September 2011 Archives

A Korean group led by Professor J. H. Je (Pohang University of Science and Technology) has recently reported some interesting experiments on water vaporization by X-ray photons. The experiments were done at the undulator beamline, XSD 32-ID, Advanced Photon Source in Argonne, USA. It was found that water is vaporized at a rate of 5.5 pL/sec at a 100 msec exposure of 1-Å-wavelength (~13 keV) X-ray irradiation of around 10⁷ photons/μm² (0.1 photons/Ų), which corresponds to a dose rate of ~50 kGy/sec. They also confirmed that water vapor is reversibly condensed during pauses in irradiation. This result suggests that photoionization induces vaporization through the reduction of the surface tension of water. For more information, see the paper, "X-ray-induced water vaporization", B. M. Weon et al., Phys. Rev. E84, 032601 (2011).

A research team led by Professor I. Robinson (London Centre for Nanotechnology, University College London) recently analyzed how gold nanocrystal changes after the adsorption of organic molecules because of the strain field. So far, it has been difficult to observe such influence of adsorbed molecules on the particle structure. The team employed the coherent X-ray diffraction method, which is extremely sensitive to displacement of atoms, and therefore to adsorption-induced near-surface stress in a single nanocrystal. It was discovered that the stress generated by thiol adsorption on gold has a fundamentally different nature in the curved, nominally spherical, regions of the crystal surface than in its flat facets. The magnitude of surface stress was also quantitatively analyzed and discussed. The experiments were done with coherent X-rays of 8.92 keV from the 34-ID-C beamline of the Advanced Photon Source (APS), Argonne, USA. For more information, see the paper, "Differential stress induced by thiol adsorption on facetted nanocrystals", M. Watari et al., Nature Materials 10, 862 (2011).

rofessor Young-June Kim (University of Toronto, Canada) and his colleagues have recently reported a Fe Kβ X-ray emission spectroscopy study on iron-based superconductors, such as PrFeAsO, Ba(Fe,Co)₂As₂, LiFeAs, Fe_1+x(Te,Se), and A₂Fe₄Se₅ (where A=K, Rb, and Cs). They found that the materials possess local magnetic moments even in their paramagnetic phases. By analyzing Fe Kβ_1,3 and Kβ' spectra using the integrated absolute difference method, the local moment size for each sample was determined. It was found that the value is independent of temperature or carrier concentration but varies significantly across different families. Specifically, all iron pnictide samples have local moments of about 1μ_B/Fe, while FeTe and K₂Fe₄Se₅ families have much larger local moments of ~2μ_B/Fe and ~3.3μ_B/Fe, respectively. Such differences point to the importance of considering the contribution of multi-orbital physics in describing magnetism of these compounds. For more information, see the paper, "Revealing the dual nature of magnetism in iron pnictides and iron chalcogenides using X-ray emission spectroscopy", H. Gretarsson et al., Phys. Rev. B84, 100509(R) (2011).

The European Molecular Biology Laboratory (EMBL) and the European XFEL have signed a Memorandum of Understanding, thereby laying the foundation for close future collaboration in deciphering the structure and dynamics of biomolecules. For further information, visit the Web page, http://www.xfel.eu/

A German group led by Professor U. Klemradt (Aachen University) has recently performed an X-ray photon correlation spectroscopy (XPCS) experiment on martensitic transformation of a Au_50.5Cd_49.5 single crystal. XPCS experiments basically consist of the observation of a time-dependent speckle pattern caused by scattering of coherent X-ray photons, and give information on the dynamics of phase transformations in soft and hard condensed matter at atomic length scales. The measurement was done at ID 10A, European Synchrotron Radiation Facility (ESRF) in Grenoble, France. A standard Bragg scattering geometry was employed to see the fluctuations of the symmetric (0 0 1) Bragg reflection from the polished surface of the Au-Cd single crystal. The research team observed slow non-equilibrium-dynamics in a narrow temperature interval in the direct vicinity of the otherwise athermal phase transformation. For more information, see the paper, "Slow Aging Dynamics and Avalanches in a Gold-Cadmium Alloy Investigated by X-Ray Photon Correlation Spectroscopy", L. Muller et al., Phys. Rev. Lett. 107, 105701 (2011).