@article{PietschHesseZhuangetal.2003,
  author    = {Pietsch, Ullrich and Hesse, A. and Zhuang, Y. and Hol{\´y}, Vaclav and Stangl, Jochen and Zerlauth, S. and Schaffler, F. and Bauer, G{\"u}nther},
  title     = {X-ray grazing-incidence study of inhomogeneous strain relaxation in Si/SiGe wires},
  issn      = {0168-583X},
  year      = {2003},
  language  = {en}
}
@article{PietschZeimerGrenzeretal.2003,
  author    = {Pietsch, Ullrich and Zeimer, Ute and Grenzer, J{\"o}rg and Grigorian, Souren A. and Fricke, J. and Gramlich, S. and Bugge, F. and Weyers, Markus and Trankle, G.},
  title     = {Influence of lateral patterning geometry on lateral carrier confinement in strain-modulated InGaAs- nanostructures},
  year      = {2003},
  language  = {en}
}
@article{PietschBhattacharyaMukherjeeetal.2003,
  author    = {Pietsch, Ullrich and Bhattacharya, M. K. and Mukherjee, M. and Sanyal, M. K. and Geue, Thomas and Grenzer, J{\"o}rg},
  title     = {Energy dispersive x-ray reflectivity technique to study thermal properties of polymer films},
  year      = {2003},
  language  = {en}
}
@article{PietschGrigorianGrenzeretal.2003,
  author    = {Pietsch, Ullrich and Grigorian, Souren A. and Grenzer, J{\"o}rg and Feranchuk, S. and Zeimer, Ute},
  title     = {Grazing-incidence diffraction study of strain-modulated single quantum well nanostructures},
  year      = {2003},
  language  = {en}
}
@article{PietschPanznerLeitenbergeretal.2003,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Leitenberger, Wolfram and Grenzer, J{\"o}rg and Bodenthin, Th. and Geue, Thomas and M{\"o}hwald, Helmuth},
  title     = {Coherence experiments at the energy-dispersive reflectometry beamline at BESSY II},
  year      = {2003},
  language  = {en}
}
@article{PietschGeueHennebergetal.2003,
  author    = {Pietsch, Ullrich and Geue, Thomas and Henneberg, Oliver and Saphiannikova, Marina},
  title     = {X-ray investigations of formation efficiency of buried azobenzene polymer density gratings},
  doi       = {10.1063/1.1554753},
  year      = {2003},
  language  = {en}
}
@article{Pietsch2003,
  author    = {Pietsch, Ullrich},
  title     = {Evidence of a density grating under light induced formation of surface relief gratings at polymers containing azobenzene moieties},
  year      = {2003},
  language  = {en}
}
@article{PietschGrigorianGrenzeretal.2003,
  author    = {Pietsch, Ullrich and Grigorian, Souren A. and Grenzer, J{\"o}rg and Vartanyants, Ivan A.},
  title     = {Thermal diffuse scattering in grazing incidence diffraction},
  year      = {2003},
  language  = {en}
}
@article{PietschTsirelsonGorfan2003,
  author    = {Pietsch, Ullrich and Tsirelson, Vladimir G. and Gorfan, S. V.},
  title     = {X-ray scattering amplitude of an atom in a permanent external electric field},
  year      = {2003},
  language  = {en}
}
@article{Pietsch2003,
  author    = {Pietsch, Ullrich},
  title     = {Thin Layers of columns of an amphiphilic hexa-peri-hexabenzocoronene at silicon wafer surfaces},
  year      = {2003},
  language  = {en}
}
@article{HennebergGeueSaphiannikovaetal.2003,
  author    = {Henneberg, Oliver and Geue, Thomas and Saphiannikova, Marina and Pietsch, Ullrich and Rochon, Paul},
  title     = {X-ray and VIS light scattering from light-induced polymer gratings},
  doi       = {10.1088/0022-3727/36/10A/350},
  year      = {2003},
  language  = {en}
}
@article{PietschLeitenbergerWendrocketal.2003,
  author    = {Pietsch, Ullrich and Leitenberger, Wolfram and Wendrock, Horst and Bischoff, Lothar and Panzner, Tobias and Grenzer, J{\"o}rg and Pucher, Andreas},
  title     = {Double pinhole diffraction of white synchrotron radiation},
  year      = {2003},
  language  = {en}
}
@article{PietschDavaasambuuPucheretal.2003,
  author    = {Pietsch, Ullrich and Davaasambuu, Jav and Pucher, Andreas and Kochin, V.},
  title     = {Atomistic origin of the inverse piezoelectric effect in alpha-SiO2 and alpha-GaPO4},
  issn      = {0295-5075},
  year      = {2003},
  language  = {en}
}
@article{PietschMukhopadhyaySanyaletal.2004,
  author    = {Pietsch, Ullrich and Mukhopadhyay, M. K. and Sanyal, M. K. and Datta, A. and Mukherjee, M. and Geue, Thomas and Grenzer, J{\"o}rg},
  title     = {Transition from two-dimensional to three-dimensional melting in Langmuir-Blodgett films},
  year      = {2004},
