Ullrich Pietsch

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Fourth generation accelerator-based light sources, such as VUV and X-ray Free Electron Lasers (FEL), deliver ultra-brilliant ( 10–10 photons per bunch) coherent radiation in femtosecond ( 10– 100 fs) pulses and, thus, require novel focal plane instrumentation in order to fully exploit their unique capabilities. As an additional challenge for detection(More)
An increase in random molecular vibrations of a solid owing to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. Here we report on a light-induced isothermal transition of a polymer film from an isotropic solid to an anisotropic liquid state in(More)
Using scanning X-ray diffraction microscopy with a spot size of 220 x 600 nm, it was possible to inspect individual GaAs nanorods grown seed-free through circular openings in a SiN(x) mask in a periodic array with 3 microm spacing on GaAs[111]B. The focused X-ray beam allows the determination of the strain state of individual rods and, in combination with(More)
InAs nanowires that grow catalyst-free along the [111] crystallographic orientation are prone to wurtzite-zincblende polytypism, making the control of the crystal phase highly challenging. In this work, we explore the dynamic relation between the growth conditions and the structural composition of the nanowires using time-resolved X-ray scattering and(More)
Bragg diffraction is often used as a tool to assess the structural quality of two-dimensional and three-dimensional (3D) photonic crystals. However, direct conclusions from the Laue diagrams to the underlying crystals structure cannot be drawn, as multiple scattering due to the high index contrast takes place. Here we systematically study the scattering of(More)
A phase transition in an amphiphilic mesophase is explored to deliberately induce mechanical strain in an assembly of tightly coupled metal ion coordination centers. Melting of the alkyl chains in the amphiphilic mesophase causes distortion of the coordination geometry around the central transition metal ion. As a result, the crystal field splitting of the(More)
Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Young's modulus by at(More)
We employed nanobeam X-ray diffraction using an X-ray spot size of 150 nm to investigate the local structure of P3HT thin films. We derived nanoscale real space maps of the X-ray diffraction properties at the π-π (020) diffraction peak. The X-ray data reveal a complex nanoscale structure of the polymer network with strong local variation where some areas of(More)
High piezoelectric activity of many ferroelectrics has been the focus of numerous recent studies. The structural origin of this activity remains poorly understood due to a lack of appropriate experimental techniques and mixing of different mechanisms related to ferroelectricity and ferroelasticity. Our work reports on the study of a uniaxial(More)
For the first time, site-selective distortion has been investigated for two different structural units in the ternary compound alpha-GaPO(4) under the influence of a permanent external electric field. Based on 54 measured reflection intensities, the electric-field-induced distortion of PO(4) and GaO(4) tetrahedra in alpha-GaPO(4) crystals is evaluated using(More)