Session X33: Focus Session: X-ray, Gamma Ray, and Electron Diffraction

First-principles study of $\gamma$-ray detectors: Cs-based compounds

In an effort to find good candidate materials for $\gamma$-ray detectors, Cs-based compounds containing heavy elements, such as in Cs$_2$Hg$_6$S$_7$, were investigated. We performed $ab$-$initio$ density functional theory calculations using the full-potential linearized augmented plane wave method\footnote {Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)}. The screened-exchange local density approximation (sX-LDA) scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the band gap of Cs$_2$Hg$_6$S$_7$ is predicted to be 1.23 eV by sX-LDA, corrected from 0.51 eV by LDA. Therefore, Cs$_2$Hg$_6$S$_7$ seems to be useful as a $\gamma$-ray detecting material in terms of the high density and the band gap. Not only the pristine bulk but also several defect configurations were calculated, which affects the transport properties. Defect formation energy determinations allow us to predict which defect configuration is most likely in Cs$_2$Hg$_6$S$_7$.   

Resolving Interface Interactions in Layered Structures in 3 Dimensions

3D micro-Laue diffraction was used to probe interface interactions in layered structures. Indented Cr/NiAl composite with alternating submicron size Cr and NiAl lamellae was chosen as a model material. Differential aperture microscopy revealed a twin orientation relationship at the interface between the Cr and NiAl lamellae in the as grown state. The indentation-induced alternation of compressive/tensile residual strains in the neighboring Cr and NiAl lamellae was observed. Line broadening analysis found a two orders of magnitude increase of dislocation density in the near-surface zone in the center of the indent.   

X-ray Studies of Quantum Fluctuations in SrTiO3 across its Quantum Paraelectric Phase Transition

Strontium titanate (SrTiO3), with a simple cubic perovskite structure at room temperature, displays a number of interesting phase transitions as a function of temperature and pressure. It exhibits an antiferrodistortive transition at Tc = ~105 K, resulting in a cubic-to-tetragonal structural distortion. This transition has been studied in detail by M. Holt and H. Hong using thermal diffuse scattering and inelastic x-ray scattering methods. Another phase transition takes place at 37 K, below which the system assumes a quantum paraelectric phase. It is generally believed that the classical free energy of the system favors a ferroelectric phase, but this transition never fully develops because of quantum fluctuation. Despite a large number of prior investigations, this quantum phenomenon is still not well understood in terms of the underlying lattice dynamics. Using inelastic x-ray scattering, we have obtained new information about this classic system, including mode softening as a function of temperature.   

X-ray nanotomography study of insulator-coated tips with sub-micron conducting apex for the combination of scanning probe microscopy and synchrotron radiation

Hard X-ray nanotomography provides an important three-dimensional view of insulator-coated ``smart tips'' that can be utilized for modern emerging scanning probe techniques. Tips, entirely coated by an insulating SiO2 film except at the very tip apex, are fabricated by means of electron beam physical vapor deposition, focused ion beam milling and ion beam-stimulated oxide growth. Although x-ray tomography studies confirm the structural integrity of the oxide film, transport measurements suggest the presence of defect-induced states in the SiO2 film [1]. The development of insulator-coated tips can facilitate nanoscale analysis with electronic, chemical, and magnetic contrast by synchrotron-based scanning probe microscopy. \\[4pt] [1] Rose at al., Appl. Phys. Lett. 99, 173102 (2011).   

Grazing-incidence coherent x-ray imaging in true reflection geometry

The development of the 3$^{rd}$ and 4$^{th}$ generation synchrotrons has stimulated extensive research activities in x-ray imaging techniques. Among all, coherent diffractive imaging (CDI) shows great promise, as its resolution is only limited by the wavelength of the source. Most of the CDI work reported thus far used transmission geometry, which however is not suitable for samples on opaque substrates or in which only the surfaces are the regions of interest. Even though two groups have performed CDI experiments (using laser or x-ray) in reflection geometry and succeeded in reconstructing the planar image of the surface, the theoretical underpinnings and analysis approaches of their techniques are essentially identical to transmission CDI. Most importantly, they couldn't obtain the structural information along sample thickness direction. Here, we introduce a reflection CDI technique that works at grazing-incidence geometry. By visualizing Au nanostructures fabricated on Si substrate, we demonstrate that this innovative imaging technique is capable of obtaining both 2D and 3D information of surfaces or buried structures in the samples. In the meanwhile, we will also explain the grazing-incidence-scattering based-algorithm developed for 3D phase retrieval.   

Soft x-ray absorption spectroscopy measurement methods with using x-ray scattering techniques

Methodology via x-ray absorption spectroscopy (XAS) has been actively employed for exploring the microscopic aspects of materials. In particular, such method within soft x-ray energy range is very useful for investigating strongly correlated systems, such as high T$_C$ superconductor, and multiferroic, including heterostructures. While XAS approach on such materials has been used, however we sometimes confront a few of experimental difficulties; electron motion distortion under external fields, charging effect, and saturation effect. In this presentation, we introduce an alternative approach for overcoming the difficulties in conventional XAS measurement, which uses soft x-ray scattering techniques, i.e., reflection and diffraction. Due to photon-in and photon-out nature, probing depth becomes longer and possible to reduce several problems in conventional total electron yield method. The results of demonstrations on simple monoxide CoO and multiferroic Y-type hexaferrites will be given.   