  abstract  = {Results of energy-dispersive x-ray reflectivity and grazing incidence diffraction studies of Langmuir-Blodgett films exhibited evolution of conventional three-dimensional melting from continuous melting, characteristic of two- dimensional systems, as a function of deposited monolayers. Continuous expansion followed by a sharp phase transition of the in-plane lattice was observed before the melting point and found to be independent of number of deposited layers. Evolution of conventional melting with an increase in the number of monolayers could be quantified by measuring stiffness against tilting of the vertical stack of molecules, which are kept together by an internal field. The internal field as defined in this model reduces as the in-plane lattice expands and the sample temperature approaches melting point. The sharpness of the melting transition, which has been approximated by a Langevin function, increases with the number of deposited monolayers},
  language  = {en}
}
@article{PietschDavaasambuuKochinetal.2004,
  author    = {Pietsch, Ullrich and Davaasambuu, Jav and Kochin, V. and Schwarz, K. H. and Blaha, Pawel},
  title     = {The atomistic origin of the inverse piezoelectric effect in a-quartz},
  year      = {2004},
  abstract  = {Ab initio calculations have been carried out using the FP-APW+lo method in order to understand the atomic origin of the inverse piezoelectric effect in x-quartz. The external electric field was modelled by a saw-like potential V-ext in order to achieve translational symmetry within a supercell (SC) containing 72 atoms. The original trigonal quartz structure was repeated along the [110] direction, which corresponds to the direction of the external field. An electric field with 550 kV/mm was applied and the atomic positions of the SC were relaxed until the forces acting on the atoms vanished. In parts of the SC, V-ext changes almost linearly and thus the relaxed atomic positions can be used to determine the structural response due to the external electric field. The calculations provide the piezoelectric modulus of the correct order of magnitude. In contrast to previous models and in agreement with recent experimental results, the atomic origin of the piezoelectric effect can be described by a rotation of slightly deformed SiO4 tetrahedra against each other. The change of the Si-O bond lengths and the tetrahedral O-Si-O angles is one order of magnitude smaller than that of the Si-O-Si angles between neighbouring tetrahedra. The calculated changes of X-ray structure factors are in agreement with experiment when the theoretical data are extrapolated down to the much smaller field strength that is applied in the experiment (E < 10 kV/mm). (C) 2004 Elsevier Ltd. All rights reserved},
  language  = {en}
}
@article{SaphiannikovaGeueHennebergetal.2004,
  author    = {Saphiannikova, Marina and Geue, Thomas and Henneberg, Oliver and Morawetz, Knut and Pietsch, Ullrich},
  title     = {Linear viscoelastic analysis of formation and relaxation of azobenzene polymer gratings},
  doi       = {10.1063/1.1642606},
  year      = {2004},
  abstract  = {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 least three orders of magnitude. Force densities of about 10(11) N/m(3) were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility. (C) 2004 American Institute of Physics},
  language  = {en}
}
@article{PietschJarreSaldittetal.2004,
  author    = {Pietsch, Ullrich and Jarre, A. and Salditt, T. and Panzner, Tobias and Pfeiffer, F.},
  title     = {White beam x-ray waveguide optics},
  year      = {2004},
  abstract  = {We report a white beam x-ray waveguide (WG) experiment. A resonant beam coupler x-ray waveguide (RBC) is used simultaneously as a broad bandpass (or multibandpass) monochromator and as a beam compressor. We show that, depending on the geometrical properties of the WG, the exiting beam consists of a defined number of wavelengths which can be shifted by changing the angle of incidence of the white x-ray synchrotron beam. The characteristic far-field pattern is recorded as a function of exit angle and energy. This x-ray optical setup may be used to enhance the intensity of coherent x-ray WG beams since the full energetic acceptance of the WG mode is transmitted. (C) 2004 American Institute of Physics},
  language  = {en}
}
@article{HennebergPanznerPietschetal.2004,
  author    = {Henneberg, Oliver and Panzner, Tobias and Pietsch, Ullrich and Geue, Thomas and Saphiannikova, Marina and Rochon, Paul and Finkelstein, Kenneth D.},
