10,001 research outputs found
Exchange and correlation energy functionals for two-dimensional open-shell systems
We consider density functionals for exchange and correlation energies in
two-dimensional systems. The functionals are constructed by making use of exact
constraints for the angular averages of the corresponding exchange and
correlation holes, respectively, and assuming proportionality between their
characteristic sizes. The electron current and spin are explicitly taken into
account, so that the resulting functionals are suitable to deal with systems
exhibiting orbital currents and/or spin polarization. Our numerical results
show that in finite systems the proposed functionals outperform the standard
two-dimensional local spin-density approximation, still performing well also in
the important limit of the homogeneous two-dimensional electron gas
Effects of Physical Exercise on Working Memory and Attention-Related Neural Oscillations
Cognitive functions, such as working memory (WM) and attention, have been shown to benefit from physical exercise. Quantifying frequency-band-specific neural oscillatory patterns during the use of such cognitive functions can provide insight into exercise-induced benefits in the brain. Specifically, we investigated whether a 4-month physical exercise training influenced theta and alpha power measured in visual WM and attention tasks. The delayed match-to-sample (DMS) task required mnemonic discrimination of similar visual stimuli, akin to pattern separation, while the visual-attention search (VAS) task required detecting the presence of a specific object (i.e., target) in an image. Behavioral and electroencephalographic data were acquired during a DMS visual WM task and VAS task both before and after the intervention. Forty-three sedentary young adults (19–34 years) were pseudorandomly assigned to a training group (indoor treadmill, n = 20) or to a control group (n = 23). Compared to the preintervention baseline, the exercise group showed increased frontal alpha power (9–12 Hz) during the VAS task after the intervention. In addition, alpha power changes correlated positively with fitness changes. Behaviorally, there were no exercise-related effects on reaction times or accuracy in either task. Our findings substantiate that aerobic training of sedentary young adults may influence neural dynamics underlying visual attention rather than visual WM and mnemonic discrimination
Timesaving Double-Grid Method for Real-Space Electronic-Structure Calculations
We present a simple and efficient technique in ab initio electronic-structure
calculation utilizing real-space double-grid with a high density of grid points
in the vicinity of nuclei. This technique promises to greatly reduce the
overhead for performing the integrals that involves non-local parts of
pseudopotentials, with keeping a high degree of accuracy. Our procedure gives
rise to no Pulay forces, unlike other real-space methods using adaptive
coordinates. Moreover, we demonstrate the potential power of the method by
calculating several properties of atoms and molecules.Comment: 4 pages, 5 figure
Suggestions de priorités d’une efficience et d’une adéquation accrue de l’intervention gérontologique en santé communautaire
Edge Electron Gas
The uniform electron gas, the traditional starting point for density-based
many-body theories of inhomogeneous systems, is inappropriate near electronic
edges. In its place we put forward the appropriate concept of the edge electron
gas.Comment: 4 pages RevTex with 7 ps-figures included. Minor changes in
title,text and figure
Self-assembled guanine ribbons as wide-bandgap semiconductors
We present a first principle study about the stability and the electronic
properties of a new biomolecular solid-state material, obtained by the
self-assembling of guanine (G) molecules. We consider hydrogen-bonded planar
ribbons in isolated and stacked configurations. These aggregates present
electronic properties similar to inorganic wide-bandgap semiconductors. The
formation of Bloch-type orbitals is observed along the stacking direction,
while it is negligible in the ribbon plane. Global band-like conduction may be
affected by a dipole-field which spontaneously arises along the ribbon axis.
Our results indicate that G-ribbon assemblies are promising materials for
biomolecular nanodevices, consistently with recent experimental results.Comment: 7 pages, 3 figures, to be published in Physica
Heart Rate Variability During Physical Exercise Is Associated With Improved Cognitive Performance in Alzheimer's Dementia Patients-A Longitudinal Feasibility Study
Heart rate variability (HRV) rapidly gains attention as an important marker of cardiovascular autonomic modulation. Moreover, there is evidence for a link between the autonomic deficit measurable by reduced HRV and the hypoactivity of the cholinergic system, which is prominently affected in Alzheimer's disease (AD). Despite the positive influence of physical exercise on cognition and its promising association with HRV, previous studies did not explore the effect of long-term physical exercise in older adults with AD. Taking advantage of a longitudinal study we analyzed the effect of a 20-week dual task training regime (3 Ă— 15-min per week) on the vagal mediated HRV index RMSSD (root mean square of successive RR interval differences) during physical exercise and the short-term memory performance in a AD cohort (N = 14). Each training contained physical exercise on a bicycle ergometer while memorizing 30 successively presented pictures as well as the associated post-exercise picture recognition memory test. Linear-mixed modeling revealed that HRV-RMSSD significantly increased over the intervention time. Moreover, the reaction time in the picture recognition task decreased while the accuracy remained stable. Furthermore, a significantly negative relationship between increased fitness measured by HRV-RMSSD and decreased reaction time was observed. This feasibility study points to the positive effects of a dual task regime on physical and cognitive fitness in a sample with impaired cognitive performance. Beyond this, the results show that the responsiveness of parasympathetic system as measured with HRV can be improved in patients with dementia
The mechanism for the 3 x 3 distortion of Sn/ge (111)
We show that two distinct ground states, one nonmagnetic,
metallic, and distorted, the other magnetic, semimetallic (or insulating) and
undistorted, compete in -phase adsorbates on semiconductor (111)
surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with
experiment, that the distorted metallic ground state prevails. The reason for
stability of this state is analysed, and is traced to a sort of bond density
wave, specifically a modulation of the antibonding state filling between the
adatom and a Ge-Ge bond directly underneath
Electronic structure of crystalline binary and ternary Cd-Te-O compounds
The electronic structure of crystalline CdTe, CdO, -TeO,
CdTeO and CdTeO is studied by means of first principles
calculations. The band structure, total and partial density of states, and
charge densities are presented. For -TeO and CdTeO, Density
Functional Theory within the Local Density Approximation (LDA) correctly
describes the insulating character of these compounds. In the first four
compounds, LDA underestimates the optical bandgap by roughly 1 eV. Based on
this trend, we predict an optical bandgap of 1.7 eV for CdTeO. This
material shows an isolated conduction band with a low effective mass, thus
explaining its semiconducting character observed recently. In all these oxides,
the top valence bands are formed mainly from the O 2p electrons. On the other
hand, the binding energy of the Cd 4d band, relative to the valence band
maximum, in the ternary compounds is smaller than in CdTe and CdO.Comment: 13 pages, 15 figures, 2 tables. Accepted in Phys Rev
Long-range/short-range separation of the electron-electron interaction in density functional theory
By splitting the Coulomb interaction into long-range and short-range
components, we decompose the energy of a quantum electronic system into
long-range and short-range contributions. We show that the long-range part of
the energy can be efficiently calculated by traditional wave function methods,
while the short-range part can be handled by a density functional. The analysis
of this functional with respect to the range of the associated interaction
reveals that, in the limit of a very short-range interaction, the short-range
exchange-correlation energy can be expressed as a simple local functional of
the on-top pair density and its first derivatives. This provides an explanation
for the accuracy of the local density approximation (LDA) for the short-range
functional. Moreover, this analysis leads also to new simple approximations for
the short-range exchange and correlation energies improving the LDA.Comment: 18 pages, 14 figures, to be published in Phys. Rev.
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