Principles of Electron Tunneling Spectroscopy

Transcription

1 Principles of Electron Tunneling Spectroscopy Second Edition E. L. Wolf Polytechnic Institute of New York University, USA OXFORD UNIVERSITY PRESS

2 Contents 1 Introduction 1.1 Concepts of quantum mechanical tunneling 1.2 Occurrence of tunneling phenomena 1.3 Electron tunneling in solid-state structures 1.4 Superconducting (quasipartide) and Josephson (pair) tunneling 1.5 Tunneling spectroscopies 1.6 The scanning tunneling microscope (STM): spectroscopic images 1.7 Atomic spatial resolution in the scanning tunneling microscope 1.8 Density of electron states (DOS) measurement in STM: STS 1.9 Perspective, scope, and organization 2 Tunneling in normal-state structures: I 2.1 Introduction 2.2 Calculational methods and models Stationary-state calculations Transfer Hamiltonian calculations Ideal barrier transmission 2.3 Basic junction types Metal-insulator-metal junctions Metal-insulator-semiconductor junctions Schottky barrier junctions pn junction (Esaki diode) direct case and the Si-Ge diode Vacuum tunneling Vacuum tunneling from a spherical STM tip 2.4 Dependence of J(V) and G(V) on band structure and density of states Fermi surface integrals Prefactors: wavefunction matching at boundaries 2.5 Nonideal barrier transmission Approach to ideal behavior Resonant barrier levels Two-step tunneling Barrier interactions 2.6 Assisted tunneling processes 2.7 Comments on the time for tunneling 2.8 Resolution obtained from a scanning tunneling microscope tip Tersoff and Hamann's model of STM resolution C. Julian Chen's atomic model of STM resolution 3 Spectroscopy of the superconducting energy gap: quasipartide and pair tunneling 3.1 Basic experiments of Giaever and Josephson tunneling 3.2 Superconductivity 3.3 Electron-phonon coupling and the BCS theory xi

3 3.3.1 The pair ground state Elementary excitations of superconductors Generalizations of BCS theory Theory of quasiparticle and pair tunneling Gap spectra of equilibrium BCS superconductors Gap spectra in more general homogeneous equilibrium superconductor cases Strong-coupling superconductors Gap anisotropy Multiple gaps, two-band superconductivity Excess currents, subharmonic structure Effects of magnetic field Magnetic impurities Pressure effects Interactions with electromagnetic radiation Superconducting fluctuations Ultrathin-film and small-particle superconductors Transition from tunnel junction to metallic contact Model of Klapwijk, Blonder, and Tinkham 171 Conventional tunneling spectroscopy of strong-coupling superconductors Introduction Eliashberg-Nambu strong-coupling theory of superconductivity Tunneling density of states Quantitative inversion for a 2 F(<w): test of Eliashberg theory Extension to more general cases Finite temperature Anisotropy Spin fluctuations Electronic density-of-states variation Limitations of the conventional method 194 Inhomogeneous superconductors: the superconducting proximity effect Introduction: continuity of the pair wavefunction Andreev reflection and specular SNS junctions Survey of phenomena in proximity tunneling structures Specular theory of tunneling into proximity structures McMillan's tunneling model of bilayers The Usadel equations and diffusive SNS junctions Reduction of Gor'kov's equations by Eilenberger and Usadel Application of reduced Gor'kov theory to tunneling problems The experiment of Truscott and Dynes confirming the bound state in clean NS junctions The experiment of le Sueur et al.: phase dependence of the density of states Proximity effects in a ferromagnetic N layer, in an NS structure Proximity electron tunneling spectroscopy (PETS) Effects of elastic scattering in the N layer Proximity corrections to conventional results Further applications of proximity effect models 251

4 xiii 6 Superconducting phonon spectra and a 2 F(co) Introduction s-p band elements Crystalline s-p band alloys and compounds Crystalline s-p band alloy superconductors s-p band compounds Amorphous metals Transition metals, alloys, and compounds Extreme weak-coupling metals Local-mode and resonance-mode superconductors Systematics of superconductivity Effects of external conditions and parameters on strong-coupling features Eliashberg inversion of bismuthate and cuprate superconductor tunneling data High-7^ electron-coupled superconductivity: cuprate and iron-based superconductors The discovery of cuprate superconductivity by Bednorz and Müller The Mott antiferromagnetic CuÜ2 insulator and its doping to a metal Paired holes in copper oxide planes Hubbard and t-j models in two dimensions Hole-doped cuprates Bi2212 and YBCO Phase diagram for superconductivity in hole-doped cuprate Crystal structures of common cuprates: I Early tunneling measurements on hole-doped superconductors Crystal structures of common cuprates: II Range of T c vs. number of copper oxide planes Disorder sites and doping of cuprate superconductors Comments on disorder and inhomogeneity in STS images Andreev-St. James tunneling spectroscopy Experimental signatures of nodal superconductivity Specific heat at transition Josephson junctions in d-wave cases Further examples of non-bcs superconductors Tunneling in normal-state structures: II Introduction Final-state effects: I Two-dimensional final states Quantum size effects in metal films Accumulation layers at semiconductor surfaces Spin-polarized tunneling as a probe of ferromagnets Julliere's model of ferromagnetic tunnel junctions Other bulk band structure effects Assisted tunneling: threshold spectroscopies Phonons Inelastic electron tunneling spectroscopy of molecular vibrations Inelastic excitations of spin waves (magnons) Inelastic excitation of surface and bulk plasmons Light emission by inelastic tunneling Spin-flip and Kondo scattering Excitation of electronic transitions 378

