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Introduction to High-Resolution Inelastic X-Ray Scattering

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Introduction to High-Resolution Inelastic X-Ray Scattering Revised, 2020 (29b), http://arxiv.org/abs/1504.01098 See also Synchrotron Light Sources & Free Electron Lasers, edited by E. Jaeschke, et al. Springer International Publishing, Cham, 2016, p. 1643-1757 https://doi.org/10.1007/978-3-319-14394-1_52 http://dx.doi.org/10.1007/978-3-319-14394-1_41 Introduction to High-Resolution Inelastic X-Ray Scattering Alfred Q.R. Baron Materials Dynamics Laboratory, RIKEN SPring-8 Center, RIKEN, 1-1-1 Kouto, Sayo, Hyogo 679-5148 Japan [email protected] Abstract This paper reviews non-resonant, meV-resolution inelastic x-ray scattering (IXS), as applied to the measurement of atomic dynamics of crystalline materials. It is designed to be an introductory, though in-depth, look at the field for those who be may interested in performing IXS experiments, or in understanding the operation of IXS spectrometers, or those desiring a practical introduction to harmonic phonons in crystals at finite momentum transfers. The treatment of most topics begins from ground-level, with a strong emphasis on practical issues, as they have occurred to the author in two decades spent introducing meV-resolved IXS in Japan, including designing and building two IXS beamlines, spectrometers and associated instrumentation, performing experiments, and helping and teaching other scientists. After an introduction that compares IXS to other methods of investigating atomic dynamics, some of the basic principles of scattering theory are described with the aim of introducing useful and relevant concepts for the experimentalist. That section includes a fairly detailed discussion of harmonic phonons. The theory section is followed by a brief discussion of calculations and then a longer section on spectrometer design, concepts, and implementation, including a brief introduction to dynamical diffraction and a survey of presently available instruments. Finally, there is discussion of the types of experiments that have been carried out, with a focused discussion on measurements of superconductors, and brief discussion (but with many references) of other types of crystalline samples. Table of Contents 0. Preface to the 2018 Revision 1. Preliminaries 1.1 Introduction 1.2 IXS: Concept and Spectral Gallery 1.3 Overview of Facilities & Future Options 1.4 Comparison with Other Methods 1.4.1 Inelastic Neutron Scattering (INS) 1.4.2 Inelastic UV spectroscopy (IUVS) 1.4.3 meV-Resolution Resonant IXS (RIXS) 1.4.4 Nuclear Resonant Scattering (NRS) 1.4.5 Thermal Diffuse Scattering (TDS) 1.4.6 Fourier Transform IXS (Pump-Probe TDS) 1.4.7 Atom and Electron Scattering 1.4.8 X-ray Photon Correlation Spectroscopy (XPCS) 1.4.9 Raman and IR spectroscopy 1.4.10 Other Methods for Acoustic Modes 2. Scattering Theory, Phonons. 2.1 Introduction 2.2 The Dynamic Structure Factor, S(Q, ω ) 2.3 The Static Structure Factor S(Q) 2.4 The X-Ray Form Factor and Charge Density Operator 2.5 Coherent and Incoherent Scattering 2.6 Other Correlation Functions, Sum Rules 2.7 The Incoherent Limit: Ideal Gas, Densities of States, Impulse Approximation 2.8 The Phonon Expansion: Formulas for (Q, ) S ω 2.9 Harmonic Phonons: The Dynamical Matrix, Eigenpolarizations, Acoustic & Optical Modes, Participation Ratio 2.10 The Real-Space Force-Constant Matrix and Acoustic Sum Rule 2.11 Practical (Harmonic) Phonons 2.11.1 One Phonon Cross-section, Transverse and Longitudinal Geometries, Q vs. q 2.11.2 On the intensity of the terms in the phonon expansion. 2.11.3 Anti-crossings 2.11.4 Two-Phonon Scattering 2.12 Non-harmonic Phonons 2.13 Detailed Balance and Symmetrizing Spectra 1 3. Calculations of Atomic Dynamics 4. IXS Spectrometers 4.1 Design Issues and Existing Spectrometer Layout 4.2 Dynamical Bragg Diffraction 4.3 Silicon & Non-Silicon Optics 4.4 High Resolution Monochromator (HRM) Design 4.5 Spherical Analyzers 4.6 Efficient Use of an Analyzer Array 4.7 Post-Sample Collimation (PSC) Optics 4.8 Comparison of 9.1 keV PSC and Spherical Analyzers 4.9 Electronic Excitation with a High Resolution Spectrometer 5. Sample Science 5.1 Introduction and Overview 5.2 Superconductors 5.2.1 Background 5.2.2 Conventional Superconductors 5.2.3 Conventional Superconductors - Temperature Dependence 5.2.4 Cuprate Superconductors 5.2.5 Iron Pnictide Superconductors 6. Concluding Comments 7. Acknowledgements 8. Electronic Supplementary Materials 9. References Table Of Abbreviations CDW Charge Density Wave DAC Diamond Anvil Cell DHO Damped Harmonic Oscillator HRM High Resolution Monochromator INS Inelastic Neutron Scattering IFC Interatomic Force Constant IUVS Inelastic Ultraviolet Spectroscopy Inelastic X-Ray Scattering IXS (In the present paper: specifically meV resolved non-resonant investigations) Nuclear