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Photoemission Spectroscopy Studies of New Topological PHOTOEMISSION SPECTROSCOPY STUDIES OF NEW TOPOLOGICAL INSULATOR MATERIALS A DISSERTATION IN Physics and Chemistry Presented to the Faculty of the University of Missouri-Kansas City in partial fulfillment of the requirements for the degree DOCTOR OF PHILOSOPHY by ANDREW PATTON WEBER B.S. University of South Alabama, 2011 M.S. University of Missouri-Kansas City, 2013 Kansas City, Missouri 2015 © 2015 ANDREW PATTON WEBER ALL RIGHTS RESERVED PHOTOEMISSION SPECTROSCOPY STUDIES OF NEW TOPOLOGICAL INSULATOR MATERIALS Andrew Patton Weber, Candidate for the Doctor of Philosophy Degree* University of Missouri-Kansas City, 2015 ABSTRACT As the size of a solid shrinks, the ratio of surface area to bulk volume grows and surface effects become more important. In a world where technologies advance with the shrinking size of electronic devices, one phase of matter has emerged which is fit for the near future of surface-dominated performance. Moreover, it has brought a new set of ideas to solid-state physics and chemistry, especially the understanding that the discipline of topology can be applied to classify the electron band structures. The topological insulator phase yields an exotic metal surface state in which the orientation of the electron’s spin is locked perpendicular to its momentum. This property suppresses backscattering (making it possible to pass spin-polarized currents through the material without loss), offers a crucial ingredient for innovative approaches to quantum computation, and provides the basis for observing unique magnetoelectric effects. However, the surface states of materials in the topological insulator phase can wildly differ, so it is of interest to systematically characterize new materials to understand how the structure in position-space is related to the spin-resolved structure of electrons in energy- and momentum-space. We will discuss this relationship as it is probed through spin- and angle-resolved photoemission spectroscopy experiments on three topological (Bi2)m(Bi2Se3)n superlattices: (a) Bi2Se3 (m = 0, n = 1), (b) Bi4Se3 (m = 1, n = 1), and (c) BiSe (m = 1, n = 2). Our studies have not only proven the topological nature of these iii materials, but also demonstrate how bulk band structure and polar chemical bonding control the surface metal’s concentration, dispersion, and spin-orbital character. Case (a) is considered to provide an ideal model of the topological surface metal. Case (b) provides the three important findings: (1) the chemical identity of the surface-termination controls the orbital composition and energy distribution of the surface states, (2) there are two topological states in sequential bulk band gaps, (3) of these, one of topological state undergoes a hybridization effect that yields a momentum-dependent gap in the band structure as large as 85 meV. Case (c) has a practical significance in that the surface metal has a potentially record-breaking carrier density of ~1013cm−2 (estimated from the Fermi surface area), more than an order of magnitude higher than in Bi2Se3. This occurs as a result of charge transfer from the Bi2 layers to the Bi2Se3 layers. iv APPROVAL PAGE The faculty listed below, appointed by the Dean of the School of Graduate Studies, have examined a dissertation titled “Photoemission Spectroscopy Studies of New Topological Insulator Materials”, presented by Andrew Patton Weber, candidate for the Doctor of Philosophy degree, and certify that in their opinion it is worthy of acceptance. Supervisory Committee Anthony Caruso, Ph.D., Committee Chair Department of Physics and Astronomy Paul Rulis, Ph.D. Department of Physics and Astronomy Elizabeth Stoddard, Ph.D. Department of Physics and Astronomy Nathan Oyler, Ph.D. Department of Chemistry David Van Horn, Ph.D. Department of Chemistry v CONTENTS ABSTRACT ............................................................................................................................ iii LIST OF ILLUSTRATIONS ................................................................................................. vii LIST OF TABLES .................................................................................................................. xi ACKNOWLEDGEMENTS ................................................................................................... xii Chapter Page 1. INTRODUCTION ............................................................................................................. 1 2. ELECTRON SPECTROSCOPIES APPLIED TO BISMUTH CHALCOGENIDES ..... 23 3. TOPOLOGICAL SEMIMETAL COMPOSED OF BISMUTH-BILAYERS AND BISMUTH SELENIDE LAYERS STACKED IN A 1:1 RATIO ................................... 68 4. TOPOLOGICAL INSULATOR COMPOSED OF BISMUTH-BILAYERS AND BISMUTH SELENIDE LAYERS STACKED IN A 1:2 RATIO ................................. 