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A New Spin on Photoemission Spectroscopy Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title A New Spin on Photoemission Spectroscopy Permalink https://escholarship.org/uc/item/54b701zz Author Jozwiak, Chris Publication Date 2010-06-11 Peer reviewed eScholarship.org Powered by the California Digital Library University of California A New Spin on Photoemission Spectroscopy by Christopher Matthew Jozwiak B.S. (Duke University) 2001 M.A. (University of California, Berkeley) 2005 A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the GRADUATE DIVISION of the UNIVERSITY OF CALIFORNIA, BERKELEY Committee in charge: Professor Alessandra Lanzara, Chair Professor Dung-Hai Lee Professor Junqiao Wu Fall 2008 The dissertation of Christopher Matthew Jozwiak is approved. Chair Date Date Date University of California, Berkeley Fall 2008 A New Spin on Photoemission Spectroscopy Copyright c 2008 by Christopher Matthew Jozwiak Abstract A New Spin on Photoemission Spectroscopy by Christopher Matthew Jozwiak Doctor of Philosophy in Physics University of California, Berkeley Professor Alessandra Lanzara, Chair The electronic spin degree of freedom is of general fundamental importance to all matter. Understanding its complex roles and behavior in the solid state, particularly in highly correlated and magnetic materials, has grown increasingly desirable as technology demands advanced devices and materials based on ever stricter comprehension and control of the electron spin. However, direct and efficient spin dependent probes of electronic structure are currently lacking. Angle Resolved Photoemission Spectroscopy (ARPES) has become one of the most successful experimental tools for elucidating solid state electronic structures, bolstered by continual breakthroughs in efficient instrumentation. In contrast, spin-resolved photoemis- sion spectroscopy has lagged behind due to a lack of similar instrumental advances. The power of photoemission spectroscopy and the pertinence of electronic spin in the current research climate combine to make breakthroughs in Spin and Angle Resolved Photoemission Spectroscopy (SARPES) a high priority . This thesis details the development of a unique instrument for efficient SARPES and rep- resents a radical departure from conventional methods. A custom designed spin polarimeter based on low energy exchange scattering is developed, with projected efficiency gains of two orders of magnitude over current state-of-the-art polarimeters. For energy analysis, the 1 popular hemispherical analyzer is eschewed for a custom Time-of-Flight (TOF) analyzer of- fering an additional order of magnitude gain in efficiency. The combined instrument signifies the breakthrough needed to perform the high resolution SARPES experiments necessary for untangling the complex spin-dependent electronic structures central to today’s condensed matter physics. Chapter 1 provides an introduction to the technique with background on electron spin, its important interactions, and the benefits of spin-resolved experiments. Chap- ter 2 contains an in-depth review of ARPES, and Chapter 3 reviews the addition of spin-resolution. Chapters 4 and 5 detail the full development of the new SARPES instrument, and Chapter 6 presents the complete spectrometer and its initial promis- ing results. Chapter 7 gives a brief conclusion with a discussion of future directions. Professor Alessandra Lanzara Dissertation Committee Chair 2 To my family and especially my wife. i Contents Abstract 1 Contents v List of Figures vi Acknowledgments ix Curriculum vitae xiii 1 Introduction 1 1.1 Expansion of electron spectroscopy ....................... 1 1.2 Spin - the ups and downs of being an electron ................. 3 1.2.1 Almost discovery - the Stern-Gerlach experiment ........... 3 1.2.2 Discovery - Uhlenbeck and Goudsmit .................. 8 1.2.3 Spin-orbit coupling ............................ 11 1.2.4 Symmetry of spin ............................. 14 1.2.5 Exchange coupling ............................ 18 1.3 Probing spin in solids .............................. 23 2 Photoemission Spectroscopy 26 2.1 The photoelectric effect ............................. 27 2.2 Energy resolution ................................. 32 2.3 Angular resolution ................................ 36 2.4 Single particle spectral function ......................... 43 2.5 EDC and MDC analysis ............................. 47 2.6 Dipole matrix elements .............................. 48 ii 2.7 State-of-the-art ARPES ............................. 50 2.7.1 Spectrometer ............................... 51 2.7.2 Ultra-high vacuum system ........................ 61 2.7.3 Sample manipulation ........................... 65 2.7.4 Light source ................................ 