Federico Zontone

X-ray cross correlation analysis uncovers hidden local symmetries in disordered matter.

We explore the different local symmetries in colloidal glasses beyond the standard pair correlation analysis. Using our newly developed X-ray cross correlation analysis (XCCA) concept together with brilliant coherent X-ray sources, we have been able to access and classify the otherwise hidden local order within disorder. The emerging local symmetries are coupled to distinct momentum transfer (Q) values, which do not coincide with the maxima of the amorphous structure factor. Four-, 6-, 10- and, most prevalently, 5-fold symmetries are observed. The observation of dynamical evolution of these symmetries forms a connection to dynamical heterogeneities in glasses, which is far beyond conventional diffraction analysis. The XCCA concept opens up a fascinating view into the world of disorder and will definitely allow, with the advent of free electron X-ray lasers, an accurate and systematic experimental characterization of the structure of the liquid and glass states.

Correlation spectroscopy with coherent X-rays

X-ray photon correlation spectroscopy (XPCS) is a novel technique for the study of slow dynamics in disordered materials. It overcomes limitations of visible light scattering techniques such as multiple scattering or limitations in Q-range by using coherent X-rays from third generation synchrotron radiation sources. Applications to the static and dynamic behavior of complex fluids and to slow dynamics in hard condensed matter systems are reviewed.

Refractive x-ray lenses

Parabolic refractive x-ray lenses are novel optical components for the hard x-ray range from about 5 keV to about 120 keV. They are compact, robust, and easy to align and to operate. They can be used like glass lenses are used for visible light, the main difference being that the numerical aperture is much smaller than 1 (of the order of 10−4–10−3). They have been developed at Aachen University and are made of beryllium, boron, aluminium and silicon. Their main applications are in micro- and nanofocusing, in imaging by absorption and phase contrast. In combination with tomography they allow for three-dimensional imaging of opaque media with sub-micrometre resolution. Finally, they can be used in speckle spectroscopy by means of coherent x-ray scattering. References to a number of applications are given.

Performance of a picosecond x-ray delay line unit at 8.39 keV

A prototype device capable of splitting an x-ray pulse into two adjustable fractions, delaying one of them with the aim to perform x-ray photon correlation spectroscopy and pump-probe type studies, was designed, manufactured, and tested. The device utilizes eight perfect silicon crystals in vertical 90 degrees scattering geometry. Its performance has been verified with 8.39 keV synchrotron radiation. The measured throughput of the device with a Si(333) premonochromator at 8.39 keV under ambient conditions is 0.6%. Time delays up to 2.62 ns have been achieved, detected with a time resolution of 16.7 ps.

Development of a hard X-ray delay line for X-ray photon correlation spectroscopy and jitter-free pump–probe experiments at X-ray free-electron laser sources

A prototype device capable of splitting an X-ray pulse into two adjustable fractions, delaying one of them with the aim of performing split pulse X-ray photon correlation spectroscopy and pump–probe type studies was designed and manufactured. Time delays up to 2.95u2005ns have been demonstrated. The achieved contrast values of 56% indicate a feasibility of performing coherence-based experiments with the delay line.

Three-dimensional coherent X-ray diffractive imaging of whole frozen-hydrated cells

Since its first experimental demonstration in 1999, coherent diffractive imaging (CDI) has been applied to image a broad range of samples using advanced synchrotron radiation, X-ray free-electron lasers, high harmonic generation and electrons. Here, the first experimental demonstration of cryogenic CDI for quantitative three-dimensional imaging of whole frozen-hydrated cells is reported. As a proof of principle, the three-dimensional mass density of the sub-cellular organization of a Neospora caninum cell is determined based on its natural contrast.

Nanocluster formation in silicate glasses by sequential ion implantation procedures

Abstract Cluster formation is studied after sequential double implantation (Cu, Ni; Ag, S) in silica and soda–lime glass. The structure and properties of nanocluster composites are investigated by optical absorption spectroscopy and transmission electron microscopy, evidencing the formation of core-shell structures. The presence of metal alloy clusters is also investigated by means of synchrotron-radiation-based techniques.

Towards a metallic YH3 phase at high pressure

Abstract The switchable mirror compound YH 3 has a high-pressure behavior at variance with current band-structure models. It remains transparent at least up to 25 GPa. At high pressure, the optical gap decreases linearly extrapolating to zero for 55±8 GPa, above which pressure a metallic state is expected. This is in disagreement with a theoretically predicted insulator-to-metal phase transition at 1.5 GPa. In-situ structural studies using synchrotron radiation reveal a 2% drop in the c -lattice vector at 4 GPa, while both above and below the phase transition, the spectra are consistent with a hcp structure. Hence this phase transition is probably due to a rearrangement of the hydrogen lattice positions only and the predicted hcp to fcc transition is not observed.

Deviation from the virtual crystal approximation in disordered Au-Cu alloy nanocrystals: EXAFS and GIXRD investigation

Abstract The short-range order and the crystalline structure of AuCu and AuAg alloy nanoclusters obtained by sequential ion implantation in silica are investigated by extended X-ray absorption fine structure (EXAFS) spectroscopy and by grazing incidence X-ray diffraction (GIXRD). While in the case of AuAg alloy the nearest neighbor interatomic distances correspond to the lattice parameter, in the case of chemically disordered solid solution AuCu alloy a significant deviation from the virtual crystal approximation is observed: the Au–Au, Cu–Cu and Au–Cu nearest neighbor distances are different, only their average corresponding to the fcc lattice parameter. This result, mainly related to the different atomic radius of the two alloy components, is for the first time reported for nanostructured systems.

Double implantation in silica glass for metal cluster composite formation: a study by synchrotron radiation techniques

Abstract Silica glass containing metal clusters is studied for both basic and applied aspects, related to the physics of cluster formation and to the optical properties of these materials. To obtain such composite structure, Cuxa0+xa0Ni, Auxa0+xa0Cu, Auxa0+xa0Ag, Cuxa0+xa0Co, and Cuxa0+xa0Ag sequential implantations in fused silica were realized. The resulting systems, after possible annealing in various atmospheres, were studied by synchrotron radiation-based techniques, namely, extended X-ray absorption fine structure (EXAFS) spectroscopy, grazing incidence X-ray diffraction (GIXRD), and grazing incidence small angle X-ray scattering (GISAXS). The unique potential of these techniques is the capability to investigate dilute (volume fraction of clusters⩾0.01) and very thin (50 nm) systems. In the presented experiment, both pure and alloy clusters in the nanometer range of size were observed to form, pointing out the complexity of the cluster formation process in terms of physical and chemical driving forces.