75 research outputs found
Optical control of 4f orbital state in rare-earth metals
Information technology demands continuous increase of data-storage density.
In high-density magnetic recording media, the large magneto-crystalline
anisotropy (MCA) stabilizes the stored information against decay through
thermal fluctuations. In the latest generation storage media, MCA is so large
that magnetic order needs to be transiently destroyed by heat to enable bit
writing. Here we show an alternative approach to control high-anisotropy
magnets: With ultrashort laser pulses the anisotropy itself can be manipulated
via electronic state excitations. In rare-earth materials like terbium metal,
magnetic moment and high MCA both originate from the 4f electronic state.
Following infrared laser excitation 5d-4f electron-electron scattering
processes lead to selective orbital excitations that change the 4f orbital
occupation and significantly alter the MCA. Besides these excitations within
the 4f multiplet, 5d-4f electron transfer causes a transient change of the 4f
occupation number, which, too, strongly alters the MCA. Such MCA change cannot
be achieved by heating: The material would rather be damaged than the 4f
configuration modified. Our results show a way to overcome this limitation for
a new type of efficient magnetic storage medium. Besides potential
technological relevance, the observation of MCA-changing excitations also has
implications for a general understanding of magnetic dynamics processes on
ultrashort time scales, where the 4f electronic state affects the angular
momentum transfer between spin system and lattice.Comment: Manuscript (14 pages, 3 figures) and Supplementary Information (22
pages, 9 figures
The interplay of local electron correlations and ultrafast spin dynamics in fcc Ni
The complex electronic structure of metallic ferromagnets is determined by a balance between exchange interaction, electron hopping leading to band formation, and local Coulomb repulsion. The interplay between the respective terms of the Hamiltonian is of fundamental interest, since it produces most, if not all, of the exotic phenomena observed in the solid state. By combining high energy and temporal resolution in femtosecond time-resolved X-ray absorption spectroscopy with ab initio time-dependent density functional theory we analyze the electronic structure in fcc Ni on the time scale of these interactions in a pump-probe experiment. We distinguish transient broadening and energy shifts in the absorption spectra, which we demonstrate to be caused by electron repopulation and correlation-induced modifications of the electronic structure, respectively. Importantly, the theoretical description of this experimental result hence requires to take the local Coulomb interaction into account, revealing a temporal interplay between band formation, exchange interaction, and Coulomb repulsion
Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL
Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users
Megahertz-rate ultrafast X-ray scattering and holographic imaging at the European XFEL
The advent of X-ray free-electron lasers (XFELs) has revolutionized fundamental science, from atomic to condensed matter physics, from chemistry to biology, giving researchers access to X-rays with unprecedented brightness, coherence and pulse duration. All XFEL facilities built until recently provided X-ray pulses at a relatively low repetition rate, with limited data statistics. Here, results from the first megahertz-repetition-rate X-ray scattering experiments at the Spectroscopy and Coherent Scattering (SCS) instrument of the European XFEL are presented. The experimental capabilities that the SCS instrument offers, resulting from the operation at megahertz repetition rates and the availability of the novel DSSC 2D imaging detector, are illustrated. Time-resolved magnetic X-ray scattering and holographic imaging experiments in solid state samples were chosen as representative, providing an ideal test-bed for operation at megahertz rates. Our results are relevant and applicable to any other non-destructive XFEL experiments in the soft X-ray range
Observation of fluctuation-mediated picosecond nucleation of a topological phase
peer reviewedTopological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.Leibniz Association Grant no. K162/2018 (OptiSPIN
Paléoenvironnements et sédimentologie des formations récifales à Rudistes du Sénonien inférieur de Sainte-Anne d'Evenos, Massif du Gros-Cerveau (Var-S.E. France)
The "Barre de la Jaume", in the "Massif du Gros Cerveau" (Var , S.E. France) is a coniacian limestone formation that overlies unconformably the top of the "La Toulousanne" palaeostructure. This palaeostructure acted probably as a submarine palaeohigh that favoured the settlement of localised rudist-coral communities, in an environment with permanent terrigenous input. However, the constant break-down of the reef builders, due to the combined action of micro-organisms and hydrodynamics, led to the deposition of bioclastic debris units that form the main body of the coniacian limestones. Micritic mounds and lenses are interbedded with the coarse debris apron deposits of the flanks of the structure.
The Santonian sandstones and marls drape the Toulousanne structure but rudist biostromes develop locally within the terrigenous sequence. These biostromes abut against the northern flank of the coniacian build-up.La Barre de la Jaume, dans le Massif du Gros Cerveau (Var), est une formation calcaire d'âge coniacien reposant en discordance sur la paléostructure crétacée de La Toulousanne. Cette paléostructure localisée a probablement joué un rôle de haut-fond durant le Coniacien, conditionnant l'implantation de communautés à Rudistes et Coraux dans un régime actif d'apports terrigènes. Ces communautés récifales ont cependant été démantelées par l'action conjuguée des micro-organismes et de l'hydrodynamisme pour ne plus laisser subsister que des épandages bioclastiques et de très rares bioconstructions. Des calcaires micritiques alternent avec des coulées biodétritiques grossières sur les flancs de la structure.
Le Santonien marno-gréseux ennoie la structure de La Toulousanne, mais des biostromes à Rudistes se développent localement au sein de la série terrigène. Ces biostromes s'appuyent contre le flanc Nord de la bio-accummulation coniacienne.Mercadier C. G. L. Paléoenvironnements et sédimentologie des formations récifales à Rudistes du Sénonien inférieur de Sainte-Anne d'Evenos, Massif du Gros-Cerveau (Var-S.E. France). In: Géologie Méditerranéenne. Tome 12-13, numéro 1-2, 1985. pp. 65-121
Observation of fluctuation-mediated picosecond nucleation of a topological phase
Topological states of matter exhibit fascinating physics combined with an intrinsic stability. A key challenge is the fast creation of topological phases, which requires massive reorientation of charge or spin degrees of freedom. Here we report the picosecond emergence of an extended topological phase that comprises many magnetic skyrmions. The nucleation of this phase, followed in real time via single-shot soft X-ray scattering after infrared laser excitation, is mediated by a transient topological fluctuation state. This state is enabled by the presence of a time-reversal symmetry-breaking perpendicular magnetic field and exists for less than 300 ps. Atomistic simulations indicate that the fluctuation state largely reduces the topological energy barrier and thereby enables the observed rapid and homogeneous nucleation of the skyrmion phase. These observations provide fundamental insights into the nature of topological phase transitions, and suggest a path towards ultrafast topological switching in a wide variety of materials through intermediate fluctuating states
A new strategy in treating acute necrotizing pancreatitis—immediate operation, retroperitoneostomy, programmed revisions
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