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Production and usage of synchrotron radiation

It is intended to create a femtosecond pulsed X-ray facility as part of JuSPARC. The aim is to provide a radiation source for X-ray spectroscopy and scattering with femtosecond time-resolution.

The high peak-power of a PW Laser system operating in the visible will be transformed into high photon energies. The envisaged energy range goes from soft to hard X-rays. Ideally within this conversion process the laser pulse width of the PW-laser (30 fs) should be preserved, thus enabling ultrafast pump-probe experiments. This ambitious wavelength coverage will be achieved by employing three complementary techniques, each with their own beam-line.

Some planned applications are listed below:

Pulsed X-rays for information technology

A key challenge of condensed-matter physics today lies in the understanding and control of electronically correlated materials on the nanoscale. This applies in the same manner to research on energy storage, energy conversion, information technology and catalysis. These complex materials exhibit properties that cannot be explained by assuming independent electrons but which arise from the correlations of the latter.

Truly deep insight at the atomistic level relies on scattering, spectroscopy, and imaging experiments, which are able to disentangle the complex interplay of electron, orbital and spin degrees of freedom.

The central challenge in the understanding of such materials is to determine and control electron and spin properties of matter with high precision in the energy, momentum, space, and time domains. This asks for a time-resolved investigation on the level of electronic time-scales, i.e. 1–100 fs, and due to the chemical complexity of the materials in question must including element selectivity [S. Mathias et al., PNAS 109, 4792 (2012), D. Rudolf et al., Nature Com. 3:1037 DOI 10.1038/ncomms2029 (2012)]. Ultrafast pump-probe techniques in the soft X-ray regime are able to cover both electronic and magnetic phenomena and excitations.

In order to investigate structural phenomena on the level of a crystalline unit cell, hard X-rays (1.5–15 keV) are needed.

The combination of femtosecond light pulses in the soft and hard X-ray regime would ultimately present a unique tool to disentangle the complex interplay between structural, electronic and magnetic properties of highly-correlated electron systems and their roles in respective phase transitions.

Element selective probing of the timescale of exchange interaction

Progress in magnetic information storage and processing technology is intimately associated with complex materials that are engineered at the nanometer scale. Next-generation devices require that the magnetic state of materials is manipulated on fast timescales and at the nanometer level. However, a complete microscopic understanding of magnetization dynamics that involves the correlated interactions of spins, electrons, photons, and phonons on femtosecond timescales has yet to be developed. The fundamental question of whether the magnetization dynamics of individual elements in a ferromagnetic alloy can differ on ultrafast timescales is very important and has not been addressed either theoretically or experimentally to date. To answer this question experimentally a sample could rapidly excited by a train of ultrashort (≈25 fs) laser pulses and the demagnetization dynamics could be probed with XUV pulses that allow one to explore the involved spin dynamics element specifically.

Due to the limited power of table top laboratory laser systems, the achievable XUV energy is limited and experimentally useful Intensities are available only up to 70 eV. This restricts this method to the shallow core levels of Fe and Ni. The full width of application, including spin dependent light scattering at absorption edges with large spin-orbit splitting can only be achieved at higher photon energies. The proposed JuSPARC X-ray facility can provide femtosecond XUV pulses with significantly higher photon energies and intensities. This would be an important step to forward magnetic information and processing technology.

Hard X-ray Photoemission (HAXPES)

ARPES (angle-resolved photoemission spectroscopy) is the most powerful and widely used tool to study the valence band electronic structure of solids, giving direct access to the k-resolved electronic bands in the direction parallel to the measured surface.

JuSPARC Fig 2Fig. 2: iSchematics of a hard X-ray photoemission spectroscopy (HAXPES) experiment of an Al/EuO/Si heterostructure probing the buried EuO and EuO/Si interface [C. Caspers et al; Phys. Status Solidi RRL 5, Vol. 12, 441 (2011)]

Going to hard X-ray photon energies (hard X-ray Photoemission (HAXPES)) and thus larger electron inelastic mean-free paths should provide a more accurate picture of electronic structure from the solid bulk, buried layers and interfaces.

