies and technological applications. They become a rising field in physics and materials science community. Transport measurements on topological insulators, especially nanostructures, are challenging because surface carriers are often outnumbered by bulk carriers. We developed a general vapor transport synthesis approach to grow nanoplates of binary and ternary topological insulators, including Bi2Se3, Bi2Te3, Sb2Te3 and (BixSb1-x)2Te3, as thin as a few nanometers. Ultrathin topological insulator nanoplates, with their large surface-to-volume ratios to enhance the contribution from surface states, provide excellent geometries for transport study. In particular for (BixSb1-x)2Te3 material system, the carrier concentration is systematically tuned with composition, allowing us to obtain nanoplates with very low carrier density. We have observed pronounced ambipolar field effect in backgate transistor devices fabricated on low density (BixSb1-x)2Te3 nanoplates, similar to that observed in graphene. The electrical manipulation of carrier type and concentration in topological insulator nanostructures paves the way to implement this novel matter in nanoelectronic and spintronic devices.
10:45 AM - L5.3
Tunability of the Critical Width of a 2D-Topological Insulator.
Parijat Sengupta 1 , Tillmann Kubis 1 , Yaohua Tan 1 , Michael Povolotskyi 1 , Gerhard Klimeck 1
1 Electrical Engineering, Purdue University, West Lafayette, Indiana, United States
Show AbstractTopological Insulators (TI) is a new class of materials that exhibit bulk-insulating properties [1] but possess conducting states on the surface. These surface states are spin-polarized, and their spin texture [2] protected through time reversal symmetry forbids electron back scattering and induces high mobility channels. TIs, by virtue of their enhanced mobility offer practical realization of faster semiconductor devices in addition to the discovery of fundamentally new physics.One of the essential material properties to produce TI like states is band inversion. An inversion of the band structure needs to pass through a Dirac like point [3] where the conduction and valence bands touch for a zero-energy gap state before splitting up (primarily due to high so-coupling) to give a negative energy-gap region. The resulting Dirac cone is the precursor to TI surface states. In two-dimensional mercury based TIs, the Dirac cone is produced at a certain, material dependent critical layer dimension. We demonstrate in this work that the critical dimension for a CdTe/HgTe/CdTe quantum well (QW) heterostructure, which has been found to support TI states [4], is tunable through suitable external parameters. Tunability presents scope for practical use of TIs either as building blocks for transistors of varying dimensions or as a switch in an electric circuit.The Dirac cone (onset of band inversion) is identified to appear for a 6.3nm thick HgTe QW flanked by CdTe barriers of equal dimensions. It is shown in this work that this critical dimension (CD) can be tuned in various ways: 1) Temperature driven alteration of the QW band-gap adjusts the CD. A lower temperature is found to be better suited. 2) Substitution of the pure HgTe and CdTe layers in the well and barrier region with CdxHg1-xTe alloys of various stoichiometric compositions at several temperatures significantly changes CD. The influence of the barrier band-gap as a boundary condition, in conjuction to that of the well material is clearly observed in the production of TI states 3) Applying a sufficiently strong external electric field can invert the band-structure and induce conducting states in a normal insulator. 4) Crystallographic orientation of the quantum well and barrier material and 5) external strain on the quantum well and the barrier interfaces allows tuning of CD as well.All of these effects are studied within an 8-band k.p model [5]. Multiple simulations of heterostructures of various dimensions, orientations, compositions and temperatures were carried out. Strain is added to the k.p Hamiltonian through Bir-Pikus deformation potentials.[1] J.Moore Nature 464,194 (2010)[2] L. Fu, C.L. Kane, and E.J. Mele, Phys. Rev. Lett. 98,106803 (2007)[3] X.L.Qi, T.L.Hughes and S.C.Zhang, Phys. Rev. B 78,195424 (2008)[4] B.A. Bernevig, T.L. Hughes and S.C. Zhang, Science 314, 1757 (2006)[5] G. L. Bir and G. E. Pikus, Symmetry and Strain-Induced Effects in Semiconductors (Wiley, 1974).
11:30 AM - **L5.4
Two Dimensional Transport Induced Linear Magneto-Resistance in Topological Insulator Bi2Se3 Nanowires and Nanoribbons.
