2014 Fall Meeting
SEMICONDUCTOR MATERIALS AND SPINTRONICST
Topological materials II
Topological materials constitute a class of quantum materials exhibiting new electronic functionalities based on specific electronic properties of 3D and 2D topological insulator crystals: topological protection from electron backscattering, surface spin polarization or conductance quantization.
The symposium is a continuation of the event held during E-MRS 2011 Fall Meeting and is meant to provide a regular European forum for scientific exchange in the world-wide very actively studied field of quantum physics, materials science and solid-state technology of topological materials.
The main topics of the symposium will review the progress in theoretical understanding of the fundamental aspects of electronic and crystal structure of topological materials as well as development of new materials, e.g.: topological crystalline insulators of IV-VI semiconductors, correlated-electron systems of hexaborides, low-dimensional semiconductors like InAs-GaSb heterostructures or HgTe quantum wells, and magnetic topological insulators like transition metal-doped (Bi,Sb)2Te3 crystals.
The scientific program will also cover recent important developments in experimental studies of canonical topological insulator materials like Bi-Sb alloys, Bi2(Se,Te)3 or HgTe-based 2D heterostructures. In particular it concerns: angle-, spin- and time-resolved photoelectron and optical spectroscopies, micro-magnetometry, STM spectroscopy of topological Dirac states and the new analysis of magneto-transport experiments.
Practical exploitation of unique electronic and spin properties of topological materials requires development of new device concepts of electronic or spintronic systems useful, e.g. as spin current sources or quantum computation electronic platforms. In this respect, the symposium will address recent progress in development of practical methods for controlling topological electronic states with electrical gates or by the application of strain lowering crystal symmetry of surface or interface.
Hot topics to be covered by the symposium
- New topological materials: theoretical proposals and experimental studies.
- Hybrid topological structures with superconductors and ferromagnets.
- Majorana fermions and topological quantum computing.
- Topological crystalline insulators.
- Real space and k-space spectroscopies of topological materials.
- Spin and charge transport in materials with topological protection.
- Device concepts exploiting topological materials.
List of invited speakers
- N.P. Armitage, Johns Hopkins University (USA), “Quantum phase transitions in topological insulators”.
- A. Brinkman, University of Twente (The Netherlands), "Josephson supercurrents through topological surface states".
- C. Bruene, Wuerzburg University (Germany), “Electron transport in the topological edge and surface states of HgTe”.
- K.S. Burch, Boston College (USA), “In hot persuit of Majoranas”.
- R.J. Cava, Princeton University (USA), “Crystal structure and chemistry of topological insulators”.
- Y. Chen, Oxford University (UK), “Dirac semimetals”.
- A. Damascelli, University of British Columbia (Canada), "Spin-orbital textures in 3D topological insulators".
- J. Denlinger, Advanced Light Source, Lawrence Berkeley National Laboratory, (USA), "Angle resolved photoemission of SmB6 as a possible topological Kondo insulator".
- Rui-Rui Du, Rice University (USA), “Edge transport in InAs/GaSb quantum wells”.
- T. Karzig, Caltech, Pasadena, USA – “Topological polaritons”
- P.D.C. King, University of St. Andrews (UK) & Cornell University (USA), "Topological surface states and superconductivity in perovskites".
- Lu Li, University of Michigan (USA), "Quantum oscillations in SmB6".
- Y. Okada, Tohoku University (Japan), “STM probing of topological crystalline insulator states”.
- J.-P. Paglione, University of Maryland (USA), "Edge transport in topological Kondo insulator SmB6".
- F. Parmigiani, University of Trieste (Italy), “Momentum and time resolved photoelectron spectroscopy for studying the out of equilibrium electronic structure of topological insulators”
- R.-J. Slager, Leiden University (The Netherlands), “Classification of topologically protected materials”.
- E. Spanton, Stanford University (USA), “Quantum spin Hall state in HgTe wells and InAs/GaSb heterostructures”.
- J.C.Y. Teo, University of Illinois (USA), “Dislocations in topological insulator/superconductor structures”.
