Published as:
Marquardt, H., S. Speziale, K. Marquardt, H. J. Reichmann, Z. Konopkova, W. Morgenroth and H. P. Liermann (2011). The Effect of Crystallite Size and Stress Condition on the Equation of State of Nano-crystalline MgO. J. Appl. Phys. 110: 113512.

Marquardt, H., A. E. Gleason, K. Marquardt, S. Speziale, L. Miyagi, G. Neusser, H. R. Wenk and R. Jeanloz (2011). Elastic Properties of MgO Nano-crystals and Grain Boundaries to High Pressures. Phys. Rev. B 84: 064131

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Published as: “Evidence of delocalized excitons in amorphous solids”, Physical Review Letters 105, 116401 (2010).

We studied the temperature dependence of the absorption coefficient of amorphous SiO₂ near and above the fundamental band gap, as obtained by Kramers-Kronig dispersion analysis of reflectivity spectra. We demonstrate the main excitonic resonance at 10.4 eV to feature a close Lorentzian shape redshifting with increasing temperature. This provides a strong evidence of excitons being almost fully delocalized notwithstanding the structural disorder intrinsic to amorphous SiO₂. Since exciton mobility is ultimately related to the translational symmetry of a lattice, this result is remarkable because it shows that, as far as exciton dynamics is concerned, an amorphous solid can actually be “much less disordered” than thought so far. [More]

Published as: “Ultrafast Melting of a Charge-Density Wave in the Mott Insulator 1T-TaS₂”, Physical Review Letters 105, 187401 (2010).

The combination of an optical laser with FEL pulses from FLASH in a pump-probe setup uniquely enables timeresolved core-level photoemission spectroscopy with high temporal and energy resolution. In a proof-ofprinciple experiment, a charge-density wave in the layered strongly correlated electron material 1T-TaS2 is driven out of equilibrium by an intense optical laser pulse and the subsequent nonequilibrium dynamics is probed by FLASH pulses on the sub-picosecond time scale. The results establish FLASH as an ultrafast movie camera for electronic structural dynamics at the surfaces of solids. [More]

Published as: “Collective Lamb Shift in Single-Photon Superradiance”, Science 328, 1248 (2010).

An atom, after being resonantly excited by a pulse of radiation, will typically de-excite with a characteristic decay time. An ensemble of many identical atoms with just one of the atoms being excited will decay much faster than a single atom. This is a cooperative effect in the interaction of light and matter known as superradiance. We have prepared a superradiant system by embedding an ultrathin layer of resonant ⁵⁷Fe nuclei in a planar X-ray cavity. The collective spectral response of the nuclei in the cavity exhibits a frequency shift, the collective Lamb shift. Cooperative emission and the collective Lamb shift reveal interesting aspects of the many-body physics of photons and atoms. [More]

Published as: “Pentacene Thin-Film Transistors Encapsulated by a Thin Alkane Layer Operated in an Aqueous Ionic Environment”, Advanced Materials 22, 4350–4354 (2010). Letters 105, 116401 (2010).

Stable operation of pentacene thin-film transistors in an aqueous ionic environment was made possible by encapsulating them with a 50 nm thick layer of tetratetracontane (TTC, C₄₄H₉₀), a long chain alkane molecule. The morphology of the TTC film deposited on a pentacene thin film surface by thermal vacuum deposition strongly depends on the nature of the substrate and on the deposition parameters. X-ray diffraction measurements show that TTC deposition rate of 4.0 Å/s and a substrate temperature of about 25°C result in densest and smoothest TTC layers with the alkane chains oriented along the surface. The pentacene thin film structure is not changed during TTC deposition, which is a clear advantage of vacuum deposition against ... [More]

Published as: "Sacrificial Tamper Slows Down Sample Explosion in FLASH Diffraction Experiments", Physical Review Letters 104, 064801 (2010).

Coating samples in a sacrificial external layer slows down the rate at which samples are damaged by the ultra-intense X-ray beam provided by the FLASH free electron laser. The extremely intense light produced by FLASH enables coherent imaging using single ultrafast X-ray pulses. The same intense pulse destroys the sample, and diffraction must occur before the X-ray beam significantly alters the sample structure, requiring ever-shorter pulses as resolution is increased. Experiments at FLASH showed that it is possible to delay sample explosion for up to picosecond time duration by coating samples in a sacrificial ‘tamper’ layer. This approach enables the use of longer and more intense pulses for high-resolution single-particle imaging, lowering the barrier to high-resolution structural studies on isolated ... [More]

Published as: “Molecular basis of the death-associated protein kinase-calcium/calmodulin regulator complex”. Sci. Signal. 3, ra6 (2010).