  title     = {X-ray and VIS light scattering from light-induced polymer gratings},
  issn      = {0044-2968},
  year      = {2004},
  abstract  = {Sinusoidally shaped surface relief gratings made of polymer films containing, azobenzene moieties can be created by holographic illumination with laser light of about lambda approximate to 500 nm. The remarkable material transport takes place at temperatures far (100 K) below the glass transition temperature of the material. As probed by visible light scattering the efficiency of grating formation crucially depends on the polarization state of the laser light and is maximal when circular polarization is used. In contrast to VIS light scattering X-ray diffraction is most sensitive for periodic surface undulations with amplitudes below 10 nm. Thus, combined in-situ X-ray and visible light scattering at CHESS were used to investigate the dynamics of surface relief grating formations upon laser illumination. The time development of grating peaks up to 9th order at laser power of P = 20 mW/cm(2) could be investigated, even the onset of grating formation as a function of light polarization. A linear growth of grating amplitude was observed for all polarizations. The growth velocity is maximal using circularly polarized light but very small for s-polarized light},
  language  = {en}
}
@article{HennebergGeuePietschetal.2004,
  author    = {Henneberg, Oliver and Geue, Thomas and Pietsch, Ullrich and Winter, Bernd},
  title     = {Investigation of azobenzene side group orientation in polymer surface relief gratings by means of photoelectron spectroscopy},
  year      = {2004},
  abstract  = {The molecular orientation of azobenzene side groups in polymer films before (nonpatterned) and after (patterned) development of a surface relief grating has been investigated by photoelectron spectroscopy using synchrotron radiation. The photoemission spectra obtained for 60-100 eV photons of a patterned and a nonpatterned surface are similar when the polarization vector of the synchrotron light is parallel to the grating vector. However, for perpendicular excitation, considerable spectral intensity differences can be observed for 9-14 eV electron binding energy. The observed changes are attributed to the formation of well-oriented azobenzenes at the surface. (C) 2004 American Institute of Physics},
  language  = {en}
}
@article{PietschHazraChinietal.2004,
  author    = {Pietsch, Ullrich and Hazra, S. and Chini, T. K. and Sanyal, M. K. and Grenzer, J{\"o}rg},
  title     = {Ripple structure of crystalline layers in ion beam induced Si wafers},
  year      = {2004},
  abstract  = {Ion-beam-induced ripple formation in Si wafers was studied by two complementary surface sensitive techniques, namely atomic force microscopy (AFM) and depth-resolved x-ray grazing incidence diffraction (GID). The formation of ripple structure at high doses (similar to7x10(17) ions/cm(2)), starting from initiation at low doses (similar to1x10(17) ions/cm(2)) of ion beam, is evident from AFM, while that in the buried crystalline region below a partially crystalline top layer is evident from GID study. Such ripple structure of crystalline layers in a large area formed in the subsurface region of Si wafers is probed through a nondestructive technique. The GID technique reveals that these periodically modulated wavelike buried crystalline features become highly regular and strongly correlated as one increases the Ar ion-beam energy from 60 to 100 keV. The vertical density profile obtained from the analysis of a Vineyard profile shows that the density in the upper top part of ripples is decreased to about 15\% of the crystalline density. The partially crystalline top layer at low dose transforms to a completely amorphous layer for high doses, and the top morphology was found to be conformal with the underlying crystalline ripple},
  language  = {en}
}
@article{PietschGuptaPauletal.2004,
  author    = {Pietsch, Ullrich and Gupta, Amod and Paul, A. and Meneghini, C. and Mibu, K. and Maddalena, S. and Dal Toe, S. and Principi, G.},
  title     = {Structural characterization of epitaxial Fe/Cr multilayers using anomalous x-ray scattering and neutron reflectivity},
  year      = {2004},