66 3 Spin-Resolved Photoemission Spectroscopy 69 3.1 Three-step model revisited ............................ 70 3.1.1 Step one .................................. 70 3.1.2 Steps two & three ............................ 72 3.2 Spin resolved PES from non-magnetic samples ................ 74 3.3 Spin resolved PES from ferromagnets ...................... 78 3.4 Current state-of-the-art technique ........................ 80 3.5 Novel approach .................................. 85 4 Time of Flight Spectroscopy 88 4.1 TOF basics .................................... 89 4.2 TOF advantages ................................. 94 4.2.1 Efficiency ................................. 94 4.2.2 Sensitivity ................................. 95 4.2.3 Ultimate resolution ............................ 96 4.3 Previous uses of TOF .............................. 96 4.4 Prototype TOF .................................. 99 4.4.1 MCP detector ............................... 99 4.4.2 Timing electronics ............................ 105 4.4.3 Lens column ............................... 109 4.4.4 Beamtime, data acquisition, and time-to-energy conversion ..... 111 4.4.5 A few remarks on TOF overlap ..................... 119 4.5 Electrostatic lens system ............................. 121 4.5.1 General design considerations ...................... 121 4.5.2 Magnetic shielding ............................ 124 4.5.3 Mechanical and material considerations ................ 128 4.5.4 Assembly and inspection ......................... 131 4.6 Summary ..................................... 135 iii 5 Spin Polarimetry 136 5.1 Spin Polarization ................................. 136 5.2 Spin-orbit polarimetry .............................. 142 5.2.1 Mott scattering .............................. 142 5.2.2 Polarimeter efficiency .......................... 146 5.2.3 Mott polarimeter design and usage ................... 148 5.2.4 Other techniques ............................. 151 5.3 Exchange polarimetry .............................. 154 5.3.1 Exchange scattering ........................... 154 5.3.2 Qualitative understanding of exchange asymmetry .......... 159 5.3.3 More details: spin-orbit contributions ................. 160 5.3.4 More details: instrumental asymmetry ................. 164 5.3.5 Previous exchange polarimeters ..................... 166 5.4 New exchange polarimeter ............................ 167 5.4.1 Target characterization ......................... 168 5.4.2 Scattering geometry and detection ................... 174 5.4.3 Full polarimeter layout .......................... 179 5.4.4 Target manipulation and magnetization ................ 181 5.5 Summary ..................................... 185 6 SARPES with the spin-TOF spectrometer 186 6.1 The spectrometer ................................. 186 6.1.1 Combining the TOF lens system and exchange polarimeter ..... 186 6.1.2 Instrumental control ........................... 190 6.1.3 Experimental endstation ......................... 193 6.1.4 Bandpass filter details .......................... 193 6.2 Spin-resolved ARPES .............................. 197 6.2.1 Sample preparation ........................... 197 6.2.2 Fe(110) .................................. 200 6.2.3 Au(111) surface state .......................... 206 6.3 Summary ..................................... 211 7 Outlook, applications, and conclusions 213 7.1 Future outlook .................................. 213 iv 7.2 Exciting applications ............................... 214 7.2.1 CMR manganites ............................. 215 7.2.2 Rashba effect ............................... 219 7.2.3 Time-resolved spin dynamics and phase transitions .......... 221 7.3 Conclusions .................................... 222 Bibliography 224 v List of Figures 1.1 Angular momentum and space quantization .................. 4 1.2 The Stern-Gerlach experiment .......................... 6 1.3 SGE original results ............................... 7 1.4 Identical particles ................................. 15 2.1 Hertz’s photoelectric effect ............................ 28 2.2 Early photoemission instruments of Lenard and Millikan ........... 29 2.3 Early photoemission experiment ........................ 31 2.4 Energy diagram of photoemission spectroscopy ................ 33 2.5 Examples of Siegbahn’s ESCA ......................... 35 2.6 ARPES schematic and geometry ........................ 38 2.7 Universal curve of electron mean free paths .................. 39 2.8 Example of ARPES bandmapping ....................... 42 2.9 Spectral function picture of ARPES ...................... 46 2.10 EDC and MDC ARPES analysis ........................ 48 2.11 Hemispherical analyzer schematic ........................ 53 2.12 Operation of a hemispherical analyzer ....................
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