Structure uncovering from fluctuation x-ray scattering of randomly oriented nanoparticles

We have carried out a fluctuation x-ray scattering experiment on platinum coated gold nanoparticles randomly oriented on a substrate. A complete algorithm for determining the electron density of an individual particle from diffraction patterns of many particles, randomly oriented about a single axis is demonstrated. This algorithm operates on angular correlations among the measured intensity distributions and recovers the angular correlation functions of a single particle from measured diffraction patterns. Taking advantage of the cylindrical symmetry of the nanoparticles, we proposed a cylindrical model to reconstruct the structure of the nanoparticle by fitting both the experimental ring angular auto-correlation and the small angle scattering data. The physical meaning of the resulted structure is discussed in terms of their statistical distributions of the shape and electron density profile.   

Hard X-ray resonance in sapphire crystal cavities using back diffraction

The Fabry-Perot type resonators using back diffraction from sapphire crystals for hard X-ray was investigated. On the basis of its less absorption and hexagonal structure, the resonator in sapphire crystals underwent a pure 2-beam diffraction which could enhance the resonance interference and improve finesse compared with the one in silicon crystals. The resonators were manufactured from sapphire crystals using microelectronic lithography process with thickness of a few tens $\mu $m. With synchrotron radiation of energy resolution $\Delta $E=0.82 meV at 14.315 keV, X-ray back diffraction from two monolithic sapphire crystal plates shows resonance fringes clearly resulting from coherent interaction inside the energy gap of the (0 0 30) reflection. These experimental results of sapphire cavities imply the potential application for X-ray optics.   

X-ray wave guiding using three-beam Bragg-surface diffraction

A diffraction-type of X-ray wave guide, in contrast to refraction-type, is proposed using three-beam diffraction geometry to generate a surface diffracted beam propagating along the direction of the wave guide. The three-beam Bragg-Surface diffraction involves a symmetric Bragg reflection and a surface diffraction. The former is used to guide a wide-angle incident beam into a silicon crystal. The simultaneously occurring surface diffraction then guides the diffracted beam propagating along the direction of the wave guide that is parallel to the crystal surface. A wave guide with a shallow ditch is then manufactured along the direction of the surface diffraction using the conventional lithographic technique. As a whole the wave guide consists of a three-layer structure of tantalum/photon resist (PMMA)/tantalum, on the Silicon substrate. The surface diffracted X-rays can then be confined in and guided along the layer of photon resist. Details of the design of the wave guide and synchrotron diffraction experiments will be reported.   

Combined surface plasmon resonance and X-ray absorption spectroscopy

We present a system for the excitation and measurement of surface plasmons in metallic films based on the Kretschmann-Raether configuration that can be installed in a synchrotron beamline. The device was mounted an tested in a hard X-ray Absorption beamline, BM25 Spline at ESRF. Whit this device it is possible to carry on experiments combining surface plasmon and X-ray absorption spectroscopies. The surface plasmons can be use to monitor in situ changes induced by the X-rays in the metallic films or the dielectric overlayer. Similarly, the changes in the electronic configuration of the material when surface plasmons are excited can be measured by X-ray absorption spectroscopy. The resolution of the system allows to observe changes in the signals of the order of 10$^{-3}$ to 10$^{-5}$ depending on the particular experiment and used configuration. The system is available for experiments at the beamline.   

Probing of Strain Mediated Hybrid Multiferroic Devices

Smart materials for sensor technology, (non) volatile device memories for information technology, and ultrasound generators in medical imaging have one thing in common, their active elements consist of ferroelectrics (FE) driven by voltages or ferromagnetics (FM) driven by magnetization. In the quest to design high functionality devices to meet today's consumer technological demands, high focus has been given to multiferroic [1]. However, the coexistence of magnetic order and ferroelectric polarization combined in a single-phase material has proven to be rear as most of these materials tend to have low magnetic ordering temperatures and are often antiferromagnets, in which the magnetoelectric (ME) coupling effect is intrinsically small. We utilize an alternative approach to design multiferroic-hybrid devices based on FE-FM composites where the ME coupling emerges from strain-mediated interaction between individual phases [2]. We develop a nonlinear thermodynamic theory for strain-mediated direct ME effect and Bragg Ptychographic Coherent Diffraction Imaging (BCDI) serves as the unique tool of choice for sub-nanometer resolution nondestructive probing of the order parameters in the devices [1] N. Spaldin and M. Fiebig, Science 309, 391 (2005). [2] E. Fohtung et al., submitted (2012)   

First-Principles Modeling for Low-Energy Electron Diffraction Spectra

We present a computational method to incorporate density-functional theory (DFT) into the calculation of the reflectivity in low-energy electron diffraction (LEED). Rapid and accurate analysis of diffraction spectra will facilitate the development of low-energy electron microscopy. The dynamical analysis of the electron reflectivity is traditionally carried out using multiple scattering theory with spherically symmetric (muffin-tin) potentials. However, for directionally bonded materials, such as semiconductors, the actual crystal potentials in the interstitial region can be significant, particularly for very low energy electrons. DFT with nonlocal pseudopotentials yields the low-energy electronic structure more accurately. In typical DFT calculations for surfaces, a finite slab is set up in a large unit cell with periodic boundary conditions. By matching the plane waves representing the LEED beams to the Kohn-Sham wave functions at the boundaries of the supercell, we determine the diffraction intensities. To demonstrate that our matching approach is not limited by the finite size of the supercell, we first consider trial models with exact solutions. We then use this approach to analyze the electron diffraction from graphene and compare with features in the band structure.