  abstract  = {The interface structure of epitaxial Fe/Cr multilayers was studied using anomalous X-ray and neutron reflectivity. The analysis of X-ray reflectivity at three different energies provided a reliable information about the interface roughnesses. It is found that the Cr-on-Fc interface is more diffused as compared to the Fe-on-Cr interface and that the roughness exhibits a significant increase with increasing depth. The magnetic roughness, as determined from neutron reflectivity, is lower than the geometrical roughness, in conformity with the behavior of a number of magnetic thin films and multilayers. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{PietschGrenzerGrigorianetal.2004,
  author    = {Pietsch, Ullrich and Grenzer, J{\"o}rg and Grigorian, Souren A. and Weyers, Markus and Zeimer, Ute and Feranchuk, S. and Fricke, J. and Kissel, H. and Knauer, A. and Tr{\"a}nkle, G.},
  title     = {Nanoengineering of lateral strain-modulation in quantum well heterostructures},
  year      = {2004},
  abstract  = {We have developed a method to design a lateral band-gap modulation in a quantum well heterostructure. The lateral strain variation is induced by patterning of a stressor layer grown on top of a single quantum well which itself is not patterned. The three-dimensional (3D) strain distribution within the lateral nanostructure is calculated using linear elasticity theory applying a finite element technique. Based on the deformation potential approach the calculated strain distribution is translated into a local variation of the band-gap energy. Using a given vertical layer structure we are able to optimize the geometrical parameters to provide a nanostructure with maximum lateral band-gap variation. Experimentally such a structure was realized by etching a surface grating into a tensile-strained InGaP stressor layer grown on top of a compressively strained InGaAs-single quantum well. The achieved 3D strain distribution and the induced band-gap variation are successfully probed by x-ray grazing incidence diffraction and low-temperature photoluminescence measurements, respectively},
  language  = {en}
}
@article{SaphiannikovaHennebergGeneetal.2004,
  author    = {Saphiannikova, Marina and Henneberg, Oliver and Gene, T. M. and Pietsch, Ullrich and Rochon, Paul},
  title     = {Nonlinear effects during inscription of azobenzene surface relief gratings},
  issn      = {1520-6106},
  year      = {2004},
  abstract  = {Surface relief gratings were inscribed on azobenzene polymer films using a pulselike exposure of an Ar+ laser. The inscription process was initiated by a sequence of short pulses followed by much longer relaxation pauses. The development of the surface relief grating was probed by a He-Ne laser measuring the scattering intensity of the first- order grating peak. The growth time of the surface relief grating was found to be larger than the length of the pulses used. This unusual behavior can be considered as a nonlinear material response associated with the trans-cis isomerization of azobenzene moieties. In this study the polymer stress was assumed to be proportional to the number of cis-isomers. One-dimensional viscoelastic analysis was used to derive the polymer deformation. The rate of trans-cis isomerization increases with the intensity of the inscribing light; in the dark it is equal to the rate of thermal cis- trans isomerization. The respective relaxation times were estimated by fitting theoretical deformation curves to experimental data},
  language  = {en}
}
@article{PietschGuptaGuptaetal.2004,
  author    = {Pietsch, Ullrich and Gupta, Ajay and Gupta, Mukul and Ayachit, S. and Rajagopalan, S. and Balamurgan, A. K. and Tyagi, A. K.},
  title     = {Iron self-diffusion in nanocrystalline FeCr thin films},
  year      = {2004},
  abstract  = {Thin films of amorphous Fe85Zr15 alloy were deposited by ion-beam sputtering of a composite target. Analogous to the melt-spun amorphous alloys of similar composition, the crystallization of the amorphous film occurs in two steps, however, with a substantially reduced thermal stability. After completion of the first crystallization step which starts at 473 K, the microstructure consists of 12 nm nanocrystals of bcc-Fe embedded in a grain boundary region of the remaining amorphous phase. At 673 K, the remaining amorphous phase transforms into the Fe2Zr alloy. The self-diffusion measurements of iron in the nanocrystalline state and in the parent amorphous state has been carried out using secondary ion mass spectroscopy (SIMS) depth profiling and neutron reflectivity techniques. In contrast to the case of finemet Fe73.5Si13.5B9Nb3Cu1 alloy, where a significant enhancement of diffusivity takes place in the nanocrystalline state, in the present case the diffusivity in the nanocrystalline state is similar to that in the parent amorphous state. It is suggested that in this system the atomic diffusion occurs mainly via the grain boundary regions. The calculated values of the pre-exponential factor and the activation energy for the diffusion are D-0 = 1 x 10(-14+/-1) m(2)/s and E = (0.7 +/- 0.1) eV respectively. (C) 2004 Published by Elsevier B.V.},
