X-Treme beamline at SLS: X-ray magnetic circular and linear dichroism at high field and low temperature

X-Treme is a soft X-ray beamline recently built in the Swiss Light Source at the Paul Scherrer Institut in collaboration with Ecole Polytechnique Federale de Lausanne. The beamline is dedicated to polarization-dependent X-ray absorption spectroscopy at high magnetic fields and low temperature. The source is an elliptically polarizing undulator. The end-station has a superconducting 7 T-2 T vector magnet, with sample temperature down to 2 K and is equipped with an in situ sample preparation system for surface science. The beamline commissioning measurements, which show a resolving power of 8000 and a maximum flux at the sample of 4.7 x 10(12) photons s(-1), are presented. Scientific examples showing X-ray magnetic circular and X-ray magnetic linear dichroism measurements are also presented

Angle- and spin-resolved photoemission on La /3Sr1/3MnO3

La2/3Sr1/3MnO3 est un des oxydes de manganèse qui suscite un vif intérêt car on s attend à ce qu il soit un demi-métal. La spectroscopie de photoélectrons résolue en angle est une technique puissante de caractérisation de la structure électronique des systèmes complexes, dans lesquels les degrés de liberté de charge, d orbitale et de spin déterminent des propriétés inattendues, comme la supraconductivité non conventionnelle ou la magnétorésistance colossale. Pour développer de nouveaux types de matériaux magnétiques adaptés à la spintronique, il est important d accéder au degré de liberté de spin. Cette thése porte sur la spectroscopie haute résolution des photoélectrons, résolue en spin et en angle, de La2/3Sr1/3MnO3. Des échantillons sous forme de couches minces (< 100 nm) ont été préparés par ablation laser. Les spectres de photoélectrons ont été simulés à partir de calculs de structure de bandes en prenant en compte les effets de durées de vie du phototrou et l élargissement de la composante perpendiculaire du vecteur d onde. Les simulations reproduisent l existence d un gap au niveau de Fermi pour les électrons minoritaires et confirment que La2/3Sr1/3MnO3 a bien un caractère demi-métallique, en dépit du fait que la polarisation de spin mesurée est inférieure à 100%.La2/3Sr1/3MnO3 is one of the manganese oxides that attracted a lot of interest because it is expected to be a half metal. Angle-resolved photoemission spectroscopy is a powerful technique for characterizing the electronic structure of complex systems, where charge, orbital, and spin degrees of freedom determine surprising properties, such as non-conventional superconductivity and colossal magnetoresistance. For the development of new types of magnetic materials suitable for spintronics, it is important to access to the spin degree of freedom. This thesis deals with high-resolution spin- and angle-resolved photoemission spectroscopy on La2/3Sr1/3MnO3. Thin film (< 100 nm) samples were prepared by laser ablation. The photoemission spectra were simulated from band structure calculations considering the photohole lifetime effects and the broadening of the perpendicular component of the wavevector. The simulations reproduce a gap between minority electrons at the Fermi level and confirm the half-metallic nature of La2/3Sr1/3MnO3, despite the fact that the measured spin polarization is lower than 100%.CERGY PONTOISE-BU Neuville (951272102) / SudocSudocFranceF

A spatial beam property analyzer based on dispersive crystal diffraction for low-emittance X-ray light sources

The advent of low-emittance synchrotron X-ray sources and free-electron lasers urges the development of novel diagnostic techniques for measuring and monitoring the spatial source properties, especially the source sizes. This work introduces an X-ray beam property analyzer based on a multi-crystal diffraction geometry, including a crystal-based monochromator and a Laue crystal in a dispersive setting to the monochromator. By measuring the flat beam and the transmitted beam profiles, the system can provide a simultaneous high-sensitivity characterization of the source size, divergence, position, and angle in the diffraction plane of the multi-crystal system. Detailed theoretical modeling predicts the system’s feasibility as a versatile characterization tool for monitoring the X-ray source and beam properties. The experimental validation was conducted at a bending magnet beamline at the Swiss Light Source by varying the machine parameters. A measurement sensitivity of less than 10% of a source size of around 12 µm is demonstrated. The proposed system offers a compact setup with simple X-ray optics and can also be utilized for monitoring the electron source.ISSN:2045-232

Three-Dimensional Electron Realm in VSe2 by Soft-X-Ray Photoelectron Spectroscopy: Origin of Charge-Density Waves

The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular to) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure

Unveiling the complete dispersion of the giant Rashba split surface states of ferroelectric α−GeTe(111) by alkali doping

α−GeTe(111) is a noncentrosymmetric ferroelectric material for which a strong spin- orbit interaction gives rise to giant Rashba split states in the bulk and at the surface. The detailed dispersions of the surface states inside the bulk band gap remains an open question because they are located in the unoccupied part of the electronic structure, making them inaccessible to static angle-resolved photoemission spectroscopy. We show that this difficulty can be overcome via in situ potassium doping of the surface, leading to a rigid shift of 80 meV of the surface states into the occupied states. Thus, we resolve, in great detail, their dispersion and highlight their crossing at the ... point, which, in comparison with density functional theory calculations, definitively confirms the Rashba mechanism

Field-induced ultrafast modulation of Rashba coupling at room temperature in ferroelectric α-GeTe(111)

Rashba materials have appeared as an ideal playground for spin-to-charge conversion in prototype spintronics devices. Among them, α-GeTe(111) is a non-centrosymmetric ferroelectric (FE) semiconductor for which a strong spin-orbit interaction gives rise to giant Rashba coupling. Its room temperature ferroelectricity was recently demonstrated as a route towards a new type of highly energy-efficient non-volatile memory device based on switchable polarization. Currently based on the application of an electric field, the writing and reading processes could be outperformed by the use of femtosecond (fs) light pulses requiring exploration of the possible control of ferroelectricity on this timescale. Here, we probe the room temperature transient dynamics of the electronic band structure of α-GeTe(111) using time and angle-resolved photoemission spectroscopy (tr-ARPES). Our experiments reveal an ultrafast modulation of the Rashba coupling mediated on the fs timescale by a surface photovoltage (SPV), namely an increase corresponding to a 13 % enhancement of the lattice distortion. This opens the route for the control of the FE polarization in α-GeTe(111) and FE semiconducting materials in quantum heterostructures