For example, the magnetic oxide/semiconductor model system, EuO on silicon, was investigated with HAXPES to probe the chemical quality of buried layers and interfaces [see Fig. 2,  Electronic structure of EuO spin filter tunnel contacts directly on silicon (PDF, 399 kB) C. Caspers et al.; Phys. Status Solidi RRL 5, Vol. 12, 441 (2011); C. Caspers et al.; Phys. Rev. B 84, 205217 (2011)]. This shows the applicability of HAXPES to the investigation of static electronic structures of buried interfaces. But especially the investigation of EuO/Si(001) was dedicated to the application in spintronics. Here the injection of pure spin currents into Si and its spacial and temporal decay is of further interest. This however is a dynamic process on a femtosecond time scale and its investigation requires a femtosecond pulsed hard X-ray source, which is not available these days.

The femtosecond time-resolution of the proposed JuSPARC X-ray source would allow these investigations and thus forward the applicability of Si based spintronics.

X-rays for material research

The ZEA-1 (Zentralinstitut für Engineering, Elektronik und Analytik, Central Institute for Engineering, Electronics and Analytics) investigates and develops new joining technologies for the combination of new material compositions like Carbon-metal connections or high temperature stable glass based materials for vacuum brazing. The new X-ray spectrometers can be used for the investigation of new intermetallic compounds which are created in the interface layer due to the joining process. Little is known about structure and electronic properties of those compounds. Another point of interest is the use of the X-ray beams to improve the computer tomography (CT) to another level. At the moment, CT is used to study structural defects, but the method is limited to static analysis. With the new X-ray beam and the time structure of the beam, dynamic investigations of defects are possible.

Investigation of charge carrier dynamics for optimization of materials for solar cells

The separation of photo-generated charges and the transport of these charges to the contacts are essential steps determining the function of solar cells.

For a broad range of materials and device structures for solar cells of the next and next but one generation, such as thin film solar cells, organic solar cells, bulk-heterojunction solar cells, hot-carrier solar cells and nanoparticle respectively nanostructure based solar cells, is this not the case.

The local structures can exhibit a dynamical change caused by the exited charge carriers, i.e. a simultaneous investigation of the kinetics of the charge transfer and the dynamics of the variation of the electronic and structural properties is essential. The investigation of charge transfer processes which typically take place on a fs-timescale therefore requires experiments which allow one to detect changes on this timescale with the respective resolution and high sensitivity.

The availability of optical and hard x-ray pulses of a few fs duration as well as a precise time correlation between the pulses will help to address fundamental questions related to the functioning of future solar cells and will foster the development of new solar cells.

Imaging of human brain diseases using synchrotron radiation

Well-known techniques for brain imaging include magnetic resonance imaging (MRI) and computer tomography (CT), which can give an overview of the degree and type of tissue damage, but fall short in depicting the morphology in an adequate manner. Pulsed synchrotron radiation is ideally suited to evaluate tissue morphology in high resolution without destroying the tissue and endangering the patient’s health.

The JuSPARC facility would open new ways of working on longitudinal and high resolution monitoring strategies of brain tissue in animal models of central nervous system (CNS) diseases and eventually also in patients. To guarantee tissue integrity it is crucial to have the shortest possible transportation times between tissue asservation and analysis. This requirement will be fulfilled by the collaboration between the laboratories at the University Hospital of Cologne and the JuSPARC facility.

The projects will eventually cover a variety of common CNS diseases, but will first be focused on the chronic autoimmune disease multiple sclerosis (MS).

One of the main future projects will be concerned with longitudinal measurements of B cell activity in individual MS patients in the peripheral blood and in the CNS itself to determine when a pathogenic contribution of B cells occurs in the disease process and whether this contribution is functionally relevant.

Overall, the project can be considered as a highly innovative diagnostic-prognostic approach, aimed at improving the still imperfect understanding of the immune pathogenesis of MS and other neurological diseases. Also, it is meant to advance the understanding of the highly heterogeneous individual disease courses in MS patients and to facilitate the development of better treatment strategies that encompass the role of B cells in the disease process.