  language  = {en}
}
@article{PietschSaphiannikovaHennebergetal.2004,
  author    = {Pietsch, Ullrich and Saphiannikova, Marina and Henneberg, Oliver and Geue, Thomas},
  title     = {Non-linear effects during inscription of azobenzene surface relief gratings},
  year      = {2004},
  language  = {en}
}
@book{PietschHolyBaumbach2004,
  author    = {Pietsch, Ullrich and Hol{\´y}, V{\´a}clav and Baumbach, Tilo},
  title     = {High resolution X-ray scattering from thin films and lateral nanostructures},
  series = {Advanced texts in physics : physics and astronomy online library},
  journal   = {Advanced texts in physics : physics and astronomy online library},
  edition   = {2. ed.},
  publisher = {Springer},
  address   = {New York},
  isbn      = {0-387-40092-3},
  pages     = {XVI, 408 S. : graph. Darst.},
  year      = {2004},
  language  = {en}
}
@article{HennebergRochonPanzneretal.2004,
  author    = {Henneberg, Oliver and Rochon, Paul and Panzner, Tobias and Finkelstein, Kenneth D. and Geue, Thomas and Saphiannikova, Marina and Pietsch, Ullrich},
  title     = {In-situ Investigation of Surface Relief Grating Formation in Photosensitive Polymers},
  year      = {2004},
  language  = {en}
}
@article{ZeimerPietschGrenzeretal.2005,
  author    = {Zeimer, Ute and Pietsch, Ullrich and Grenzer, Joerg and Fricke, J. and Knauer, A. and Weyers, Markus},
  title     = {Optimised two layer overgrowth of a lateral strain-modulated nanostructure},
  issn      = {0925-8388},
  year      = {2005},
  abstract  = {Recently it has been shown that lateral carrier confinement in an InGaAs quantum well (QW) embedded in GaAs can be achieved by using a laterally patterned InGaP stressor layer on top of the heterostructure. To exploit this effect in a device the structure has to be planarized by a second epitaxial step. It has been shown that the lateral strain modulation almost vanishes after overgrowth with GaAs, whereas overgrowth with a single ternary layer of opposite strain compared to the stressor layer suffers from strain induced decomposition. Here we show that the lateral carrier confinement of the initially free standing nanostructure can almost be maintained using a two step process for overgrowth, where a strained thin ternary layer is grown first followed by GaAs up to complete planarization of the patterned structure. Thickness and composition of the ternary layer are adjusted on the basis of finite element calculations of the strain distribution (FEM). The strain field achieved after overgrowth is probed by X-ray grazing- incidence diffraction (GID). (c) 2005 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{PietschPanznerLeitenbergeretal.2005,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Leitenberger, Wolfram and Vartanyants, Ivan A.},
  title     = {Coherence experiments using white synchrotron radiation},
  year      = {2005},
  abstract  = {Experiments at the bending magnet beamline at BESSY II (EDR beamline) profit from the excellent coherence properties of third generation synchrotron sources. Considering the exponentially decaying incident spectrum, and because no optical elements are installed except slits and vacuum windows, coherence experiments can be performed between 5 keV < E < 15 keV. First, the energy dependence of spatial coherence properties were determined measuring diffraction at single and double pinholes. Next, the coherent white radiation was used to probe the morphology of thin films in reflection geometry. The recorded intensity maps (reflectivity versus sample position) provide speckle patterns which reveal the locally varying sample morphology. Setting the incident angle, alpha(i), smaller or larger than the critical angle of total external reflection, alpha(c), one should be able to separate the surface height profile from the subsurface density modulation of a sample. The validity of this approach is verified at the example of reciprocal space maps taken from a polymer surface where we could reconstruct the lateral height profile from speckle data. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{PietschPanznerPfeifferetal.2005,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Pfeiffer, Franz and Robinson, Ian K.},
