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Atomic scale step structure and orientation of a curved surface ZnO single crystal
No full textWe have investigated the surface structure of a curved ZnO-crystal, going from the (0001)-facet at 0° miscut to the (101¯4)-facet at a miscut of 24.8° using scanning tunneling microscopy and low energy electron diffraction. We find that the surface separates locally into (0001)-terraces and (101¯4)-facets, where the ratio between the facets depends on the miscut angle. In X-ray photoemission spectroscopy (XPS) the intensity of an O 1s component scaling with the step density of the surface is observed. No other facets were observed and the surface maintains a high degree of order over all angles. Such a curved ZnO crystal can be used for systematic studies relating the step density to the chemical reactivity using XPS to probe the curved surface at different positions
Understanding the Conversion Process of Magnetron-Deposited Thin Films of Amorphous ReO to Crystalline ReO upon Thermal Annealing
No full textThin films of rhenium trioxide (ReO) were produced by reactive DC magnetron sputtering from a metallic rhenium target, followed by annealing in the air in a range of temperatures from 200 to 350 °C. Nanocrystalline single-phase ReO films were obtained after being annealed at about 250 °C. The thin films appeared bright red in reflected light and blue-green in transmitted light, thus showing an optical transparency window in the spectral range of 475–525 nm. The film exhibited a high conductivity as evidenced by macro- and nanoscale conductivity measurements. The long-range and local atomic structures of the films were studied in detail by structural methods, such as X-ray diffraction and X-ray absorption spectroscopy. The oxidation state (6+) of rhenium was confirmed by X-ray photoemission and X-ray absorption spectroscopy. The nanocrystalline morphology of the annealed films was evidenced by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The obtained results allowed us to propose a mechanism of rhenium oxide conversion from the initially amorphous ReO phase to cubic ReO
Alexandrite: an attractive thin-disk laser material alternative to Yb:YAG?
No full textYb:YAG thin-disk (TD) technology has enabled construction of laser/amplifier systems with unprecedented average/peak power levels, and has become the workhorse of many scientific investigations. On the other hand, for some applications, the narrow emission bandwidth of Yb:YAG limits its potential, and the search for alternative broadband TD gain media with suitable thermo-optomechanical parameters is ongoing. The alexandrite gain medium has a broad emission spectrum centered around 750 nm, possesses thermomechanical strength that even outperforms Yb:YAG, and has unique spectroscopic properties enabling efficient laser operation even at elevated temperatures. In this work, we have numerically investigated the power scaling potential of continuous-wave (cw) alexandrite lasers in TD geometry for the first time. Using a detailed laser model, we have compared the potential cw laser performance of Yb:YAG, Ti:Sapphire, Cr:LiSAF, Cr:LiCAF, and alexandrite thin-disk lasers under similar conditions and show that among the investigated transition metal-doped gain media, alexandrite is the best alternative to Yb:YAG in power scaling studies at room temperature. Our analysis further demonstrates that potentially Ti:Sapphire is also a good alternative TD material, but only at cryogenic temperatures. However, in comparison with Yb:YAG, the achievable laser gain is relatively low for both alexandrite and Ti:Sapphire, which then requires usage of low-loss cavities with small output coupling for efficient cw operation
In situ cell for x-ray absorption spectroscopy of low volatility compound vapors
No full textTechnologically relevant gas phase processes rely on reactants in vapor form for the production of thin films and nanoparticles. An instrument is described which enables the investigation of such vapors by x-ray absorption spectroscopy. Corresponding in situ studies provide information about gas phase precursor chemistry and optimized synthesis processes. The setup consists of a sealed vapor container heated by a hot air bath. Inert gas filling and temperature monitoring are implemented. Fluid dynamic simulations reveal a homogeneous temperature distribution without hot or cold spots. Temperature stability better than 1 K for at least 190 min allows time-dependent measurements or improved signal to noise ratios by averaging of datasets. Iron acetylacetonate is studied as a model system. X-ray absorption spectra measured by fluorescence are of high quality, allowing a detailed analysis of X-ray Absorption Near Edge Structure (XANES) and extended x-ray absorption fine structure. A molecular structure transformation is observed in XANES spectra of iron acetylacetonate vapor above 480 K probably due to the loss of one ligand. The setup allows the investigation of low volatility compounds with vapor pressures above 2 kPa at temperatures up to 520 K
Cryo-EM, X-ray diffraction, and atomistic simulations reveal determinants for the formation of a supramolecular myelin-like proteolipid lattice
No full textMyelin protein P2 is a peripheral membrane protein of the fatty acid–binding protein family that functions in the formation and maintenance of the peripheral nerve myelin sheath. Several P2 gene mutations cause human Charcot-Marie-Tooth neuropathy, but the mature myelin sheath assembly mechanism is unclear. Here, cryo-EM of myelin-like proteolipid multilayers revealed an ordered three-dimensional (3D) lattice of P2 molecules between stacked lipid bilayers, visualizing supramolecular assembly at the myelin major dense line. The data disclosed that a single P2 layer is inserted between two bilayers in a tight intermembrane space of ∼3 nm, implying direct interactions between P2 and two membrane surfaces. X-ray diffraction from P2-stacked bicelle multilayers revealed lateral protein organization, and surface mutagenesis of P2 coupled with structure-function experiments revealed a role for both the portal region of P2 and its opposite face in membrane interactions. Atomistic molecular dynamics simulations of P2 on model membrane surfaces suggested that Arg-88 is critical for P2-membrane interactions, in addition to the helical lid domain. Negatively charged lipid headgroups stably anchored P2 on the myelin-like bilayer surface. Membrane binding may be accompanied by opening of the P2 β-barrel structure and ligand exchange with the apposing bilayer. Our results provide an unprecedented view into an ordered, multilayered biomolecular membrane system induced by the presence of a peripheral membrane protein from human myelin. This is an important step toward deciphering the 3D assembly of a mature myelin sheath at the molecular level