Xuan Gao 1
1 Department of Physics, Case Wastern Reserve University, Cleveland, Ohio, United States
Show AbstractBulk Bi2Se3 has been proposed and confirmed as a type of three-dimensional (3D) topological insulators (TI’s) with a single Dirac cone for the surface state. Although the existence of topological surface state in Bi2Se3 has been established by surface sensitive techniques (ARPES, STM), the transport properties of two dimensional (2D) surface state in 3D TI’s has been plagued by the dominating conductivity from bulk carriers. We will describe our study of a novel linear magneto-resistance (MR) under perpendicular magnetic fields in chemically synthesized Bi2Se3 nanowires and nanoribbons, and show that this linear MR is purely due to 2D transport by angular dependence experiments. The 2D magneto-transport induced linear MR in Bi2Se3 nanoribbons is in agreement with the recently discovered linear MR from topological surface state in bulk Bi2Te3, and the MR of other gapless semiconductors including graphite and graphene. We further demonstrate that the linear MR of Bi2Se3 nanowires/nanoribbons persists up to room temperature, underscoring the potential of exploiting TI nanomaterials for room temperature magneto-electronic applications.
12:00 PM - L5.5
Liquid Phase Exfoliation and Characterization of Layered Thermoelectric Metal Chalcogenides.
Mustafa Lotya 1 2 , Jonathan Coleman 1 2
1 School of Physics, Trinity College Dublin, Dublin Ireland, 2 , Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Dublin Ireland
Show AbstractRecent work has shown that the layered Transition Metal Dichalcogenides (TMDs) such as MoS2, MoSe2, WS2 and others can be readily exfoliated with the assistance of ultrasonication in specially selected solvents [1]. Such liquid-phase exfoliation, without the use of intercalating species, yields few-layer and mono-layered flakes for a wide range of TMDs [1]. Following on from this work we have examined other related layered metal chalcogenides; Bi2Te3, Bi2Se3, Sb3Te3 and Sb2Se3. These materials are noted for their electrical characteristics as topological insulators [2, 3]. Importantly, they have useful thermoelectric properties for use in waste heat recovery or refrigeration applications [4]. We will explore the results of liquid phase exfoliation of these materials. Liquid phase processing allows the facile formation of thin films and the potential for enhanced composite materials using conductive nanocarbons as a filler - such hybrid systems could prove useful in enhancing the thermoelectric properties of these materials. Transmission electron microscopy analysis of the flakes will be presented along with scanning probe characterization. The composition of the flakes will be studied by SEM/EDX. Latest results on the electrical and thermoelectric properties of these exfoliated materials and their composites will be shown.1.Coleman, J.N., et al., Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science, 2011. 331(6017): p. 568-571.2.Zhang, H.J., et al., Topological insulators in Bi2Se3, Bi2Te3 and Sb2Te3 with a single Dirac cone on the surface. Nature Physics, 2009. 5(6): p. 438-442.3.Hasan, M.Z. and C.L. Kane, Colloquium: Topological insulators. Reviews of Modern Physics, 2010. 82(4): p. 3045-3067.4.Snyder, G.J. and E.S. Toberer, Complex thermoelectric materials. Nat Mater, 2008. 7(2): p. 105-114.
12:15 PM - L5.6
Microstructure Evolution of Bi-Te Films with Post Annealing for Thermoelectric Applications.
Seungmin Hyun 1 , Seong-jae Jeon 1 2 , Minsub Oh 1 2 , Hoo-jeong Lee 2 , Ho-ki Lyeo 3
1 Department of Nano-Mechanics, Korea Institute of Machinery and Materials, Daejeon Korea (the Republic of), 2 Materials Science and Engineering, Sungkyunkwan University, Suwon Korea (the Republic of), 3 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractRecently, Bismuth-telluride (Bi-Te) has attracted attention as promising topological insulators. In addition, Bi-Te has attractive applications due to its good thermoelectric properties. Bi-Te, among several thermoelectric materials, has been a key material owing to their high figure of merit ZT. The good thermoelectric properties are heavily dependent on the microstructure of Bi-Te materials. In this study, we have investigated the microstructure changes of the Bi-Te film with post annealing and its effects on thermoelectric properties. Bi-Te films were sputtered on a SiO2/Si substrate at room temperature from Bi and Te targets in a radio frequency (RF) magnetron sputtering system. The films were post-annealed at 200 °C for different durations under N2 ambient. The microstructures of bismuth-tellurium films were characterized using TEM (JEOL JEM-2100F and JEM-ARM200F ) and XRD (Bruker, D8 Focus). The film was grown as the rock-salt structure during deposition, and the microstructure of the film was transformed to a graphene like layered structure (Bi2Te3-type phase) with post annealing. The nucleation and growth of some grains of the Bi2Te3-type phase are observed on the interface with SiO2. The seebeck coefficient of the film is also increased from 56 μV/K for the as-deposited film to 250 μV/K for the annealing film. The power factor (α2ρ, α : Seebeck coefficient and ρ: electrical resistivity) increases to 12 μW/K2cm for the annealed film, which is showing an impact of the microstructure change on the thermoelectric properties. Thermal conductivity of the film gradually decreased with microstructure change after post-annealing.