Investigating how cells communicate is fundamental to our understanding of biology. We have determined the first molecular structure of a protein kinase in complex with the universal regulator protein calmodulin and calcium. The structure, which provides a model for many other protein kinases, allows insight into how its function could be specifically blocked. Since the selected protein kinase is associated with a number of frequently occurring tumours, our data provide a basis for developing new anti-cancer therapies. [More]

Published as: “Spontaneous Symmetry Breaking by Charge Stripes in the High Pressure Phase of Superconducting La_1.875Ba_0.125CuO₄”, Phys. Rev. Lett. 104, 057004 (2010).

Competing magnetic and electronic interactions in the cuprate high temperature superconductors often result in nanoscale inhomogeneity of the charge and spin density, a property that could be relevant to the unconventional superconductivity. A very interesting example is the spin and charge stripe order in the copper-oxide planes of La_1.875Ba_0.125CuO₄. Theoretically, stripe order would break the rotational and translational symmetry of the planes, but so far it was observed only in distorted phases, where the reduced crystal symmetry allows for alternative explanations. If stripe order represented a fundamental instability, one would expect to see it develop in otherwise undistorted planes. We have used pressure to ... [More]

Published as: “Single-pulse resonant magnetic scattering using a soft x-ray free-electron laser”, Phys. Rev. B 81, 100401 (R) (2010).

Single-pulse resonant magnetic scattering experiments were performed by using soft X-ray pulses generated by the free-electron laser FLASH. A magnetic diffraction pattern was recorded from a Co/Pt multilayer sample at the Co M_2,3 edge with a single 30 femtosecond long FEL pulse, without destroying the sample. [More]

Published as: “Coherent x-ray imaging of defects in colloidal crystals’’, Phys. Rev. B Vol. 81, 224105 (2010).

Real crystalline materials, in contrast to the idealized model of a perfect crystal, contain a broad spectrum of defects. These defects determine most of the mechanical, optical and electronic properties of the crystals. Visualization of the defect core in a bulk material with X-ray methods still remains a challenge. Here we demonstrate how to use coherent X-ray diffractive imaging (CXDI) to map such defects in colloidal crystals. The inversion of the diffraction patterns reveals the arrangement of colloidal particles in a face-centered cubic (fcc) lattice as well as defects in the form of stacking faults in the (111) planes. [More]

Published as: “Analysis of periodic dislocation networks using x-ray diffraction and extended finite element modeling”, Appl. Phys. Lett. 96, 131905 (2010).

Self organization provides a possibility to create dislocation free regions in heteroepitaxial semiconductor structures in order to improve the quality of subsequently grown devices. In this work the statistical properties of a periodic array of dislocations in thin PbSe films deposited on a PbTe buffer were investigated by means of X-ray diffraction. A novel method for the calculation of the displacement field due to the dislocations was used in order to circumvent the limitations of analytical solutions. A short range order model was applied successfully for the description of the dislocation distribution and verified by computer simulations. [More]

Published as: “Study of the Interface between Rhodium and Carbon Nanotubes” ACS Nano 4, 1680 – 1686 (2010).

Carbon nanoelectronics require the development of reproducible, high-quality ohmic contacts between carbon nanomaterials and their contact electrodes. The hunt is on for new candidate metals. We have followed the interface formation between rhodium and carbon nanotubes using X-ray photoelectron spectroscopy at 3.5 keV photon energy in combination with high-resolution transmission electron microscopy. Rh nucleates at defect sites whether initially present or induced by oxygen plasma treatment. Experimental results compared to density functional theory calculations show that Rh strongly interacts with the nanotube surface and is less sensitive to the presence of oxygen than other metals that form ohmic nanotube contacts. [More]

Published as: “Cryptotomography: Reconstructing 3D Fourier Intensities from Randomly Oriented Single-Shot Diffraction Patterns”, Phys. Rev. Lett. 104, 239902 (2010).

