Abstract:The experimental characterization of the spatial and temporal coherence properties of the free-electron laser in Hamburg (FLASH) at a wavelength of 8.0 nm is presented. Double pinhole diffraction patterns of single femtosecond pulses focused to a size of about 10×10 µm 2 were measured. A transverse coherence length of 6.2 ± 0.9 µm in the horizontal and 8.7 ± 1.0 µm in the vertical direction was determined from the most coherent pulses. Using a split and delay unit the coherence time of the pulses produced in the same operation conditions of FLASH was measured to be 1.75 ± 0.01 fs. From our experiment we estimated the degeneracy parameter of the FLASH beam to be on the order of 10 10 to 10 11 , which exceeds the values of this parameter at any other source in the same energy range by many orders of magnitude. (4) were well satisfied in our experimental geometry. The maximum time delay introduced through the path length difference was τ max ≈ 0.6 fs and was smaller than the temporal coherence length τ c = (1.75 ± 0.0.01) fs measured by the split and delay unit (see below). Therefore, we could safely assume that in transverse coherence measurements |γ eff 12 (τ)| ≈ |γ eff 12 (0)| and α 12 (τ) ≈ α 12 (0). 36. An unconstrained fit yields a value of |γ eff 11 | ≈ 0.8 in both directions and provides slightly larger values for the transverse coherence length. We attribute this to inhomogenities in the transmission through the pinholes. 37. J. Chalupsky, J. Krzywinski, L. Juha, V. Hajkova, J. Cihelka, T. Burian, L. Vyain, J. Gaudin, A. Gleeson, M.Jurek, A. R. Khorsand, D. Klinger, H. Wabnitz, R. Sobierajski, M. Störmer, K. Tiedtke, and S. Toleikis, "Spot size characterization of focused non-Gaussian X-ray laser beams", Opt. Express 18, 27836 (2010). 38. We attribute this positional uncertainty to both, instabilities of the sample stages and beam positional jitter. 39. In our experiment the maximum of |γ 12 (τ)| did not reach unity but rather a value of 0.14. The reason for this is that the full beam was split in the middle and overlapped again meaning that parts of the center of the beam were overlapped with parts of the edge of the beam (see [27]

Abstract.We report the commissioning of the novel holography endstation for the P10 coherence beamline at PETRA III at DESY. The experimental imaging scheme is based on a highly coherent and divergent (cone) beam illumination, achieved by fixed-curvature focusing mirrors with additional spatial and coherence filtering by x-ray waveguides. The optical elements along the beam path and the instrument in commissioning are described. First experimental results are shown.

Electrical-transport properties of ferromagnetic GaGdN layers grown by molecular-beam epitaxy on highly resistive 6H-SiC͑0001͒ substrates have been investigated. It is found that doping with low concentrations of Gd increases the resistivity by several orders of magnitude as compared to unintentionally doped GaN. In the measurable temperature range between 5 and 120 K two different temperature dependences of the resistivity ͑T −1/2 and T −1/4 ͒ are observed, both of which are characteristic of variable-range-hopping conductivity in an impurity band of localized states. The experimental evidence, that room-temperature ferromagnetism and impurity-band hopping transport are coexistent, is discussed in connection with recent models proposing defect-induced magnetism in wide-band-gap III-Nitrides.

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In this study, we report on the incorporation of dilute Gd amounts into GaN films grown by molecular beam epitaxy. A combination of x-ray fluorescence with x-ray absorption spectroscopic techniques enabled us to examine not only the distribution of rare earth atoms in the GaN matrix but also the short-range structural order. Our results show Gd atoms in a trivalent state with tetrahedral coordination, thus substituting Ga in the wurtzite GaN structure.