195 research outputs found
Managing the supercell approximation for charged defects in semiconductors: finite size scaling, charge correction factors, the bandgap problem and the ab initio dielectric constant
The errors arising in ab initio density functional theory studies of
semiconductor point defects using the supercell approximation are analyzed. It
is demonstrated that a) the leading finite size errors are inverse linear and
inverse cubic in the supercell size, and b) finite size scaling over a series
of supercells gives reliable isolated charged defect formation energies to
around +-0.05 eV. The scaled results are used to test three correction methods.
The Makov-Payne method is insufficient, but combined with the scaling
parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma
point corrections for defect level dispersion are completely incorrect, even
for shallow levels, but re-aligning the total potential in real-space between
defect and bulk cells actually corrects the electrostatic defect-defect
interaction errors as well. Isolated defect energies to +-0.1 eV are then
obtained using a 64 atom supercell, though this does not improve for larger
cells. Finally, finite size scaling of known dopant levels shows how to treat
the band gap problem: in less than about 200 atom supercells with no
corrections, continuing to consider levels into the theoretical conduction band
(extended gap) comes closest to experiment. However, for larger cells or when
supercell approximation errors are removed, a scissors scheme stretching the
theoretical band gap onto the experimental one is in fact correct.Comment: 11 pages, 3 figures (6 figure files). Accepted for Phys Rev
Laserlight visual cueing device for freezing of gait in Parkinson's disease: a case study of the biomechanics involved
AbstractBackground: Freezing of gait (FOG) is a serious gait disorder affecting up to two-thirds of people with Parkinson's disease (PD). Cueing has been explored as a method of generating motor execution using visual transverse lines on the floor. However, the impact of a laser light visual cue remains unclear. Objective: To determine the biomechanical effect of a laser cane on FOG in a participant with PD compared to a healthy age- and gender-matched control. Methods: The participant with PD and healthy control were given a task of initiating gait from standing. Electromyography (EMG) data were collected from the tibialis anterior (TA) and the medial gastrocnemius (GS) muscles using an 8-channel system. A 10-camera system (Qualisys) recorded movement in 6 degrees of freedom and a calibrated anatomical system technique was used to construct a full body model. Center of mass (COM) and center of pressure (COP) were the main outcome measures. Results: The uncued condition showed that separation of COM and COP took longer and was of smaller magnitude than the cued condition. EMG activity revealed prolonged activation of GS, with little to no TA activity. The cued condition showed earlier COM and COP separation. There was reduced fluctuation in GS, with abnormal, early bursts of TA activity. Step length improved in the cued condition compared to the uncued condition. Conclusion: Laserlight visual cueing improved step length beyond a non-cued condition for this patient indicating improved posture and muscle control
Barrier-free subsurface incorporation of 3d metal atoms into Bi(111) films
By combining scanning tunneling microscopy with density functional theory it is shown that the Bi(111) surface provides a well-defined incorporation site in the first bilayer that traps highly coordinating atoms such as transition metals (TMs) or noble metals. All deposited atoms assume exactly the same specific sevenfold coordinated subsurface interstitial site while the surface topography remains nearly unchanged. Notably, 3d TMs show a barrier-free incorporation. The observed surface modification by barrier-free subsorption helps to suppress aggregation in clusters. It allows a tuning of the electronic properties not only for the pure Bi(111) surface, but may also be observed for topological insulators formed by substrate-stabilized Bi bilayers. © 2015 American Physical Society.DFG/SFB/616DFG/SPP/1601DFG/Pf238/3
Defects in SiO2 as the possible origin of near interface traps in the SiC∕SiO2 system: A systematic theoretical study
A systematic study of the level positions of intrinsic and carbon defects in SiO2 is presented, based on density functional calculations with a hybrid functional in an alpha-quartz supercell. The results are analyzed from the point of view of the near interface traps (NIT), observed in both SiC/SiO2 and Si/SiO2 systems, and assumed to have their origins in the oxide. It is shown that the vacancies and the oxygen interstitial can be excluded as the origin of such NIT, while the silicon interstitial and carbon dimers give rise to gap levels in the energy range inferred from experiments. The properties of these defects are discussed in light of the knowledge about the SiC/SiO2 interface
Quadrupole Susceptibility and Elastic Softening due to a Vacancy in Silicon Crystal
We investigate the electronic states around a single vacancy in silicon
crystal by using the Green's function approach. The triply degenerate vacancy
states within the band gap are found to be extended over a large distance
from the vacancy site and contribute to the reciprocal
temperature dependence of the quadrupole susceptibility resulting in the
elastic softening at low temperture. The Curie constant of the quadrupole
susceptibility for the trigonal mode () is largely
enhanced as compared to that for the tetragonal mode ().
