E. A. Soares

The generalized simulated annealing algorithm in the low energy electron diffraction search problem

We present in this work results concerning the application of the generalized simulated annealing (GSA) algorithm to the LEED search problem. The influence of the visiting distribution function (defined by the so-called q(V) parameter) in the effectiveness of the method was investigated by the application of the algorithm to structural searches for optimization of two to ten parameters in a theory-theory comparison for the CdTe(110) system. Results, obtained with the scaling relation and probability of convergence as a function of the number of parameters to be varied, indicate the fast simulated annealing (FSA) (q(V) = 2.0) approach as the best search machine.Non-reversing relaxation enthalpies (∆Hnr) at glass transitions Tg(x) in the PxGexSe1-2x ternary display a wide, sharp and deep global minimum (~0) in the 0.09 < x < 0.145 range, within which Tg becomes thermally reversing. In the reversibility window these glasses are found not to age, in contrast to aging observed for fragile glass compositions outside the window. Thermal reversibility and lack of aging are paradigms that molecular glasses in the window share with proteins in transition states, which result from structural self-organization in both systems. In proteins the self-organized structures appear to be at places where life sustaining repeating foldings and unfoldings occur.

XPS AND EELS STUDY OF THE BISMUTH SELENIDE

Abstract A Bi 2 Se 3 crystal was studied by XPS (X-Ray Photoelectron Spectroscopy) and EELS (Electron Energy Loss Spectroscopy). Al Kα radiation and an electron beam energy in the range of 0.1 to 2 keV were used, respectively, to probe a Se-terminated (0001) surface. Samples of the constituent elements (Bi and Se) have also been measured with the same setup. The core level chemical shifts obtained show that a charge transfer occurs in Bi 2 Se 3 . The spectra in the valence band region suggest that the density of states in the compound may be obtained by combining the spectra of the constituent elements, that the electronic states in the vicinity of the gap region consist of a mixture of the metal and calcogen p -orbitals and that the band arising from the valence s -orbitals occurs about 12 eV below the valence band maximum. The EELS spectra allow to identify the bulk plasmons for the three materials and the Bi5 d 3 and Bi5 d 5 interband transitions. Considerations of the energies of the Bi5 d transitions as measured by XPS and EELS indicate that the bottom of the conduction band of the compound is 1.2 eV above the Fermi level. The EELS results also shows evidence that the losses occurring at 6.4 eV in the compound and at 5.4(5.5) eV in Bi(Se) have their origins in some surface process. We suggest that they may be associated to a surface plasmon.

Advances on surface structural determination by LEED

In the last 40 years, low energy electron diffraction (LEED) has proved to be the most reliable quantitative technique for surface structural determination. In this review, recent developments related to the theory that gives support to LEED structural determination are discussed under a critical analysis of the main theoretical approximation-the muffin-tin calculation. The search methodologies aimed at identifying the best matches between theoretical and experimental intensity versus voltage curves are also considered, with the most recent procedures being reviewed in detail.

Surface structure of Bi2Se3(111) determined by low-energy electron diffraction and surface x-ray diffraction

The surface structure of the prototypical topological insulator Bi2Se3 is determined by low-energy electron diffraction and surface x-ray diffraction at room temperature. Both approaches show that the crystal is terminated by an intact quintuple layer. Specifically, an alternative termination by a bismuth bilayer is ruled out. Surface relaxations obtained by both techniques are in good agreement with each other and found to be small. This includes the relaxation of the van der Waals gap between the first two quintuple layers.

Global search in photoelectron diffraction structure determination using genetic algorithms

Photoelectron diffraction (PED) is an experimental technique widely used to perform structural determinations of solid surfaces. Similarly to low-energy electron diffraction (LEED), structural determination by PED requires a fitting procedure between the experimental intensities and theoretical results obtained through simulations. Multiple scattering has been shown to be an effective approach for making such simulations. The quality of the fit can be quantified through the so-called R-factor. Therefore, the fitting procedure is, indeed, an R-factor minimization problem. However, the topography of the R-factor as a function of the structural and non-structural surface parameters to be determined is complex, and the task of finding the global minimum becomes tough, particularly for complex structures in which many parameters have to be adjusted. In this work we investigate the applicability of the genetic algorithm (GA) global optimization method to this problem. The GA is based on the evolution of species, and makes use of concepts such as crossover, elitism and mutation to perform the search. We show results of its application in the structural determination of three different systems: the Cu(111) surface through the use of energy-scanned experimental curves; the Ag(110)–c(2 × 2)-Sb system, in which a theory–theory fit was performed; and the Ag(111) surface for which angle-scanned experimental curves were used. We conclude that the GA is a highly efficient method to search for global minima in the optimization of the parameters that best fit the experimental photoelectron diffraction intensities to the theoretical ones.

Bulk band structure of Bi 2 Te 3

The bulk band structure of Bi

Bulk band structure ofBi2Te3

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The Simulated Annealing Global Search Algorithm Applied To The Crystallography Of Surfaces By Leed

Te

A layer-by-layer study of CdTe(110) surface Debye temperature and thermal vibrations by low energy electron diffraction

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Significant Dzyaloshinskii–Moriya interaction at graphene–ferromagnet interfaces due to the Rashba effect

has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot