Magnetism and Accelerator-Based Light Sources

Springer Proceedings in Physics 262 Hervé Bulou Loïc Joly Jean-Michel Mariot Fabrice Scheurer Editors Magnetism and Accelerator-Based Light Sources Proceedings of the 7th International School ‘‘Synchrotron Radiation and Magnetism’’, Mittelwihr (France), 2018 Springer Proceedings in Physics Volume 262 Indexed by Scopus The series Springer Proceedings in Physics, founded in 1984, is devoted to timely reports of state-of-the-art developments in physics and related sciences. Typically based on material presented at conferences, workshops and similar scientific meetings, volumes published in this series will constitute a comprehensive up-to-date source of reference on a field or subfield of relevance in contemporary physics. 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Please contact: For Americas and Europe: Dr. Zachary Evenson; [email protected] For Asia, Australia and New Zealand: Dr. Loyola DSilva; [email protected] More information about this series at http://www.springer.com/series/361 Hervé Bulou • Loïc Joly • Jean-Michel Mariot • Fabrice Scheurer Editors Magnetism and Accelerator-Based Light Sources Proceedings of the 7th International School ‘‘Synchrotron Radiation and Magnetism’’, Mittelwihr (France), 2018 123 Editors Hervé Bulou Loïc Joly IPCMS IPCMS Université de Strasbourg, CNRS Université de Strasbourg, CNRS Strasbourg, France Strasbourg, France Jean-Michel Mariot Fabrice Scheurer LCPMR IPCMS Sorbonne Université Université de Strasbourg, CNRS Paris, France Strasbourg, France ISSN 0930-8989 ISSN 1867-4941 (electronic) Springer Proceedings in Physics ISBN 978-3-030-64622-6 ISBN 978-3-030-64623-3 (eBook) https://doi.org/10.1007/978-3-030-64623-3 © The Editor(s) (if applicable) and The Author(s) 2021. 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This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland To Eric Beaurepaire Foreword In October 2018, as the last epiphany of a long-standing tradition, the 7th International School “Magnetism and Synchrotron Radiation”, most commonly known as “Mittelwihr School of Magnetism”, gathered more than 100 young (and older) scientists from the whole Europe as well as from the USA and Japan. For the first time of this long series, one of the organizers of the very first Mittelwihr schools, the recently departed Eric Beaurepaire was not attending the school and, in many occasions, lectures were given with a very special attention to Eric’s pioneering work. This textbook is dedicated to Eric and can be regarded as a tribute to his scientific achievements. This version of the “Mittelwihr School” was not much different from the pre- vious ones so that the Lecture Notes contain most of the expected, basic ingredients. It starts with an introduction to the physics of modern X-ray sources (Chap. 1) coupled to a profound deep presentation of light/matter interaction in the X-ray range (Chap. 3) with a complete overview of all different types of angular dependence for X-ray absorption spectroscopy (Chap. 4). A broad and general introduction to magnetism (Chap. 2) is followed by an involved description of spintronics (Chap. 5) and physics of superconductivity (Chap. 6) with emphasis brought to how spintronics or superconductivity can be understood thanks to X-ray spectroscopies and X-ray scattering. The 7th International School “Magnetism and Synchrotron Radiation” took place in a context where many storage rings were either upgraded or looking forward to be upgraded. For most X-ray beamlines on third-generation storage rings, the vertical emittance of the electron beam was close to the diffraction limit imposed by the X-ray wavelength, i.e. k=4p. Not much could be gained in this direction. On the contrary, for most 3rd generation storage rings, the horizontal emittance was much larger than the diffraction limit. At SOLEIL, the horizontal emittance of the electrons was 4000 pm rad so that it was more than one order of magnitude larger than the diffraction limit for X-rays with energy larger than 250 eV. At ESRF, a similar situation existed. Reducing the horizontal emittance was then regarded as a target to follow in order to increase the brilliance and this was first obtained at MAX-IV by replacing the “long” dipole bending magnets by a vii viii Foreword series of smaller dipole magnets yielding the multibend achromat lattice. Brilliance is defined as the number of photons per 0.1% bandwidth, per second, per surface of the source and per horizontal and vertical angular divergences of the source. Reducing the size of the source was an efficient way to increase the brilliance. Brilliance is a driving parameter for experiments where the photon source is focused on the sample and in such cases, the increase of brilliance is fully related to an increase of useful photons. It should nevertheless be kept in mind that for experiments where the beam is not refocused on the sample, the gain in brilliance is not accompanied by a high gain in useful photons: this is indeed what is observed for experiments where the figure of merit is the flux at the sample position (the flux, expressed in J mÀ2 sÀ1, is the energy of all the photons crossing a surface unit per second). As a consequence of increasing the brilliance, the new X-ray beams of the diffraction-limited storage rings have a much higher level of coherence since one estimates that a source with size r and divergence r' has a 50% coherence for k such that rr' ¼ k=4p. Not regarding the controversy between brilliance and flux, the reduction of the emittance in the horizontal plane opens new perspectives for experiments taking advantage of coherence such as X-ray phase-contrast imaging, ptychography, Fourier transform holography, X-ray photon-correlation spectroscopy, coherent diffractive imaging, or fluctuation microscopy. Chapter 1, “X-ray sources at large scale facilities”, gives a brief introduction to the physics of storage rings with special attention to the physics of electron accelerators and to the devices developed to produce X-rays: bending magnets, wigglers, undulators. One can notice an introduction to the diffraction-limited storage rings that are presently blooming in Lund (MAX-IV) or Grenoble (upgraded ESRF) and that will replace the old, common structure of conventional storage rings. The X-ray free-electron lasers in conjunction with the self-amplified spontaneous emission of X-rays are also presented and the characteristics of theses new X-ray beams are described so that it is accessible to most, future potential users. Based on the knowledge of the X-ray beams, Chap. 3, “Electronic structure theory for X-ray absorption and photoemission spectroscopy” presents the con- nection between the electronic structure of matter and core-hole spectroscopies such as X-ray absorption spectroscopy and X-ray photoemission spectroscopy. I would like to attract the attention of the reader concerning the recently developed, state of the art pieces of theory that go beyond the common density functional theory (DFT). The time-dependent density functional theory, the Green's function approach (with special care to treatment of the self-energy within the GW approximation), or the Bethe–Salpeter equation are presented in a concise way so that the reader interested in learning beyond the DFT can get a flavour of these actual topics, still under development. Chapter 4, “X-ray dichroisms in spherical tensor and Green’s function formalism”, calculates the various types of angular dependence present in XAS. It first starts with a presentation of the multi-electronic, many-body calculation of the X-ray absorption cross sections. Then connections are made with the atomic multi-electronic approach as it is developed in the ligand-field Foreword ix multiplet approach or with the monoelectronic DFT approach up to a brief survey of the many-body extended theories. The second part is dedicated to the angular dependence of electric dipole, E1:E1 and electric quadrupole E2:E2 terms, present in the X-ray absorption cross sections where the emphasis is brought to their tensor expansions.

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