- J. Tworzydlo, University of Warsaw (Poland), “Disorder in topological insulators”.
- B.M. Wojek, KTH Stockholm (Sweden), “Topological crystalline insulators”.
Tentative list of scientific committee members
- C. Beenakker, Leiden University (The Netherlands).
- R. Buczko, Institute of Physics PAS (Poland).
- H. Buhmann, Wuerzburg University (Germany).
- C. Felser, University of Mainz (Germany).
- E. Frantzeskakis, University of Amsterdam (The Netherlands)
- M. Kaminska, University of Warsaw (Poland).
- J. Moodera, MIT (USA).
- O. Tjernberg, KTH Stockholm (Sweden)
- A. Wojs, Wroclaw University of Technology (Poland)
- J. Zaanen, Leiden University (The Netherlands)
The symposium will be co-organized by the EU 7th Framework Programme under the project REGPOT-CT-2013-316014 (EAgLE)
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Authors : V.K. Lazarov, Y. Liu, D. Gilks, L. Lari, A. Ghasemi, , Q. Ramasse, D. Kepaptsoglou, M. Guerrero-Lebrero, P. L. Galindo, M. Weinert, and L. Li
Affiliations : University of York; University of Wisconsin-Milwaukee; SuperSTEM; University of Cadiz
Resume : Tailoring the functionality of 3D topological insulators in thin films and heterostructures is strongly correlated to ability on atomic scale to control their surface and interface structures. Hence understanding the growth mechanism and atomic structure of thin films growth defects is crucial for future device applications of the 3D TIs. In this work, on the case of Bi2Se3, we show how defects such as antiphase domain boundaries and misfit edge dislocations modify Dirac surface states. Based on in-situ scanning tunnelling microscopy and scanning-transmission electron microscopy we show that MBE grown Bi2Se3 initiates with two-dimensional nucleation, and that spiral growth ensues with pinning of the 2D growth fronts at jagged steps of the substrate. Coalescence of the film grains results in grain boundaries with modified atomic surface structure. In particular, the low-angle tilt grain boundaries, consisting of arrays of alternating edge dislocation pairs, are of special interest since these dislocations introduce periodic in-plane compressive and tensile strains. From tunnelling spectroscopy experiments and first-principles calculations, we find that whereas the energy of the Dirac state shifts in regions under tensile strain, a gap opens in regions under compressive strain, indicative of the destruction of the Dirac states at the surface. These results demonstrate that Dirac states can be tuned by strain at the atomic scale.
Authors : C. Rinaldi, S. Bertoli, M. Cantoni, R. Bertacco, J. Krempasky, H. Dil, I. Vobornik, G. Panaccione, R.-N. Wang, J. Boschker, A. Giussani, R. Calarco, D. Di Sante, S. Picozzi
Affiliations : Department of Physics - Politecnico di Milano, Milano, Italy; Swiss Light Source at the Paul Scherrer Institut, Villigen, Switzerland; TASC Laboratory - Elettra Synchrotron IOM-CNR, Trieste, Italy; Paul-Drude-Institut für Festkörperelektronik, Berlin, Germany; CNR-SPIN, L'Aquila, Italy;
Resume : Germanium telluride (GeTe) belongs to the new class of materials called FerroElectric Rashba SemiConductors (FERSCs) . The remanent ferroelectric polarization vector breaks the inversion symmetry in GeTe and leads to a giant bulk Rashba spin splitting of the bands , which can be controlled via external electric field, providing a unique way to modulate the spin transport properties in novel spintronic devices with non-volatile logic functions associated with remanent ferroelectric states. In this paper, by piezo-force microscopy measurements we demonstrate the presence of an intrinsic remanent outward ferroelectric polarization, which provides the inversion symmetry breaking needed to observe a net Rashba effect. By angular resolved photoemission spectroscopy we provide evidence for a huge Rashba splitting of the valence band in GeTe(111) thin films . The bands map and the k-splitting are in nice agreement with DFT calculations . Finally, preliminary electrical measurements of the spin-Hall effect in fully epitaxial Fe/GeTe(111) heterostructures suggest that GeTe can be used as source and modulator of spin polarized currents. This work provides the experimental proof of the existence FERSC materials displaying bulk Rashba and paves the way to their use in novel spinorbitronics devices.  S. PICOZZI et al., FRONTIERS IN PHYSICS 2, 10 (2014)  DI SANTE et al., ADV. MAT. 25, 509-513 (2013)  A. GIUSSANI et al., PHYS. STATUS SOLIDI B 249, 19391944 (2012)
