Volume 230, Issue 4 - Special Issue: Element Specific Structure Determination Using Modern X-ray and Neutron Techniques, Part 2 / Wolf-Christian Pilgrim, Koichi Hayashi, Shinya Hosokawa, Pawel Korecki

A fast calculation algorithm for three-dimensional atomic image reconstruction from X-ray fluorescence holography (Barton's method) is described. This method employs HEALPix, an algorithm that is used to subdivide a spherical surface in which each pixel covers an identical surface area. Using this algorithm, the computing time for atomic image reconstruction is reduced by a factor of 14.5. The time can be reduced further by parallel processing and an optimization of the resolution parameter of HEALPix.

The influence of matrix effects i.e. beam attenuation (BA) and indirect excitation (IE) on local structure imaging with multiple-energy x-ray fluorescence holography is studied using computer simulations. An analytic formalism is developed which allows for the description of BA and IE when the thickness of the sample is arbitrary. It is shown that beyond the thin-sample approximation, in specific cases, the measured holograms cannot be treated as entirely element sensitive. Consequently, it is demonstrated that due to the reduction of element sensitivity, spurious maxima can arise in the holographic reconstruction which can be misinterpreted as atomic images. The proposed formalism allows one to analyse BA and IE and to correct for them. It opens way for quantitative interpretation of x-ray fluorescence holograms.

X-ray fluorescence holograms of ZnSe and Zn 0.74 Mn 0.2 Be 0.06 Se single crystals were measured by means of a table-top laboratory setup. Holograms were recorded simultaneously for Zn K, Se K and, in the case of the mixed crystal, for Mn K X-ray fluorescence. Analysis of the holograms was based on a linear regression algorithm preceded by the matrix effects correction. It enabled the determination of real and imaginary parts of the structure factors from which atomic structures around different fluorescing atoms could be reconstructed. In particular, direct imaging of Mn lattice sites in Zn 0.74 Mn 0.2 Be 0.06 Se unambiguously demonstrated that the majority of Mn atoms occupy substitutional Zn positions. However, it was shown that the precise analysis of Mn lattice site distribution can be hindered due to extinction effects which have to be included in the analysis of XFH in the case of non-centrosymmetric samples.

To observe the difference of atomic heights between the Se and Te layers with respect to the Fe layer in FeSe 0.4 Te 0.6 single crystal, a Fe Kα fluorescence X-ray holography (XFH) experiment was performed at room temperature. The crystal structure of superconductor FeSe 0.4 Te 0.6 obtained by X-ray diffraction (XRD) at a low temperature has distinct z -coordinates of Se and Te, remarkably different from each other. The reconstructed atomic image around central Fe atoms by XFH, however, reveals the different and complex results.

The crystalline structure and quality of ZnSnAs 2 :Mn films grown by molecular beam epitaxy (MBE) on InP substrates are two of the most important issues in preparing spintronic structures such as magnetic quantum wells and spin-based transistors. We conducted high-resolution X-ray diffraction (XRD) measurements to clarify the crystalline structure of ZnSnAs 2 :Mn thin films. The XRD measurements reveal with very high accuracy that the samples have a sphalerite structure with high epitaxial quality. These results, within the limits of measurement accuracy support the previous assumption that ZnSnAs 2 :Mn thin films are coherently clustered ferromagnetic semiconductors without breaking the continuity of the sphalerite structure, leading to ferromagnetism with a high Curie temperature close to that of zincblende MnAs.

The local atomic structure near an Nb atom in a Ti–20 at. %Nb alloy single crystal, which consists of the β , α ′′ and ω Ti phases, was investigated by means of X-ray fluorescence holography (XFH). The atomic images were reconstructed in the vicinity of an Nb atom, which is one of the typical β stabilizing elements in β Ti alloys. Most atoms in the β Ti primary phase were reconstructed clearly. The atoms in the α ′′ Ti martensite phase could not be reconstructed, because the α ′′ Ti martensite has the crystal structure with low symmetry. Some atoms in the ω Ti fine precipitates were reconstructed successfully, although the amount of the ω Ti phase was much smaller than that of the β phase. An Nb atom and its first nearest neighbors tend to keep the BCC structure even upon the ω Ti phase formation.

