Structures, mechanical properties and applications of silk fibroin materials

High-resolution angle-resolved photoemission spectroscopy reveals bosonic modes in a SrTiO3 substrate coupling to electrons in an FeSe overlayer to facilitate high-temperature superconductivity. Bulk iron selenide (FeSe) is a superconductor with a critical temperature Tc = 8 K, but superconductivity is substantially enhanced in single-unit cell films of FeSe grown on strontium titanate (SrTiO3 or STO) substrates, where superconducting energy gaps open at temperatures close to the boiling point of liquid nitrogen (77 K). This raises the question of whether the substrate has a contributory role in this enhancement. Zhi-Xun Shen and colleagues report high-resolution angle-resolved photoemission spectroscopy (ARPES) results that reveal bosonic modes (thought to be oxygen optical phonons) in the SrTiO3 substrate coupling to electrons in the FeSe overlayer to facilitate high-temperature superconductivity. Such coupling helps superconductivity in most channels, so the pairing enhancement described here may well work for other superconducting materials, as well as for FeSe. Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO3 or STO) substrates have recently shown1,2,3,4 superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature Tc, as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO3 (refs 5, 6, 7), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations8,9,10,11,12,13,14. Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO3.

Interfacial mode coupling as the origin of the enhancement of T(c) in FeSe films on SrTiO3.

X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle’s size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research w...

Small Angle X-ray Scattering for Nanoparticle Research

Traditional nanostructured design of cerium oxide catalysts typically focuses on their shape, size, and elemental composition We report a different approach to enhance the catalytic activity of cerium oxide nanostructures through engineering high density of oxygen vacancy defects in these catalysts without dopants The defect engineering was accomplished by a low pressure thermal activation process that exploits the nanosize effect of decreased oxygen storage capacity in nanostructured cerium oxides

http://ncesr.unl.edu/wordpress/wp-content/uploads/2011/01/11-07-11-511-Zeng-Defect-Engineering-in-Cubic-Cerium-article.pdf

Defect Engineering in Cubic Cerium Oxide Nanostructures for Catalytic Oxidation

Thermo-mechanical behavior of low-dimensional systems: The local bond average approach

1. Introduction.- 2. Geometry, Peak Indexing, and Experimental Techniques.- 2.1 Geometry of Multiple Diffraction.- 2.1.1 Real-Space and Reciprocal-Space Representations.- 2.1.2 Persistent and Coincidental Multiple Diffractions.- 2.1.3 Lattice-Symmetry Dependence-Intrinsic Multiple Diffraction.- 2.1.4 Multiple-Diffraction Possibilities.- 2.1.5 Decomposition of Multiple Diffraction.- 2.2 Experimental Techniques for Obtaining Multiple Diffraction.- 2.2.1 Collimated-Beam Technique.- 2.2.2 Divergent-Beam Techniques.- 2.3 Indexing Multiple Diffraction Patterns.- 2.3.1 Reference Vector Method.- 2.3.2 Orientation Matrix Method.- 2.4 Indexing Kossel Patterns.- 2.4.1 Stereographic Projection Method.- 2.4.2 Combined Gnomonic-Stereographic Projection Method.- 3. Kinematical Theory of Diffraction.- 3.1 Equation of Power Transfer for Multi-Beam Cases.- 3.2 Approximate Solutions to the Equation of Power Transfer.- 3.3 Integrated Intensity and the Lorentz-Polarization Factors.- 3.4 Path Lengths of X-Ray Beams in Crystals.- 3.5 Exact Solution to the Power-Transfer Equation.- 3.6 Iterative Calculation for Reflection Power.- 3.7 Dynamical Treatment for Kinematical Reflections.- 3.8 Diffraction in Multi-Layered Crystals.- 3.9 Peak Width, Beam Divergence, and Mosaic Spread.- 4. Dynamical Theory of X-Ray Diffraction.- 4.1 Fundamental Equation of Wavefields.- 4.2 Polarization of Wavefields.- 4.3 Dispersion Surface.- 4.3.1 Geometry of the Dispersion Surface.- 4.3.2 Excitation of the Dispersion Surface.- 4.4 Energy Flow.- 4.4.1 Poynting Vectors and the Dispersion Surface.- 4.4.2 Group Velocity and Energy Flow.- 4.5 Modes of Wave Propagation.- 4.5.1 Wavefields of Modes.- 4.5.2 Number of Permitted Modes.- 4.6 Absorption.- 4.7 Boundary Conditions.- 4.8 Excitation of Mode and Excitation of Beam.- 4.9 Intensity of Wavefield (Standing-Wave) in Crystal.- 4.10 Consideration of the Spherical-Wave Nature of the Incident X-Rays.- 5. Approximations, Numerical Computing, and Other Approaches.- 5.1 Two-Beam Approximation for Three-Beam Diffraction.- 5.1.1 General Considerations.- 5.1.2 Three-Beam Transmission Case (Borrmann Diffraction).- 5.1.3 Diffracted Intensity of Three-Beam Bragg Reflection.- 5.1.4 Integrated Reflection for Non-Absorbing Crystals.- 5.1.5 Diffraction in Absorbing Crystals.- 5.2 Procedures for Numerical Computing.- 5.3 Quantum Mechanical Approach.- 5.4 N-Beam Diffraction in Other Types of Interaction.- 5.4.1 Two-Beam Diffraction with Specular Reflection.- 5.4.2 Interaction of X-Rays with Phonons.- 6. Case Studies.- 6.1 Bragg-Type Multiple Diffraction from Gallium Arsenide, Indium Arsenide and Indium Phosphide-Kinematical Interpretation.- 6.2 Three-Beam Borrmann Diffraction-Dynamical Calculation.- 6.3 Simultaneous Four-Beam Borrmann Diffraction.- 6.4 Three-Beam Bragg-Laue and Bragg-Bragg Diffraction.- 6.5 Four-Beam Bragg-Laue Diffraction.- 7. Applications.- 7.1 Experimental Determination of X-Ray Reflection Phases Application to Crystal Structure Determination.- 7.1.1 General Consideration of the X-Ray Phase Problem.- 7.1.2 Reflection Phases and Multiple Diffraction.- 7.1.3 Experimental Methods for Phase Determination Using Multiple Diffraction.- 7.1.4 Determination of Centrosymmetric Crystal Structures in Practice.- 7.1.5 Phase Determination for Non-Centrosymmetric Crystals.- 7.2 Determination of Lattice Constants of Single Crystals.- 7.2.1 Divergent-Beam Photographic Methods.- 7.2.2 Collimated-Beam Method.- 7.3 Determination of Lattice Mismatch in Thin Layered Materials.- 7.4 Multi-Beam X-Ray Topography.- 7.5 Multi-Beam X-Ray Interferometer.- 7.6 Monochromatization of X-Ray Beams.- 7.7 Plasma Diagnosis.- 7.8 Determination of Mosaic Spread of Crystals.- 7.9 Multi-Beam X-Ray Standing-Wave Excited Fluorescence Technique for Surface Studies-A Proposed Method.- 7.10 Possible Future Trend of Development.- References.