Authors : A. Hruban1, A1. Materna1, S.G. Strzelecka1, M. Piersa1, E. Jurkiewicz-Wegner1, W. Orłowski1, W. Dalecki1, R. Diduszko1, M. Kamińska2, A. Wołoś2
Affiliations : 1 Instytut Technologii Materiałów Elektronicznych, ul. Wólczyńska 133, 01-919 Warszawa; 2 Instytut Fizyki Doświadczalnej, Uniwersytet Warszawski, ul. Hoża 69, 00-681 Warszawa
Resume : We report on crystal growth and electrical characterization of binary and ternary topological insulators Bi2Te3, Bi2Se3, and Bi2Te2Se obtained by the modified vertical Bridgman technique. These crystals should present p-type conductivity and high resistivity at low temperatures [1,2]. We obtained undoped Bi2Te3 crystals of resistivity 0.003 ohmcm at T=10 K as well as p-type Bi2Se3:Ca crystals with carrier concentration (3 - 6) x 10^17 cm-3 and resistivity 0.12 ohmcm at T=10 K . Bi2Te2Se crystals displayed high resistivity (4 - 8) ohmcm and hole concentration 3 x 10^15 cm-3 at 10 K. These parameters were achieved as a result of thermal annealing in the solid state [4,5]. Measurements of resistivity vs. temperature of ternary BTS crystals revealed semiconductor behavior below T < 200 K. Observed saturation of resistivity at low temperatures is attributed to metallic conductivity component of the surface state of topological insulator.  S. Jia, H. Ji, E. Climent-Pascual, M. K. Fuccillo, M. E. Charles, Jun Xiong, N.P. Ong, R.J. Cava, Phys. Rev B 84, 235206 (2011).  Z. Ren, A.A. Taskin, S. Sasaki, K. Segawa, Y. Ando, Phys. Rev B 82, 241306 (R), (2010).  A. Hruban, A. Materna, G. Strzelecka, et al., Acta Phys. Pol. A 120, 950 (2011).  O.B. Sokolov, S.Y. Skipidarov, N.I. Duvankov, G.G. Shabunina, J. Crystal Growth 262, 442-448 (2004).  S.N. Chizhevskaja, L.E. Schelimova, W.I. Kosjakov, W.A. Schestakov: Inorganic Materials 33, 903-911 (1997).
Authors : M. Dobrzański1, M. Waśniowska2, M. Sikora1,3, T. Eelbo3, M.M. Soares,5, M. Rams6, I. Miotkowski7, R. Wiesendanger3, Z. Kąkol1, and A. Kozłowski,1
Affiliations : 1Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, al. Mickiewicza 30, 30059 Cracow, Poland 2Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany 3Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, al. Mickiewicza 30, 30059 Cracow, Poland 4Institute of Applied Physics, University of Hamburg, Jungiusstr. 11, D-20355 Hamburg, Germany 5 ESRF - The European Synchrotron, CS 40220, 38043 Grenoble, France 6 Institute of Physics, Jagiellonian University, Reymonta 4, 30059 Cracow, Poland 7Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, Indiana, USA
Resume : Metallic surface of topological insulators (TI) essentially differs from that of isostructural, but non-TI materials and a simple test of this difference is still necessary. We treat the electronic structure of Co adatoms as a probe to observe the difference between surfaces of topological insulators, such as Bi2Se3, Bi2Te3 and Bi1.98Fe0.02Se3, and non-topological insulators, here represented by the magnetically ordered Bi1.9Mn0.1Te3. The parent materials electronic states were inspected by STM and STS. The existence of two distinct Co positions on the surface of this class of compounds was confirmed and differences between local density of electronic states on these two positions were found. Also, the Dirac cone was found on TI, including Fe-doped Bi2Se3, while the evident gap was observed by STS on the non-TI Bi1.9Mn0.1Te3. Electronic structure of Co adatoms were measured by XAS and SXMCD. We found that the global electronic properties of Co are different depending on the surface they were deposited on, but the main difference is the in-plane easy axis for Co adatoms for all but the ferromagnetic Bi1.9Mn0.1Te3 surface where the out-of-plane easy axis was found. We think that not the magnetism of Mn doped Bi2Te3, but the peculiar electronic properties of TI materials resulting in strongly enhanced surface magnetic interactions are responsible for this effect. If independently confirmed, it might mean that the very clear indicator for the Dirac surface states was found.