Upon a core level excitation by circularly polarized light (CPL), the angular momentum of light, i.e. helicity, is transferred to the emitted photoelectron. This phenomenon can be confirmed by the parallax shift measurement of the forward focusing peak (FFP) direction in a stereograph of the atomic arrangement. The angular momentum of the emitted photoelectron is the sum of CPL helicity and the magnetic quantum number (MQN) of the initial state that define the quantum number of the core hole final state. The core hole may decay via Auger electron emission, where in this two electron process the angular momentum has to be conserved as well. Starting from a given core hole, different Auger decay channels with different final state energies and angular momenta of the emitted Auger electrons may be populated. Here we report the observation and formulation of the angular momentum transfer of light to Auger electrons, instead of photoelectrons. We measured photoelectron and Auger electron intensity angular distributions from Cu(111) and Cu(001) surfaces as a function of photon energy and photoelectron kinetic energy. By combining Auger electron spectroscopy with the FFP shift measurements at absorption threshold, element- and MQN-specific hole states can be generated in the valence band.

Extended X-ray absorption fine structure (EXAFS) spectroscopy and molecular dynamics (MD) simulations are two complementary techniques widely used to study the atomic structure of materials. Their combined use, known as the MD-EXAFS approach, allows one to access the structural information, encoded in EXAFS, far beyond the nearest coordination shells and to validate the accuracy of the interaction potential models. In this study we demonstrate the use of the MD-EXAFS method for a validation of several force-field models on an example of the cubic-perovskite SrTiO 3 and hexagonal wurtzite-type ZnO crystals.

EXAFS spectroscopy is an element-specific method that can provide perhaps the most extensive information on the local atomic structure and lattice dynamics for a broad class of materials. Conventional methods of EXAFS data treatment are often limited to the nearest coordination shells of the absorbing atom due to the difficulties in accurate accounting for the large number of correlated structural parameters that have to be included in the analysis. In this study we overcome this problem by applying novel simulation-based method: reverse Monte Carlo simulations, coupled with the evolutionary algorithm and with a powerful signal processing technique – wavelet transform. This complex approach was applied to the analysis of the W L 3 -edge and Co K-edge EXAFS spectra of crystalline CoWO 4 , which exists in antiferromagnetic state below 55 K. Temperature dependence of the local environment up to 4.3 Å around both metal ions was reconstructed in the range from 10 K to 300 K, and the rigidity of the tungstate structure due to zigzag chains of WO 6 and CoO 6 octahedra was analyzed.

A cobalt ultrathin film, which shows a large change in Curie temperature by an electric field application, has been studied by x-ray reflectometry with applying electric fields. The cobalt film was made by the sputtering method on top of a Pt buffer layer, and capped with a MgO layer. X-ray reflectometry shows that the change in Co thickness caused by the applied voltage up to ±10 V was less than 0.06 Å. The reflectivity signal intensity shows a characteristic kink at the Co K -absorption edge. The spectrum does not show any change with applying voltage. As a result, electric field effects on the structure and chemical states of Co in the Co ultrathin film were found to be minor.

The interface between the N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) ionic liquid and a gold (111) surface was investigated with time-resolved X-ray reflectivity in order to clarify the dynamics of structural changes of the electric double layer after changing the electrode potential. In the experiment, the potential was switched repeatedly between +1.5 V and −1.5 V every 2 s or every 0.3 s, while measuring the specular X-ray reflectivity. When the potential was switched every 2 s, the time dependence of the reflectivity was different from that of the accumulated charge. This indicates structural relaxation processes that occur on a slower time scale than the acummulation of the charge at the electric double layer. When the potential was switched every 0.3 s, on the other hand, the reflectivity changes followed the evolution of the charge of the electric double layer within the experimental precision, indicating that slow relaxation processes without charge transfer do not contribute significantly to structural changes at this time scale.