  title     = {Substrate morphology repetition in "thick" polymer films},
  year      = {2005},
  abstract  = {Using Grazing-incidence small-angle scattering (GISAXS) technique we investigated the surface morphology of polymer films spin-coated on different silicon substrates. As substrates we used either technologically smooth silicon wafers or the same silicon wafer coated with thin aluminium or gold films which show a granular structure at the surface. Although the polymer thickness exceeds 300 nm the GISAXS pattern of the film shows the same in-plane angle distribution Delta2theta as the underlying substrate. Annealing the polymer films at a temperature above its glass transition temperature Delta2theta changed from a broad to a narrow distribution as it is typically for films on pure silicon. The experiment can be interpreted by roughness replication and density fluctuation within the polymer film created while spin-coating at room temperature. Due to the low segment mobility there are density fluctuations which repeat the surface morphology of the substrate. Above the glass temperature the polymer density can be homogenized independently from the morphology of the substrate. (C) 2004 Elsevier B.V. All rights reserved},
  language  = {en}
}
@article{KarageorgievNeherSchulzetal.2005,
  author    = {Karageorgiev, Peter and Neher, Dieter and Schulz, Burkhard and Stiller, Burkhard and Pietsch, Ullrich and Giersig, Michael and Brehmer, Ludwig},
  title     = {From anisotropic photo-fluidity towards nanomanipulation in the optical near-field},
  issn      = {1476-1122},
  year      = {2005},
  abstract  = {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 which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the film behaves as an isotropic viscoelastic fluid, it shows considerable fluidity only in the direction parallel to the light field vector under linear polarized light. The fluidization phenomenon is related to photoinduced motion of azobenzene-functionalized molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also describe here how the photofluidization allows nanoscopic elements of matter to be precisely manipulated},
  language  = {en}
}
@article{GorfmanTsirelsonPietsch2005,
  author    = {Gorfman, S. V. and Tsirelson, Vladimir G. and Pietsch, Ullrich},
  title     = {X-ray diffraction by a crystal in a permanent external electric field : general considerations},
  issn      = {0108-7673},
  year      = {2005},
  abstract  = {The variations of X-ray diffraction intensities from a crystal in the presence of a permanent external electric field is modeled analytically using a first-order stationary perturbation theory. The change in a crystal, induced by an external electric field, is separated into two contributions. The first one is related to a pure polarization of an electron subsystem, while the second contribution can be reduced to the displacements of the rigid pseudoatoms from their equilibrium positions. It is shown that a change of the X-ray diffraction intensities mainly originates from the second contribution, while the influence of the pure polarization of a crystal electron subsystem is negligibly small. The quantities restored from an X-ray diffraction experiment in the presence of an external electric field were analyzed in detail in terms of a rigid pseudoatomic model of electron density and harmonic approximation for the atomic thermal motion. Explicit relationships are derived that link the properties of phonon spectra with E-field-induced variations of a structure factor, pseudoatomic displacements and piezoelectric strains. The displacements can be numerically estimated using a model of independent atomic motion if the Debye - Waller factors and pseudoatomic charges are known either from a previous single-crystal X-ray diffraction study or from density functional theory calculations. The above estimations can be used to develop an optimum strategy for a data collection that avoids the measurements of reflections insensitive to the electric-field-induced variations},