A new method called cryptotomography combines two dimensional (2D) diffraction patterns of identical particles, each collected in a random and unknown orientation, to unravel the full three-dimensional (3D) image of the average particle. We carried out the first demonstration of cryptotomography by collecting diffraction patterns of ellipsoidal iron oxide nanoparticles at FLASH. These particles were shot across the FEL beam, in vacuum, using the techniques being developed for single-molecule imaging. This demonstration is an important step towards the 3D imaging of noisy and weakly scattering biological samples at X-ray free electron laser sources. [More]

Published as: “Laser-Induced Alignment of Self-Assembled Films of an Oligopeptide β-Sheet on the Water Surface”, Angew. Chem. Int. Ed. 122, 2404–2407 (2010).

A pulsed infrared laser beam was used to align oligopeptide molecules at the air-water interface. The oligopeptide was designed to form a cyclic β-strand dimer in a volatile solution. After spreading the solution onto the water surface, illumination with linearly polarized laser light during solvent evaporation induced formation of an aligned crystalline film, whereas circularly polarized laser light did not. [More]

Published as: “Quaternary structure of the human Cdt1-Geminin complex regulates DNA replication licensing”, Proc. Natl. Acad. Sci. USA 106, 19807 (2009).

Small angle scattering becomes a streamline tool in modern structural molecular biology providing valuable information about overall structure and conformational changes of native individual proteins and functional complexes. The variety of questions addressed by the technique ranges from evaluation of the overall geometrical parameters and low-resolution shape reconstruction to structure validation and molecular modelling. Recently the method was applied in combination with X-ray crystallography for a structural study of the human Cdt1-Geminin complex regulating DNA replication licensing which is essential for genomic integrity. It was found that the transition of the complex from heterohexamer to heterotrimer acts as a switch between inhibitory and permissive states for DNA replication. [More]

Published as: “The liquid-liquid phase transition in silicon revealed by snapshots of valence electrons”, Proc. Natl. Acad. Sci. USA 107, 39, 16772–16776 (2010).

Microscopic models for the „anomalies of water“ are still lacking an experimental proof, although anomalous thermodynamic behaviour is common for a class of matter that forms tetrahedral networks - like water, diamond or silicon. Their phase diagrams are very rich, but the exploration of large areas has been limited mostly to theoretical studies. And yet, remainders of those experimentally unaccessible areas contribute to the properties at standard conditions. For example a possible explanation for the anomalies is based on the existence of a liquid-liquid phase transition in the supercooled region. With the combination of ultrashort optical laser pulses and soft X-ray pulses from FLASH, we study the melting dynamics of silicon in detail. We find two distinct melting steps separated ... [More]

Published as: "Decoherence in attosecond photoionization", Phys. Rev. Lett. 106, 053003 (2011)

Attosecond science holds the promise of controlling electron motion to manipulate physical processes at the atomic level. One way of inducing electron motion is photoionization using an attosecond laser pulse. The typical time scale of electronic motion in atoms, molecules, and condensed matter systems ranges from a few attoseconds (1as = 10 ^-18s) to tens of femtoseconds (1fs = 10 ^-15s) [1]. Remarkable progress in high-harmonic generation made it possible to generate attosecond pulses as short as 80 as [2]. Attosecond pulses have opened the door to real-time observations of the most fundamental processes on the atomic scale like photoionization [3], Auger decay [4], and valence ... [More]

Published as: “Structural insights into eRF3 and stop codon recognition by eRF1”, Genes and Development Vol. 23 No. 9, 1106-1118, (2009).

Developments in Small-Angle X-ray Scattering (SAXS) at the EMBL, Hamburg Outstation made this method a high-throughput, low resolution tool providing information about overall structure and conformational changes of biological macromolecules in solution. SAXS is often employed along with protein crystallography to elucidate structure of functional complexes, such as that of proteins eRF1 and eRF3, which assure fast release of peptides from the ribosome during termination of protein synthesis. It was found that the M domain of eRF3 stimulates a GTPase activity of eRF1. The latter protein recognizes the stop codon of the messenger RNA, and in concert with eRF3 mediates dissociation of ribosome, which plays a critical role in the recycling of the ribosome before the next translation event. [More]

Published as: “Stabilization of Antiferromagnetic Order in FeO Nanolayers”, Physical Review Letters 103, 097201 (2009).