Inelastic Scattering NIS (also, sometimes, NRVS, or, occasionally, NRIXS, is used instead) Non-Resonant Inelastic X-Ray Scattering NRIXS (Generally with resolution on the 0.1 eV scale for electronic interactions) NRS Nuclear Resonant Scattering PSC Post Sample Collimation RIXS Resonant Inelastic X-Ray Scattering SA Spherical Analyzer SIXS Soft (X-ray) Inelastic X-Ray Scattering TDS Thermal Diffuse Scattering XAFS X-Ray Absorption Fine Structure XPCS X-Ray Photon Correlation Spectroscopy 2 0. Preface to the 2018 and 2020 Revisions This paper is a revised version of a review prepared in 2014 and disseminated in 2015 and 2016 (Baron), and then revised in 2018 and 2020. The 2018 version updated references, and generally tried to make the paper more readable and precise. It also extended the discussion of spectrometers with a more detailed comparison of spherical analyzers (SA) and post-sample collimation (PSC) optics (section 4.8). The 2020 revision includes further updates in references and changes to the discussion about multi-phonon scattering, and detailed balance. 1. Preliminaries 1.1 Introduction Inelastic x-ray scattering (IXS), broadly interpreted, refers to a large collection of photon-in -> photon-out measurement techniques that are used to investigate many disparate aspects of materials dynamics*. The breadth of the science involved, and the sophistication of the instrumentation, can make the field somewhat daunting for newcomers. Table 1 lists some of the techniques that fall under the broad interpretation of the heading, while figure 1 schematically shows some of the excitations that may be probed. Methods range from Compton scattering that may be used to probe electron momentum densities, to x-ray Raman scattering that is, to a first approximation, similar to XAFS, probing atomic bonding, to resonant inelastic scattering which can probe collective and localized electronic states, as well as magnetism, to non-resonant techniques that, with sufficiently high flux (and, usually, poorer resolution), can be used to probe band-structure and electronic excited state structure, and, with higher resolution, to probe atomic dynamics. Figure 1. Conceptual diagram of excitations that may be probed using non-resonant IXS. Energy scales and intensities are approximate. Magnons (say 0.01-1 eV) are not shown but may be accessed via resonant inelastic x-ray scattering. The energy scale of the core excitations will depend significantly on the material and that for Compton scattering on the x-ray energy and momentum transfer. Valence excitations include both band structure and localized and non- localized excitations. Momentum transfer is not considered, but can have a huge impact on intensity. The present paper will focus on high-, meV-resolution, non-resonant IXS as applied to the investigation of atomic dynamics, especially phonons in crystalline materials. It will provide an introduction to x-ray scattering appropriate for the consideration of these measurements, a reasonably detailed description of the instrumentation and optical issues relevant to spectrometer performance, and a selection of examples drawn from recent literature. The goal is to provide an introduction that is appropriate for a motivated but uninitiated reader - we will try to clearly define most of the concepts used, and provide a selection of references for more information. Further, while discussing the background for the scattering theory, constant effort is made to make contact with practical issues and examples. The driving impetus for this paper is the present opportunity afforded by meV-resolved inelastic x-ray scattering. The concepts for understanding IXS largely date back to the 50s and 60s when neutron sources and spectrometers began to allow measurement of dynamical properties of materials over atomic-scale correlation lengths. However, with the development of synchrotron sources, x-rays have undergone an increase in source brilliance of more than 10 orders of magnitude in the last 50 years, so, at least in some respects, the x-ray spectrometers have now out-stripped the neutron spectrometers. In particular, with the increased specialization of synchrotron beamlines over the last 2 decades, there are now multiple facilities where one, as a user, can show up with a small piece of crystal - say 1 mm, or 0.1 mm, or * A comprehensive treatment of many aspects of IXS, but not the meV-resolved work discussed here, can be found in (Schülke, 2007). 3 even 0.01 mm in diameter - and walk away with extremely clean, meV-resolved measurements of the dynamic structure factor, S(Q,ω ) . One can investigate phonon dispersion, interactions, and line shapes, doping dependencies, etc., with relative ease: in IXS, the backgrounds tend to be small, there are no "spurions", there is no complex resolution coupling, and (for better or worse)
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