  language  = {en}
}
@article{PietschGrenzerBischoff2005,
  author    = {Pietsch, Ullrich and Grenzer, J{\"o}rg and Bischoff, Lothar},
  title     = {Grazing-incidence diffraction strain analysis of a laterally patterned Si wafer treated by focused Ge and An ion beam implantation},
  year      = {2005},
  abstract  = {Strain analysis of a laterally patterned Si-wafer was carried out utilizing X-ray grazing-incidence diffraction with synchrotron radiation. The lateral patterning was done by focused ion beam implantation using an ion source of liquid AuGeSi alloy. Samples were prepared by either 35 keV Au+ ions (dose: 0.2, 2 x 10(14) cm(-2)) or 70 keV Ge++ ions (dose: 8 X 10(14) cm(-1)). It was shown that due to implantation a periodical defect structure is created consisting of both implanted and not implanted stripes. The evaluated depth distribution of defects within the implanted stripes corresponds to that obtained by TRIM calculation. The induced strain distribution, however, shows no periodicity. This can be explained by an overlap of the strain fields created in each implanted stripe. (c) 2005 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinheim},
  language  = {en}
}
@article{PietschBodenthinGrenzeretal.2005,
  author    = {Pietsch, Ullrich and Bodenthin, Yves and Grenzer, J{\"o}rg and Geue, Thomas and M{\"o}hwald, Helmuth and Kurth, Dirk G.},
  title     = {Structure and temperature behavior of metallo-supramolecular assemblies},
  year      = {2005},
  abstract  = {A detailed structural analysis of a Langmuir-Blodgett (LB) multilayer composed of a polyelectrolyte-amphiphile complex (PAC) is presented. The PAC is self-assembled from metal ions, ditopic bis-terpyridines, and amphiphiles. The vertical structure of the LB multilayer is investigated by X-ray reflectometry. The multilayer has a periodicity of 57 A, which corresponds to an architecture of flat lying metallo-supramolecular coordination polyelectrolyte (MEPE) rods and upright-standing amphiphiles (dihexadecyl phosphate, DHP). In-plane diffraction reveals hexagonal packing of the DHP molecules. Using extended X-ray absorption fine structure (EXAFS) experiments, we prove that the central metal ion is coordinated to the terpyridine moieties in a pseudo-octahedral coordination environment. The Fe-N bond distances are 1.82 and 2.0 angstrom, respectively. Temperature resolved measurements indicate a reversible phase transition in a temperature range up to 55 degrees C. EXAFS measurements indicate a lengthening of the average Fe-N bond distance from 1.91 to 1.95 angstrom. The widening of the coordination cage upon heating is expected to lower the ligand field stabilization, thus giving rise to spin transitions in these composite materials},
  language  = {en}
}
@article{PietschBhattacharyaSanyaletal.2005,
  author    = {Pietsch, Ullrich and Bhattacharya, M. K. and Sanyal, M. K. and Geue, Thomas},
  title     = {Glass transition in Ultrathin Polymer Films : a Thermal Expansion Study},
  year      = {2005},
  abstract  = {The glass transition process gets affected in ultrathin films having thickness comparable to the size of the molecules. We observe systematic broadening of the glass transition temperature (T-g) as the thickness of an ultrathin polymer film reduces below the radius of gyration but the change in the average T-g was found to be very small. The existence of reversible negative and positive thermal expansion below and above T-g increased the sensitivity of our thickness measurements performed using energy-dispersive x-ray reflectivity. A simple model of the T-g variation as a function of depth expected from sliding motion could explain the results},
  language  = {en}
}
@article{PietschBodenthinMoehwaldetal.2005,
  author    = {Pietsch, Ullrich and Bodenthin, Yves and M{\"o}hwald, Helmuth and Kurth, Dirk G.},
  title     = {Inducing spin crossover in metallo-supramolecular polyelectrolytes through an amphiphilic phase transition},
  year      = {2005},