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Authors : A. Wolos, A. Drabinska, M. Kaminska, A. Hruban, S. G. Strzelecka, A. Materna, M. Piersa
Affiliations : Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland; Faculty of Physics, University of Warsaw, ul. Hoza 69, 00-681 Warsaw, Poland; Institute of Electronic Materials Technology, ul. Wólczyńska 133, 01-919 Warsaw, Poland
Resume : Three-dimensional topological insulators, Bi2Te3, Bi2Se3, and Bi2Te2Se, were grown by vertical Bridgman method. The influence of the growth from non-stoichiometric melt and doping with Ca and Mn acceptors on carrier concentration and crystal morphology will be presented. The doping with acceptors reduces successfully initially high carrier concentration in Bi2Se3, originating from native crystal lattice defects. Strong doping, however, leads to phase separation, and technological compromise needs to be maintained in order to obtain desired crystal quality. Microwave spectroscopy study was performed using conventional Electron Paramagnetic Resonance spectrometer on samples with the Fermi level engineered. Paramagnetic resonance signals (conduction electron spin resonance, shallow acceptor resonance, Mn high-spin resonance), together with signals related to magnetoconductivity (cyclotron resonance, weak localization and anti-localization, Shubnikov-de Haas oscillations) allowing investigations of electron transport properties were detected. The distinction of the signals originating from bulk and surface states will be presented and discussed. Funded by NCN, grant No. 2011/03/B/ST3/03362.
Session 6 : chair - Rui-Rui Du
Authors : Christoph Bruene
Affiliations : Department of Physics, Wuerzburg University, 97074 Wuerzburg, Germany
Resume : HgTe is a II-VI zinc blende crystal with an inverted band ordering of the Г6 and Г8 bands. The band inversion enables the observation of topological insulator (TI) physics in this material. Both 2- and 3-dimensional TIs can be realized by growing quantum well structures or strained bulk layers, respectively. Due to the well-established MBE growth of such layers bulk carrier contributions are negligible and the carrier mobilities exceed 100000 cm²/Vs in HgTe TI structures. Here we report on the transport properties of the topological edge and surface states in HgTe quantum wells and strained bulk HgTe layers. 2-dimensional TIs are characterized by the formation of two counter-propagating oppositely spin polarized edge states, the so called quantum spin Hall effect. The transport signatures of this edge states will be discussed, especially regarding their spin polarization. 3-dimensional TIs exhibit 2 dimensional Dirac surface states. The topological surface states in strained bulk HgTe are investigated utilizing the quantum Hall effect.