5 8.4 Final-state effects: II More general many-body theories of tunneling Tunneling studies of electron correlation and localization in metallic systems Phonon self-energy effects in degenerate semiconductors Electron scattering in the Kondo ground state Zero-bias anomalies Giant resistance peak Semiconductor conductance minima Assorted maxima and minima in metals The Giaever-Zeller resistance peak model Scanning tunneling spectroscopy (STS) of single atoms and molecules Theory of observation of single atoms in STS and experiment Friedel oscillations in 2-D surface state Effect of surface state: inference of wavevector Fourier-transform STM/STS Quantum corrals Elliptical corrals and focusing effects: quantum mirage Pair-breaking single adatoms on superconductors Mn and Cr on Pb Zn impurity atoms imaged in cuprate planes Spectroscopy of Kondo and spin-flip scattering Introduction Kondo spectroscopy of a single trapped electron Spectroscopy of localized moments in Si: As Schottky junctions Comparison of the two Kondo spectroscopy experiments STM spectroscopy of magnetic adatoms Molecules and their vibrational spectra Scanning tunneling spectroscopy of superconducting cuprates and magnetic manganites Gap imaging of optimally doped cuprates Site dependence of apparent gap Overdoped case Anticorrelation of gap and zero-bias density of states Internal proximity effect Localized state at Zn impurity Model for spectral distortions of noncuprate layers Superlattice modulation in Bi Fourier-transform STS (FT-STS) and application Observations of charge ordering in cuprate superconductors Relation of STS to angle-resolved photoemission spectroscopy (ARPES) Evidence for electron-spin wave coupling Colossal magnetoresistance: Mott transition in doped manganites Introduction: mechanism of colossal magnetoresistance (CMR) Pseudogap in manganite LSMO observed by ARPES Relation of cuprates to ferromagnetic CMR manganites Applications of barrier tunneling phenomena Introduction Josephson junction interferometers 477

6 11.3 SQUID detectors: the scanning SQUID microscope Establishing d-wave nature of cuprate pairing 11.4 Josephson junction logic: rapid single-flux quantum devices The single-flux quantum voltage pulse Analog to digital conversion (ADC) using RSFQ logic 11.5 Detection of radiation SIS detectors Josephson effect detectors Optical point-contact antennas (high-speed MIM junctions) 11.6 Tunnel-junction magnetoresistance sensors Appendix A Experimental methods of junction fabrication and characterization A. 1 Thin-film electrodes A. 1.1 Evaporated films A. 1.2 Film thickness measurement A. 1.3 Substrate temperature A. 1.4 Sputtered films A. 1.5 Chemical vapor-deposited films A. 1.6 Epitaxial single-crystal films A. 1.7 Atomic layer deposition A.2 Foil and single-crystal electrodes A.3 Characterization of tunneling electrodes A.4 Preparation of oxide tunneling barriers A.4.1 Thermal oxide barriers A.4.2 Plasma oxidation processes A.5 Artificial barriers A.5.1 Totally oxidized metal overlayers A.5.2 Directly deposited artificial barriers A.5.3 Polymerized organic films A.6 Point-contact barrier tunneling methods A.6.1 Anodized metal probes A.6.2 Schottky barrier probes A.6.3 Deformable metal vacuum tunneling probes A.6.4 Analysis of point-contact data A.7 Characterization of tunnel junctions A.7.1 Initial characterization of junctions A.7.2 Derivative measurement circuitry Appendix B Methods of scanning tunneling spectroscopy and competing approaches B.l STM basics, tip production, and characterization; single atom tips B.2 Noise-free x, y, z translation; vibration isolation B.2.1 The cryogenic STM of Wilson Ho B.2.2 The 240-mK STM design of Pan, Hudson, and J. C. Davis B.3 Atomic force microscope; combination STM/AFM B.4 Scanning tunneling potentiometry and point-contact measurements B.5 Ballistic electron emission microscopy (BEEM) B.6 Scanning charge microscopy and spectroscopy B.6.1 Scanning single-electron-transistor electrometry B.6.2 Scanning subsurface charge accumulation microscopy: STM/SCAM B.6.3 Single electron capacitance spectroscopy B.7 Scanning Hall probe microscopy

7 xvi CONTENTS Appendix C Tabulated results 542 Table C. 1 s, p elements 543 Table C.2 Alloys and unusual phases: s, p elements 544 Table C.3 d-band elements 545 Table C.4 d-band alloys, oxides, and compounds 546 Table C.5 f-band elements 548 Table C.6 Metal overlayers for barrier formation 548 Table C.7 Studies of Tomasch oscillations in thick superconducting films Table C.8 and of McMillan-Rowell oscillations in thick normal films 548 Tunneling studies of superconductor phonons under hydrostatic pressure 548 Tables C.9 Cuprate superconductors 549 Table C.9a Gap values for Bi 2 Sr2CaCu ( s (Bi2212) 549 Table C.9b Gap values for YBa 2 Cu30 7+,5 550 Table C.9c Gap values for HgBa2Ca _icu n 02 n +2+,5 551 References 553 Index 583