370 research outputs found
Electronic Structure of Sodium Cobalt Oxide: Comparing Mono- and Bilayer-hydrate
To shed new light on the mechanism of superconductivity in sodium cobalt
oxide bilayer-hydrate (BLH), we perform a density functional calculation with
full structure optimization for BLH and its related nonsuperconducting phase,
monolayer hydrate (MLH). We find that these hydrates have similar band
structures, but a notable difference can be seen in the band around
the Fermi level. While its dispersion in the direction is negligibly small
for BLH, it is of the order of 0.1 eV for MLH. This result implies that the
three dimensional feature of the band may be the origin for the
absence of superconductivity in MLH.Comment: 5 pages, 7 figures, to be published in Phys. Rev.
Structure of CAl12
The structures of an isolated CAl12 cluster and a solid composed of CAl12 clusters have been studied using the CarâParrinello method, based on the density functional theory and the local density approximation. We have compared the results of using the ultrasoft Vanderbilt pseudopotential with those of both a traditional pseudopotential and a linear combination of atomic orbitals method. We have confirmed the high stability of the cluster in its icosahedral structure. However, we show that the clusterâassembled solid is unstable against melting of the clusters, as previously found for SiAl12.Peer reviewe
A mixed ultrasoft/normconserved pseudopotential scheme
A variant of the Vanderbilt ultrasoft pseudopotential scheme, where the
normconservation is released for only one or a few angular channels, is
presented. Within this scheme some difficulties of the truly ultrasoft
pseudopotentials are overcome without sacrificing the pseudopotential softness.
i) Ghost states are easily avoided without including semicore shells. ii) The
ultrasoft pseudo-charge-augmentation functions can be made more soft. iii) The
number of nonlocal operators is reduced. The scheme will be most useful for
transition metals, and the feasibility and accuracy of the scheme is
demonstrated for the 4d transition metal rhodium.Comment: 4 pages, 2 figure
Neuropsychological intervention of dyslexia has a positive effect on aspects of psychological well-being in young adults - a randomized controlled study
Effectiveness of individual- and group-based neuropsychological intervention on aspects of psychological well-being of dyslexic adults was evaluated. Dyslexic young adults (n = 120) were randomly assigned into individual intervention, group intervention or wait-list control group. Both interventions focussed on cognitive strategy learning, supporting self-esteem, and using psychoeducation. In group format peer support was also utilized. Cognitive and behavioural strategies, mood states, quality of life and self-esteem were assessed via self-report questionnaires at baseline, after the intervention/wait-list control time at 5 months and 10 months. Results indicated that the neuropsychological interventions had a positive effect on self-evaluated cognitive and behavioural strategies, especially in increasing success expectations and to a lesser degree in diminishing task-avoidance and in group intervention in diminishing social pessimism. The interventions also improved cognition-related quality of life and, to a lesser degree, self-esteem. These results indicate that structured neuropsychological interventions can positively affect self-evaluated psychological well-being, especially on cognitive and behavioural strategies. Considering the secondary consequences of dyslexia, support among young adults is often needed beyond the cognitive and reading-based challenges dyslexia poses.Peer reviewe
Charging Induced Emission of Neutral Atoms from NaCl Nanocube Corners
Detachment of neutral cations/anions from solid alkali halides can in
principle be provoked by donating/subtracting electrons to the surface of
alkali halide crystals, but generally constitutes a very endothermic process.