Ferromagnetic order in thin films is strongly affected when their thickness is reduced to the nanometre regime. The main reason for this are thermal excitations that lead to fluctuations of the magnetic moments. It is well known that thick antiferromagnetic buffer layers with a high magnetic anisotropy can be used to stabilize magnetic order in ultra-thin ferromagnetic films. This effect is of high importance for modern data storage technology. On the way to further miniaturization one consequently asks what happens to antiferromagnetic layers in the ultra thin limit and how their magnetic properties are influenced by surrounding magnetic material. These questions are subject of an investigation involving ultra-thin layers of Fe and its native oxide. [More]

Published as: “X-ray cross correlation analysis uncovers hidden symmetries in disordered matter”, Proceedings of the National Academy of Sciences 106, 11511-11514 (2009).

The different local symmetries in colloidal glasses were investigated by coherent X-ray scattering beyond the standard pair correlation analysis. By analyzing the resulting speckle patterns with a newly developed X-ray cross correlation analysis (XCCA) concept, it is possible to access and classify the otherwise hidden local order within disorder. Four-, six-, ten- and, most prevailing, five-fold symmetries are observed coupled to distinct momentum transfer (Q) values, which do not coincide with the maxima of the amorphous structure factor. The observation of dynamical evolution of these symmetries forms a connection to dynamical heterogeneities in glasses. The XCCA concept opens up a fascinating view into the world of disorder and will definitely allow, with the advent of ... [More]

Published as: "Terminal assembly of sarcomeric filaments by intermolecular beta-sheet formation". Trends Biochem Sci. 34, 33-39 (2009). PubMed PMID: 18996015.

The contraction/relaxation cycle of muscle cells translates into large molecular movements of several filament systems in sarcomeres, requiring special molecular mechanisms to maintain their structural integrity. Recent structural and functional data from filaments with extensive arrays of immunoglobulin-like domains have for the first time unravelled a common function of their terminal domains: assembly and anchoring of the respective filaments. The available data have also revealed a number of common principles governing terminal filament assembly. In all these cases, protein-protein interactions are mediated by antiparallel dimerisation modules, via intermolecular β-sheets. These observations provide an attractive model for several other filament proteins, which have not yet been ... [More]

Published as: “Origin and temperature dependence of radiation damage in biological samples at cryogenic temperatures”, PNAS, vol. 107, no. 3, 1094-1099 (2010).

Radiation damage to biological samples has become a major limitation for experiments at brilliant 3rd generation synchrotron sources such as Petra III and free electron lasers. Radiation damage leads to sample deterioration and hence drastically limits the applicability of experimental techniques employing high doses of radiation [1, 2]. Cryo-cooling of samples down to 100 K or even lower can reduce but not prevent radiation damage [3].

Until now radiation damage to biological samples has not been completely understood. By combining different experimental techniques we have identified radiation induced C-H bond cleavage and the ... [More]

Published as: “Near edge X-ray absorption fine structure spectroscopy with X-ray free-electron lasers”, Appl. Phys. Lett. 95, 134102 (2009).

A new method to perform X-ray absorption spectroscopy experiments at a free-electron laser has been developed. Instead of selecting a narrow bandwidth of the incident beam with a grating-monochromator, a dispersive set-up is used. The incident pink radiation from the SASE-FEL is dispersed by the grating and collected by an area detector. A special sample-preparation method has been used in order to measure the intensity and energy distribution of the incident and absorbed beam simultaneously. This method can be improved in the future to perform pump-probe experiments with XAS as the probe with fs-temporal resolution at an FEL. [More]

Published as: "R.R. Fäustlin et al. Observation of Ultrafast Nonequilibrium Collective Dynamics in Warm Dense Hydrogen", Phys. Rev. Lett. 104, 125002 (2010).

The short pulse duration and high intensity of FLASH soft x-ray radiation at DESY allows us to generate and probe highly homogeneous warm dense non-equilibrium hydrogen within a single light pulse. By analyzing the spectrum of the 13.5 nm Thomson scattered light we determine the plasma temperature and density. The results are compared via simulations with different models for impact ionization, which is the main interaction on this early femtosecond time scale during the evolution of the plasma. We find that classical models of this interaction describe our dense plasma conditions better than state of the art theories. This has implications for various fields ranging from planetary astrophysics to inertial confinement fusion. [More]