  abstract  = {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 d-orbital subsets decreases resulting in a spin transition from a low-spin to a high-spin state. The diamagnetic-paramagnetic transition is reversible. This concept is demonstrated in a metallo-supramolecular coordination polyelectrolyte-amphiphile complex self-assembled from ditopic bis-terpyridines, Fe(II) as central transition metal, and dialkyl phosphates as amphiphiles. The magnetic properties are studied in a Langmuir-Blodgett multilayer. The modularity of this concept provides extensive control of structure and function from molecular to macroscopic length scales and gives access to a wide range of new molecular magnetic architectures such as nanostructures, thin films, and liquid crystals},
  language  = {en}
}
@article{TalalaevTommElsaesseretal.2005,
  author    = {Talalaev, V and Tomm, JW and Elsaesser, T and Zeimer, Ute and Fricke, J and Knauer, A and Kissel, H and Weyers, Markus and Tarasov, GG and Grenzer, J{\"o}rg and Pietsch, Ullrich},
  title     = {Carrier dynamics in laterally strain-modulated InGaAs quantum wells},
  year      = {2005},
  abstract  = {We investigate the transient recombination and transfer properties of nonequilibrium carriers in an In0.16Ga0.84As/GaAs quantum well (QW) with an additional lateral confinement implemented by a patterned stressor layer. The structure thus contains QW- and quantum-wire-like areas. At low excitation densities, photoluminescence (PL) transients from both areas are well described by a rate equation model for a three-level system with a saturable interlevel carrier transfer representing the lateral drift of carriers from the QW regions into the wires. Small-signal carrier lifetimes for QW, wires, and transfer time from QW to wire are 180, 190, and 28 ps, respectively. For high excitation densities the time constants of the observed transients increase, in agreement with the model. In addition, QW and wire PL lines merge indicating a smoothening of the potential difference, i.e., the effective carrier confinement caused by the stressor structure becomes weaker with increasing excitation. (c) 2005 American Institute of Physics},
  language  = {en}
}
@article{PietschPanznerLeitenbergeretal.2005,
  author    = {Pietsch, Ullrich and Panzner, Tobias and Leitenberger, Wolfram and Vartanyants, Ivan A.},
  title     = {Coherence experiments at the EDR-beamline of BESSY II},
  year      = {2005},
  language  = {en}
}
@article{GorfmanTsirelsonPucheretal.2006,
  author    = {Gorfman, Semen and Tsirelson, Vladimir and Pucher, Andreas and Morgenroth, Wolfgang and Pietsch, Ullrich},
  title     = {X-ray diffraction by a crystal in a permanent external electric field : electric-field-induced structural response in alpha-GaPO4},
  issn      = {0108-7673},
  doi       = {10.1107/S0108767305036111},
  year      = {2006},
  abstract  = {For the first time, site-selective distortion has been investigated for two different structural units in the ternary compound alpha-GaPO4 under the influence of a permanent external electric field. Based on 54 measured reflection intensities, the electric-field-induced distortion of PO4 and GaO4 tetrahedra in alpha-GaPO4 crystals is evaluated using a model of pseudoatomic displacements introduced recently [Gorfman, Tsirelson \& Pietsch (2005). Acta Cryst. A61, 387- 396]. A stronger variation of the P-O bond lengths in the PO4 tetrahedron was found compared to the bonds in the GaO4 tetrahedron. The different distortions of the tetrahedra owing to the electric field were analysed in terms of the valence charge density of alpha-GaPO4 and its topological characteristics. The larger charge of the P pseudoatom compared to the Ga atom was recognized as the main reason for the higher sensitivity of the PO4 tetrahedron to a permanent external electric field},
  language  = {en}
}
@article{LeitenbergerPietsch2007,
  author    = {Leitenberger, Wolfram and Pietsch, Ullrich},
  title     = {A monolithic Fresnel bimirror for hard X-rays and its application for coherence measurements},
  issn      = {0909-0495},
  doi       = {10.1107/S0909049507003846},
  year      = {2007},
  abstract  = {Experiments using a simple X-ray interferometer to measure the degree of spatial coherence of hard X-rays are reported. A monolithic Fresnel bimirror is used at small incidence angles to investigate synchrotron radiation in the energy interval 5-50 keV with monochromatic and white beam. The experimental setup was equivalent to a Young's double-slit experiment for hard X-rays with slit dimensions in the micrometre range. From the high-contrast interference pattern the degree of coherence was determined.},
  language  = {en}
}