Multiple diffraction of x-rays in crystals

The kinematical theory of X-ray diffraction follows closely the secondary ex-tinction theory, which can be applied to situations involving very weak or no interaction between the incident wave and the diffracted wave. The theory is therefore suited for diffractions from small and imperfect crystals. If the diffraction from a large perfect crystal is concerned, the primary extinction becomes important. This is because the atomic planes in a perfect crystal are well ordered. Once the atomic planes are set to the exact Bragg diffraction position for an incident wave, they are also perfectly aligned for the reflected wave to be diffracted back into the direction of the incident wave. In analogy with the optics of the visible spectrum, a phase lag of about π/2 of the reflected wave in the incident-wave direction relative to the incident wave results. Namely, the phase difference between the twice-reflected wave and the incident wave leads to the modification of the amplitude of the incident wave. Similar modification takes place for the diffracted wave. Thus, the interaction between the incident and the diffracted waves via multiple reflection dominates the diffraction process. If a relatively strong reflection is involved, this multiple reflection effect cannot be ignored. Moreover, the phase lag of the reflected wave occurs for each plane of atoms inside the crystal. The resultant phase lags of the incident and diffracted waves modify their phase velocities in the crystal, thus changing the index of refraction.

Dynamical Theory of X-Ray Diffraction

An algorithm is proposed for solving the fundamental equations of the wavefield for multiple-wave dynamical X-ray diffraction with grazing incidence and scattering geometry. The algorithm is developed based on the representation of the electric fields and wavevectors in a single Cartesian coordinate system with one of the axes in the direction normal to the crystal surface. With this representation, the fundamental equations of the wavefield can be solved as an eigenvalue–eigenvector problem involving a 4N × 4N scattering matrix in which the matrix elements are not related to the polarization, N being the number of waves. The polarization factors are absorbed in the vector components of the eigenvectors. This simplifies the process in solving the fundamental equation, avoids unnecessary approximations on polarization in the matrix calculation and makes the algorithm very generic so that it can be applied to multiple diffractions of all kinds, including grazing-angle and wide-angle geometries. The intensity distribution of an Umweganregung of a specularly diffracted wave in a polarization-forbidden state is calculated using this algorithm as a demonstration.

An Algorithm for Solving Multiple‐Wave Dynamical X‐ray Diffraction Equations

1. Introduction.- 2. Elements of X-Ray Physics and Crystallography.- 3. Diffraction Geometry.- 4. Experimental Techniques.- 5. Kinematical Theory of X-Ray Diffraction.- 6. Dynamical Theory of X-Ray Diffraction.- 7. Theoretical Approaches.- 8. Dynamical Diffraction Properties and Behaviors.- 9. Applications.- References.- Figure Acknowledgements.

X-Ray Multiple-Wave Diffraction : Theory and Application

A practical method is described for the phase determination of x-ray reflections from single crystals. Considerations on the dynamical interaction in multiple diffraction and on the relative rotation of the crystal lattice with respect to the Ewald sphere reveal both experimentally and theoretically the phase dependence of the reflected intensity. Applications can be made for a direct experimental determination of phases without carrying out the complicated dynamical calculation.

Shih-Lin Chang

Papers

Multiple diffraction of x-rays in crystals

Dynamical Theory of X-Ray Diffraction

An Algorithm for Solving Multiple‐Wave Dynamical X‐ray Diffraction Equations

X-Ray Multiple-Wave Diffraction : Theory and Application

Direct Determination of X-Ray Reflection Phases