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Register a new userPhase of the transmitted wave in the dynamical theory and quasi-kinematical approximation
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Variation of the phase of the beam transmitted through a crystalline material as a function of the rocking angle is a well known dynamical effect in x-ray scattering. Unfortunately, it is not so easy to measure directly these phase variations in a conventional scattering experiment. It was recently suggested that the transmitted phase can be directly measured in ptychography experiments performed on nanocrystal samples. Results of such experiment for different crystal thickness, reflections and incoming photon energies, in principle, can be fully described in the frame of dynamical theory. However, dynamical theory does not provide a simple analytical expression for the further analysis. We develop here quasi-kinematical theory approach that allows to describe correctly the phase of the transmitted beam for the crystal thickness less than extinction length that is beyond applicability of the conventional kinematical theory.
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The experimental STM images for the CDW phase of the blue bronze RbMoO3 have been successfully explained on the basis of first-principles DFT calculations. Although the density of states near the Fermi level strongly concentrates in two of the three types of Mo atoms Mo-II and Mo-III, the STM measurement mostly probes the contribution of the uppermost O atoms of the surface, associated with the Mo-IO6 octahedra. In addition, it is found that the surface concentration of Rb atoms plays a key role in determining the surface nesting vector and hence the periodicity of the CDW modulation. Significant experimental inhomogeneities of the b* surface component of the wavevector of the modulation, probed by STM, are reported. The calculated changes in the surface nesting vector are consistent with the observed experimental inhomogeneities.
In our previous study [Phys. Rev. B 86, 201104 (2012)] we introduced the so called quasi-non-uniform gradient-level exchange-correlation approximation (QNA) and demonstrated its strength in producing highly accurate equilibrium volumes for metals and their alloys within the density-functional theory. In this paper we extend the scheme to include the accuracy of bulk modulus as an additional figure of merit and show that this scheme is flexible enough as to allow the computation of accurate equilibrium volumes and bulk moduli at the same time. The power and feasibility of this scheme is demonstrated on NiAl and FeV binary alloys.
The optimized effective potential (OEP) method presents an unambiguous way to construct the Kohn-Sham potential corresponding to a given diagrammatic approximation for the exchange-correlation functional. The OEP from the random-phase approximation (RPA) has played an important role ever since the conception of the OEP formalism. However, the solution of the OEP equation is computationally fairly expensive and has to be done in a self-consistent way. So far, large scale solid state applications have therefore been performed only using the quasiparticle approximation (QPA), neglecting certain dynamical screening effects. We obtain the exact RPA-OEP for 15 semiconductors and insulators by direct solution of the linearized Sham-Schluter equation. We investigate the accuracy of the QPA on Kohn-Sham band gaps and dielectric constants, and comment on the issue of self-consistency.
Starting from exact expression for the dynamical spin susceptibility in the time-dependent density functional theory a controversial issue about exchange interaction parameters and spin-wave excitation spectra of itinerant electron ferromagnets is reconsidered. It is shown that the original expressions for exchange integrals based on the magnetic force theorem (J. Phys. F14 L125 (1984)) are optimal for the calculations of the magnon spectrum whereas static response function is better described by the ``renormalized magnetic force theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is confirmed by the {it ab initio} calculations for Fe and Ni.
We present a self-consistent electronic structure calculation method based on the {it Exact Muffin-Tin Orbitals} (EMTO) Theory developed by O. K. Andersen, O. Jepsen and G. Krier (in {it Lectures on Methods of Electronic Structure Calculations}, Ed. by V. Kumar, O.K. Andersen, A. Mookerjee, Word Scientific, 1994 pp. 63-124) and O. K. Andersen, C. Arcangeli, R. W. Tank, T. Saha-Dasgupta, G. Krier, O. Jepsen, and I. Dasgupta, (in {it Mat. Res. Soc. Symp. Proc.} {bf 491}, 1998 pp. 3-34). The EMTO Theory can be considered as an {it improved screened} KKR (Korringa-Kohn-Rostoker) method which is able to treat large overlapping potential spheres. Within the present implementation of the EMTO Theory the one electron equations are solved exactly using the Greens function formalism, and the Poissons equation is solved within the {it Spherical Cell Approximation} (SCA). To demonstrate the accuracy of the SCA-EMTO method test calculations have been carried out.
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