Revealing Hot and Long-Lived Metastable Spin States in the Photoinduced Switching of Solvated Metallogrid Complexes with Femtosecond Optical and X-ray Spectroscopies
No full textAn atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d4–d7 transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interest. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrays have not yet been elucidated due to the expected complications associated with the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond time scale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and long-lived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays
Deformation of the gravitational wave spectrum by density perturbations
No full textWe study the effect of primordial scalar curvature perturbations on the propagation of gravitational waves over cosmic distances. We point out that such curvature perturbations deform the isotropic spectrum of any stochastic background of gravitational waves of primordial origin through the (integrated) Sachs-Wolfe effect. Computing the changes in the amplitude and frequency of the propagating gravitational wave induced at linear order by scalar curvature perturbations, we show that the resulting deformation of each frequency bin of the gravitational wave spectrum is described by a linearly biased Gaussian with the variance σ2 ≃ ∫ d&ln; k Δ R2, where Δ; R2(k) denotes the amplitude of the primordial curvature perturbations. The linear bias encodes the correlations between the changes induced in the frequency and amplitude of the gravitational waves. Taking into account the latest bounds on Δ R2 from primordial black hole and gravitational wave searches, we demonstrate that the resulting O(σ) deformation can be significant for extremely peaked gravitational wave spectra. We further provide an order of magnitude estimate for broad spectra, for which the net distortion is O(σ2)
Discovery of terrestrial allabogdanite , and the effect of Ni and Mo substitution on the barringerite-allabogdanite high-pressure transition
No full textMinerals formed at high pressures are sensitive indicators of extreme pressure-temperature conditions that occur in nature. The discovery of the high-pressure polymorph of (Fe,Ni)P, allabogdanite in the surficial pyrometamorphic rocks of the Hatrurim Formation (the Mottled Zone) surrounding the Dead Sea basin in Israel is the first terrestrial occurrence of a mineral previously only found in iron meteorites. Stepwise annealing experiments demonstrate that allabogdanite is metastable at ambient pressure and that it irreversibly transforms into its low-pressure polymorph, barringerite, upon heating to 850±50°C. High-pressure high-temperature diamond-anvil cell (DAC) experiments confirm the results of annealing experiments. The DAC data indicate that Hatrurim allabogdanite is metastable below 7.4 GPa, and the low- to high-pressure phase transition (barringerite→allabogdanite) occurs at 25±3 GPa and 1400±100°C. The observed transition pressure of Hatrurim allabogdanite is significantly higher than that of pure synthetic FeP (8 GPa), due to partial substitution of Fe for Ni (4 wt.%) and Mo (2.5 wt.%). Because the influence of substituting impurities on the conditions of phase transitions can be unexpectedly strong, our findings confirm that caution should be exercised when extrapolating data from experiments on synthetic compounds to natural systems. Based on the discovery of terrestrial allabogdanite (Fe,Ni)P coupled with experiments probing the phase transitions in this natural composition, we contend that terrestrial allabogdanite formed via transformation from barringerite and posit potential scenarios of its formation
Accelerator-Based Tunable THz Source for Pump-and-Probe Experiments at the European X-Ray Free-Electron Laser Facility
No full textThere is a high demand for intense THz sources since “many excitation mechanisms of matter resonate in the terahertz regime” especially for condensed matters. “Accelerator-based THz sources provide the wide tunability together with high intensity and repetition rates beyond 100 kHz, that will enable broad application at the European XFEL to the most interesting scientific problems in the field” [1]. Supported by European XFEL a proof of principle study is started at the Photo-Injector Test Facility located at DESY in Zeuthen site (PITZ). Since PITZ and European XFEL electron sources are identical the X-ray and THz radiation can be produced with identical bunch train structure so that for every X-ray pulse a corresponding THz pulse can be provided for the pump-and- probe experiments
The complex structural mechanisms behind strain curves in bismuth sodium titanate–barium titanate
No full textIn this work, the lead-free composition (1-)BiNaTiO–BaTiO (BNT–BT) with x = 0.12 was investigated using in situ Synchrotron x-ray powder diffraction. With the applied electric field, the pseudo-cubic relaxor phase reversibly transforms to a ferroelectric state. The reversibility is still preserved after 104 bipolar electric field cycles. A Rietveld refinement with a structure, strain, and texture analysis using a model based on the atomic scale was applied for four frequencies from 10−4 to 101 Hz. The analysis allowed us to separately determine the two coexisting phases, their electric field dependent evolution, and the underlying strain mechanisms. For all the applied frequencies, we showed that domain switching is the only strain mechanism appearing in the tetragonal phase and the lattice strain is the only mechanism in the rhombohedral phase. The coercive field of the tetragonal phase (4 kV/mm) is found to be higher than that of the rhombohedral phase (3 kV/mm). This divergence has not been observed in previously investigated lead-containing materials and cannot be detected solely using macroscopic strain and polarization experiments. Moreover, the domain strain abruptly starts to occur only after a threshold field value and exhibits high hysteresis. The lattice strain, on the other hand, starts nearly from the beginning and increases more linearly during the bipolar field cycle. It could, therefore, be demonstrated that complex structural mechanisms underlie the apparent clear and continuous macroscopic strain curve. These findings are crucial for all actuator materials undergoing a relaxor to ferroelectric phase transformation and provide approaches and strategies to optimize lead-free materials for tailored applications