The obtained results are consistent with the recent ultrasonic experiments in
silicon crystal down to 20 mK. We also calculate the dipole and octupole
susceptibilities and find that the octupole susceptibilities are extremely
enhannced for a specific mode.Comment: 6 pages, with 5 figure
Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
Quantum ESPRESSO is an integrated suite of computer codes for
electronic-structure calculations and materials modeling, based on
density-functional theory, plane waves, and pseudopotentials (norm-conserving,
ultrasoft, and projector-augmented wave). Quantum ESPRESSO stands for "opEn
Source Package for Research in Electronic Structure, Simulation, and
Optimization". It is freely available to researchers around the world under the
terms of the GNU General Public License. Quantum ESPRESSO builds upon
newly-restructured electronic-structure codes that have been developed and
tested by some of the original authors of novel electronic-structure algorithms
and applied in the last twenty years by some of the leading materials modeling
groups worldwide. Innovation and efficiency are still its main focus, with
special attention paid to massively-parallel architectures, and a great effort
being devoted to user friendliness. Quantum ESPRESSO is evolving towards a
distribution of independent and inter-operable codes in the spirit of an
open-source project, where researchers active in the field of
electronic-structure calculations are encouraged to participate in the project
by contributing their own codes or by implementing their own ideas into
existing codes.Comment: 36 pages, 5 figures, resubmitted to J.Phys.: Condens. Matte
Know Thyself: Behavioral Evidence for a Structural Representation of the Human Body
Background: Representing one's own body is often viewed as a basic form of self-awareness. However, little is known about structural representations of the body in the brain.Methods and Findings: We developed an inter-manual version of the classical "in-between'' finger gnosis task: participants judged whether the number of untouched fingers between two touched fingers was the same on both hands, or different. We thereby dissociated structural knowledge about fingers, specifying their order and relative position within a hand, from tactile sensory codes. Judgments following stimulation on homologous fingers were consistently more accurate than trials with no or partial homology. Further experiments showed that structural representations are more enduring than purely sensory codes, are used even when number of fingers is irrelevant to the task, and moreover involve an allocentric representation of finger order, independent of hand posture.Conclusions: Our results suggest the existence of an allocentric representation of body structure at higher stages of the somatosensory processing pathway, in addition to primary sensory representation
Soil water content effects on net ecosystem CO2 exchange and actual evapotranspiration in a Mediterranean semiarid savanna of Central Chile
Biosphere-atmosphere water and carbon fluxes depend on ecosystem structure, and their magnitudes
and seasonal behavior are driven by environmental and biological factors. We studied the seasonal
behavior of net ecosystem CO2 exchange (NEE), Gross Primary Productivity (GPP), Ecosystem
Respiration (RE), and actual evapotranspiration (ETa) obtained by eddy covariance measurements
during two years in a Mediterranean Acacia savanna ecosystem (Acacia caven) in Central Chile. The
annual carbon balance was −53 g C m−2 in 2011 and −111 g C m−2 in 2012, showing that the ecosystem
acts as a net sink of CO2, notwithstanding water limitations on photosynthesis observed in this
particularly dry period. Total annual ETa was of 128 mm in 2011 and 139 mm in 2012. Both NEE and ETa
exhibited strong seasonality with peak values recorded in the winter season (July to September), as a
result of ecosystem phenology, soil water content and rainfall occurrence. Consequently, the maximum
carbon assimilation rate occurred in wintertime. Results show that soil water content is a major driver
of GPP and RE, defining their seasonal patterns and the annual carbon assimilation capacity of the
ecosystem, and also modulating the effect that solar radiation and air temperature have on NEE
components at shorter time scales.This work was funded by FONDECYT projects 1120713 and 1170429, a grant from the Inter-American Institute
for Global Change Research (IAI) [grant number CRN3056], which is supported by the US National Science
Foundation [grant number GEO-1128040], and the Spanish Ministry of Economy and Competitiveness project
GEI Spain (CGL2014-52838-C2-1-R), including ERDF founds. F. Bravo-Martínez is grateful to CONICYT for the
grants “Formación de Capital Humano Avanzado-2009′′, “Beca de Apoyo al término de la tesis doctoral-2012′′,
and CORFO INNOVA Grant N° 09CN14-5704. We thank to Enrique Pérez Sanchez-Cañete and Borja Ruíz-
Reverter for technical support. We also thank “CODELCO–División Andina” for use of the site. C. Montes
acknowledges the NASA Postdoctoral Program and to Universities Space Research Association
Keeping in Touch with One's Self: Multisensory Mechanisms of Self-Consciousness
BACKGROUND: The spatial unity between self and body can be disrupted by employing conflicting visual-somatosensory bodily input, thereby bringing neurological observations on bodily self-consciousness under scientific scrutiny. Here we designed a novel paradigm linking the study of bodily self-consciousness to the spatial representation of visuo-tactile stimuli by measuring crossmodal congruency effects (CCEs) for the full body. METHODOLOGY/PRINCIPAL FINDINGS: We measured full body CCEs by attaching four vibrator-light pairs to the trunks (backs) of subjects who viewed their bodies from behind via a camera and a head mounted display (HMD). Subjects made speeded elevation (up/down) judgments of the tactile stimuli while ignoring light stimuli. To modulate self-identification for the seen body subjects were stroked on their backs with a stick and the felt stroking was either synchronous or asynchronous with the stroking that could be seen via the HMD. We found that (1) tactile stimuli were mislocalized towards the seen body (2) CCEs were modulated systematically during visual-somatosensory conflict when subjects viewed their body but not when they viewed a body-sized object, i.e. CCEs were larger during synchronous than during asynchronous stroking of the body and (3) these changes in the mapping of tactile stimuli were induced in the same experimental condition in which predictable changes in bodily self-consciousness occurred. CONCLUSIONS/SIGNIFICANCE: These data reveal that systematic alterations in the mapping of tactile stimuli occur in a full body illusion and thus establish CCE magnitude as an online performance proxy for subjective changes in global bodily self-consciousness
Mechanical ventilation modes for respiratory distress syndrome in infants: a systematic review and network meta-analysis
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