Variational finite-difference representation of the kinetic energy operator

A potential disadvantage of real-space-grid electronic structure methods is the lack of a variational principle and the concomitant increase of total energy with grid refinement. We show that the origin of this feature is the systematic underestimation of the kinetic energy by the finite difference representation of the Laplacian operator. We present an alternative representation that provides a rigorous upper bound estimate of the true kinetic energy and we illustrate its properties with a harmonic oscillator potential. For a more realistic application, we study the convergence of the total energy of bulk silicon using a real-space-grid density-functional code and employing both the conventional and the alternative representations of the kinetic energy operator.Comment: 3 pages, 3 figures, 1 table. To appear in Phys. Rev. B. Contribution for the 10th anniversary of the eprint serve

On a stationary spinning string spacetime

The properties of a stationary massless string endowed with intrinsic spin are discussed. The spacetime is Minkowskian geometrically but the topology is nontrivial due to the horizon located on the surface $r=0$, similar with Rindler's case. For $r$ less than the Planck length $b$, $g_{\phi\phi}$ has the same sign as $g_{tt}$ and closed timelike curves are possible. We assume an elementary particles' spin originates in the frame dragging effect produced by the rotation of the source. The Sagnac time delay is calculated and proves to be constant.Comment: revised version of hep-th/0602014 v1, 7 pages, title changed, sec.5 removed, talk given at "Recent Developments in Gravity" (NEB XII), Nafplio, Greece, 29 June 200

Glass-formation and corrosion properties of Fe-Cr-Mo-C-B glassy ribbons with low Cr content

The effect of low amounts of Cr on glass forming ability and corrosion behavior of Fe(65-x)CrxMo14C15B6 (x = 0, 2, 4, 6, 8,10 at.%) ribbons have been studied. It was found that the reduced glass transition temperature (T-rg) do not change significantly with Cr content. The glass forming ability (GFA) of this system is evaluated by the gamma(m), delta and omega parameters and the alloys containing 4 and 6 at.% were found to be the best glass formers in this system. The temperature interval of the supercooled liquid region (Delta T-x) changed with Cr and was enlarged from 35 K at x = 0 to 50 K at x = 4. Corrosion rates measured by immersion tests in H2SO4 decreased with an increase of chromium content in the alloys. The electrochemical measurements indicate that the alloys containing more than 4 at.% of Cr are spontaneously passivated with low current densities in 0.1 N H2SO4 whereas the alloys with Cr content <4 at.% showed transpassive Mo dissolution. In view of these results, the optimal amount of Cr addition in Fe-Mo-C-B amorphous steels is discussed. (C) 2014 Elsevier B.V. All rights reserved.Peer ReviewedPostprint (published version

Electronic transport calculations for rough interfaces in Al, Cu, Ag, and Au

We present results of electronic structure and transport calculations for metallic interfaces, based on density functional theory and the non-equilibrium Green's functions method. Starting from the electronic structure of smooth Al, Cu, Ag, and Au interfaces, we study the effects of different kinds of interface roughness on the transmission coefficient and the I-V characteristic. In particular, we compare prototypical interface distortions, including vacancies and metallic impurities.Comment: 15 pages, 10 figure

Systematic Study of Electron Localization in an Amorphous Semiconductor

We investigate the electronic structure of gap and band tail states in amorphous silicon. Starting with two 216-atom models of amorphous silicon with defect concentration close to the experiments, we systematically study the dependence of electron localization on basis set, density functional and spin polarization using the first principles density functional code Siesta. We briefly compare three different schemes for characterizing localization: information entropy, inverse participation ratio and spatial variance. Our results show that to accurately describe defect structures within self consistent density functional theory, a rich basis set is necessary. Our study revealed that the localization of the wave function associated with the defect states decreases with larger basis sets and there is some enhancement of localization from GGA relative to LDA. Spin localization results obtained via LSDA calculations, are in reasonable agreement with experiment and with previous LSDA calculations on a-Si:H models.Comment: 16 pages, 11 Postscript figures, To appear in Phys. Rev.

Density functional study of Aun_n (n=2-20) clusters: lowest-energy structures and electronic properties

We have investigated the lowest-energy structures and electronic properties of the Au$_n$(n=2-20) clusters based on density functional theory (DFT) with local density approximation. The small Au$_n$ clusters adopt planar structures up to n=6. Tabular cage structures are preferred in the range of n=10-14 and a structural transition from tabular cage-like structure to compact near-spherical structure is found around n=15. The most stable configurations obtained for Au$_{13}$ and Au$_{19}$ clusters are amorphous instead of icosahedral or fcc-like, while the electronic density of states sensitively depend on the cluster geometry. Dramatic odd-even alternative behaviors are obtained in the relative stability, HOMO-LUMO gaps and ionization potentials of gold clusters. The size evolution of electronic properties is discussed and the theoretical ionization potentials of Au$_n$ clusters compare well with experiments.Comment: 6 pages, 7 figure

Efficient index handling of multidimensional periodic boundary conditions

An efficient method is described to handle mesh indexes in multidimensional problems like numerical integration of partial differential equations, lattice model simulations, and determination of atomic neighbor lists. By creating an extended mesh, beyond the periodic unit cell, the stride in memory between equivalent pairs of mesh points is independent of their position within the cell. This allows to contract the mesh indexes of all dimensions into a single index, avoiding modulo and other implicit index operations.Comment: 2 pages, 0 figure

Mild sonochemical exfoliation of bromine-intercalated graphite: a new route towards graphene

A method to produce suspensions of graphene sheets by combining solution-based bromine intercalation and mild sonochemical exfoliation is presented. Ultrasonic treatment of graphite in water leads to the formation of suspensions of graphite flakes. The delamination is dramatically improved by intercalation of bromine into the graphite before sonication. The bromine intercalation was verified by Raman spectroscopy as well as by x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations show an almost ten times lower interlayer binding energy after introducing Br2 into the graphite. Analysis of the suspended material by transmission and scanning electron microscopy (TEM and SEM) revealed a significant content of few-layer graphene with sizes up to 30 $mu$m, corresponding to the grain size of the starting material.Comment: 10 pages 4 figure

Algorithmic Complexity for Short Binary Strings Applied to Psychology: A Primer

Since human randomness production has been studied and widely used to assess executive functions (especially inhibition), many measures have been suggested to assess the degree to which a sequence is random-like. However, each of them focuses on one feature of randomness, leading authors to have to use multiple measures. Here we describe and advocate for the use of the accepted universal measure for randomness based on algorithmic complexity, by means of a novel previously presented technique using the the definition of algorithmic probability. A re-analysis of the classical Radio Zenith data in the light of the proposed measure and methodology is provided as a study case of an application.Comment: To appear in Behavior Research Method