However, the amount of energy required for emission is smaller for atoms
located in less favorable positions, such as surface steps and kinks. For a
corner ion in an alkali halide cube the binding is the weakest, so it should be
easier to remove that atom, once it is neutralized. We carried out first
principles density functional calculations and simulations of neutral and
charged NaCl nanocubes, to establish the energetics of extraction of
neutralized corner ions. Following hole donation (electron removal) we find
that detachment of neutral Cl corner atoms will require a limited energy of
about 0.8 eV. Conversely, following the donation of an excess electron to the
cube, a neutral Na atom is extractable from the corner at the lower cost of
about 0.6 eV. Since the cube electron affinity level (close to that a NaCl(100)
surface state, which we also determine) is estimated to lie about 1.8 eV below
vacuum, the overall energy balance upon donation to the nanocube of a zero
energy electron from vacuum will be exothermic. The atomic and electronic
structure of the NaCl(100) surface, and of the nanocube Na and Cl corner
vacancies are obtained and analyzed as a byproduct.Comment: 16 pages, 2 table, 7 figure
Ab initio simulations of liquid NaSn alloys: Zintl anions and network formation
Using the Car-Parrinello technique, ab initio molecular dynamics simulations
are performed for liquid NaSn alloys in five different compositions (20, 40,
50, 57 and 80 % sodium). The obtained structure factors agree well with the
data from neutron scattering experiments. The measured prepeak in the structure
factor is reproduced qualitatively for most compositions. The calculated and
measured positions of all peaks show the same trend as function of the
composition.\\ The dynamic simulations also yield information about the
formation and stability of Sn clusters (Zintl anions) in the liquid. In our
simulations of compositions with 50 and 57 % sodium we observe the formation of
networks of tin atoms. Thus, isolated tin clusters are not stable in such
liquids. For the composition with 20 % tin only isolated atoms or dimers of tin
appear, ``octet compounds'' of one Sn atom surrounded by 4 Na atoms are not
observed.Comment: 12 pages, Latex, 3 Figures on reques
Magnetic reconstruction at (001) CaMnO surface
The Mn-terminated (001) surface of the stable anti-ferromagnetic insulating
phase of cubic perovskite CaMnO is found to undergo a magnetic
reconstruction consisting on a spin-flip process at surface: each Mn spin at
the surface flips to pair with that of Mn in the subsurface layer. In spite of
very little Mn-O charge transfer at surface, the surface behavior is driven by
the states due to charge redistribution. These
results, based on local spin density theory, give a double exchange like
coupling that is driven by character, not additional charge, and may have
relevance to CMR materials.Comment: 4 pages, 5 figures reference added Fig. 3 modified. Caption of Fig. 5
modifie
Thermal diffusion of supersonic solitons in an anharmonic chain of atoms
We study the non-equilibrium diffusion dynamics of supersonic lattice
solitons in a classical chain of atoms with nearest-neighbor interactions
coupled to a heat bath. As a specific example we choose an interaction with
cubic anharmonicity. The coupling between the system and a thermal bath with a
given temperature is made by adding noise, delta-correlated in time and space,
and damping to the set of discrete equations of motion. Working in the
continuum limit and changing to the sound velocity frame we derive a
Korteweg-de Vries equation with noise and damping. We apply a collective
coordinate approach which yields two stochastic ODEs which are solved
approximately by a perturbation analysis. This finally yields analytical
expressions for the variances of the soliton position and velocity. We perform
Langevin dynamics simulations for the original discrete system which fully
confirm the predictions of our analytical calculations, namely noise-induced
superdiffusive behavior which scales with the temperature and depends strongly
on the initial soliton velocity. A normal diffusion behavior is observed for
very low-energy solitons where the noise-induced phonons also make a
significant contribution to the soliton diffusion.Comment: Submitted to PRE. Changes made: New simulations with a different
method of soliton detection. The results and conclusions are not different
from previous version. New appendixes containing information about the system
energy and soliton profile
Acceleration Schemes for Ab-Initio Molecular Dynamics and Electronic Structure Calculations
We study the convergence and the stability of fictitious dynamical methods
for electrons. First, we show that a particular damped second-order dynamics
has a much faster rate of convergence to the ground-state than first-order
steepest descent algorithms while retaining their numerical cost per time step.
Our damped dynamics has efficiency comparable to that of conjugate gradient
methods in typical electronic minimization problems. Then, we analyse the
factors that limit the size of the integration time step in approaches based on
plane-wave expansions. The maximum allowed time step is dictated by the highest
frequency components of the fictitious electronic dynamics. These can result
either from the large wavevector components of the kinetic energy or from the
small wavevector components of the Coulomb potential giving rise to the so
called {\it charge sloshing} problem. We show how to eliminate large wavevector
instabilities by adopting a preconditioning scheme that is implemented here for
the first-time in the context of Car-Parrinello ab-initio molecular dynamics
simulations of the ionic motion. We also show how to solve the charge-sloshing
problem when this is present. We substantiate our theoretical analysis with
numerical tests on a number of different silicon and carbon systems having both
insulating and metallic character.Comment: RevTex, 9 figures available upon request, to appear in Phys. Rev.
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