Authors : M. Majewicz1, G. Grabecki1;2 , J. Wr?bel1;3, M. Czapkiewicz1, Ł. Cywiński1, M. Papaj7, S. Gierałtowska1, M. Godlewski1,2, M. Zholudev4;5, V. Gavrilenko5, N. N. Mikhailov6, S. A. Dvoretski6, W. Knap4, F. Teppe4 and T. Dietl1;7;8
Affiliations : 1Institute of Physics, Polish Academy of Sciences, al. Lotnik?w 32/46, PL-02 668 Warszawa, Poland; 2Department of Mathematics and Natural Sciences, College of Sciences, Cardinal Wyszyński University, ul. W?ycickiego 1/3, PL 01-938 Warszawa, Poland; 3 Faculty of Mathematics and Natural Sciences, Rzesz?w University, al. Rejtana 16A, 35-959 Rzesz?w, Poland; 4L2C, UMR No5221 CNRS, Universit? Montpellier 2, GIS-TERALAB, F-34095 Montpellier, France; 5Institute for Physics of Microstructures, Russian Academy of Sciences, GSP-105, Nizhny Novgorod, 603950, Russia; 6Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, pr. Lavrentieva 13, Novosibirsk, 630090, Russia; 7Institute of Theoretical Physics, Faculty of Physics, University of Warsaw, ul. Hoza 69, PL-00 681 Warszawa, Poland; 8WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
Resume : We present electron transport data for Hall microbridges and submicrometer constrictions of HgCdTe/HgTe/HgCdTe quantum wells (QWs) of the width d=8 nm which is 2D topological insulator. Both types of samples are patterned from In-modulation doped QWs, grown by MBE on semi-insulating GaAs  substrates . In the microbridges where the edge channel length is of the order of 100 microns, nonlocal resistances data prove that in the depletion regime the current is carried by the edge states only . However, high and nonquantized values of channel resistances show that the topological protection length is much shorter than 100 microns. For the constrictions, we have observed large quasiperiodic reproducible conductance fluctuations as a function of the gate voltage. Surprisingly, the resistance measured as a function of the magnetic field does not show such fluctuations. We explain these findings in terms of the charge puddles in QW, to which the electrons from the edge channels are tunnel-coupled and give rise to aperiodic Coulomb blockade oscillations. The puddles are formed due to the potential fluctuations in QW. Electrons trapped by the puddles lose their spin memory and then may be backscattered which explains the high channel resistances. The research was partially supported by Regional Development Program (Poland), grant WND-RPPK. 01.03.00-18-053/12.  S. Dvoretsky, et al., J. Electron. Mat., 39, 918 (2010);  G. Grabecki et al., Phys. Rev. B88, 165309 (2013).
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Authors : L. Plucinski, M. Eschbach, E. Mlynczak, J. Kellner, M. Gehlmann, S. Döring, P. Gospodaric, J. Kampmeier, C. Weyrich, T. Schäpers, G. Mussler, N. Demarina, D. Grützmacher, M. Morgenstern, C. M. Schneider
Affiliations : Peter Gruenberg Institute, Forschunszentrum Juelich, Germany
Resume : Band structure engineering in 3D topological insulators has been in focus of intense research activities since several years. Various ternary compounds have been characterized in search for intrinsic semiconducting characteristics. Furthermore, electric biasing has been used to alter the population of the topological surface state, and therefore control the position of the Fermi level within the fundamental gap. We will present a thorough experimental and theoretical study of the topological p-n junction created out of Sb2Te3  and Bi2Te3  in thin film geometry by means of the MBE. In particular we will discuss the competing effects due to the built-in electric field and the interfacial intermixing.  L. Plucinski, et al, J. Appl. Phys. 113, 053706 (2013), doi: 10.1063/1.4789353.  A. Herdt et al. Phys. Rev. B 87, 035127 (2013), doi: 10.1103/PhysRevB.87.035127.
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Session 9 : chair - J.-P. Paglione
Authors : P.D.C. King, Y.F. Nie, K.E. Arpino, D.C. Wallace, T. Birol, C.J. Fennie, K.M. Shen, T.M. McQueen
Affiliations : University of St Andrews, Cornell University, Johns Hopkins University, Johns Hopkins University, Cornell University, Cornell University, Cornell University, Johns Hopkins University
Resume : There is an intense current search to identify new topological compounds, of which topological superconductors are an elusive category. Here, we report the observation by angle-resolved photoemission of Dirac-like surface states in the perovskite superconductor Tl5Te3, as well as its non-superconducting Sn-doped sister compound. We show evidence from ARPES that these are spin-polarised, consistent with our calculations which suggest them to be of topological origin, opening new opportunities to probe the interplay of topological order with bulk superconductivity. We will compare and contrast this first non-trigonal topological system with the much better studied Bi2Se3 class.
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