112 5. SUMMARY AND OUTLOOK ..................................................................................... 136 BIBLIOGRAPHY ................................................................................................................ 141 VITA ................................................................................................................................... 148 vi LIST OF ILLUSTRATIONS Figure Page 1.1 Illustration of the Rashba effect in a 2DEG ....................................................................... 8 1.2 Electronic structure of an idealized topological metal ....................................................... 9 1.3 Sketch of real-space and reciprocal-space lattices for a hexagonally packed atoms ....... 15 1.4 Schematic of the band inversion process ......................................................................... 19 1.5 Cartoon of Bloch wave symmetry varying with reciprocal-lattice-translation ................ 20 2.1 Cartoon of the density of states in a Bi-chalcogenide topological insulator .................... 26 2.2 “Universal curve” of the electron inelastic mean free path ............................................. 27 2.3 Cartoon showing the evolution of photoelectron kinetic energies as they move from sample to detector ............................................................................................................ 30 2.4 Kinematics of photoelectron refraction ........................................................................... 33 2.5 ARPES band mapping of Bi2Se3 .................................................................................... 35 2.6 Comparison of the three-step and one-step models of photoemission ............................ 37 2.7 Electron energy-momentum structure in the three-step model ........................................ 38 2.8 Photoionization cross sections for Bi and Se ................................................................... 41 2.9 Experimental geometry for photoemission employing linearly polarized light and mirror symmetry selection rules.................................................................................................. 42 2.10 Spin-resolved ARPES of hexagonally warped topological surface states on Bi2Te3 and TlBiSe2 ............................................................................................................................. 45 2.11 Orbital-selective SARPES results from the literature ................................................. 51 2.12 Drawing of the relationship between light polarization and probed spin-texture ....... 51 2.13 Temperature-dependent ARPES results for the Bi2Se3 surface state .......................... 53 vii 2.14 Cartoon of the U5UA beamline layout ........................................................................ 56 2.15 Distribution of photon intensity versus energy for the U5UA beamline with the undulator gap set to 45 mm .............................................................................................. 58 2.16 Schematic drawing of the Scienta ARPES analyzer ................................................... 59 2.17 Schematics of the photoemission experimental geometry .......................................... 61 2.18 View of the Scienta multichannel plate and transfer lens apertures ............................ 62 2.19 Views of the transfer lenses and fully assembled analyzer at U5UA ......................... 63 2.20 Schematic of the mini-Mott polarimeter design .......................................................... 64 2.21 Pictures of the one of the spin detectors and its components ...................................... 66 2.22 Spin-resolved ARPES measurements of the Bi2Se3 surface state using the analyzer at U5UA ............................................................................................................................... 67 3.1 Unit Cell and Brillouin zone of Bi4Se3 ............................................................................ 70 3.2 Schematic crystal structure and micrographs of the surface of Bi4Se2.6S0.4 .................... 72 3.3 Core-level and valence band spectra of Bi4Se2.6S0.4 and Bi2Se3 ...................................... 74 3.4 Spin- and momentum-resolved electronic structure of Bi4Se2.6S0.4 ................................. 76 3.5 Calculated bulk electronic band structure of Bi4Se3 ........................................................ 77 3.6 Cartoons of homogenously cleaved and mixed-termination Bi4Se3 ................................ 80 3.7 Energy diagrams of two isolated dissimilar metals and a junction between two dissimilar metals ..............................................................................................................................
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