331 research outputs found
Preconditioner-Based Contact Response and Application to Cataract Surgery
International audienceIn this paper we introduce a new method to compute, in real-time, the physical behavior of several colliding soft-tissues in a surgical simulation. The numerical approach is based on finite element modeling and allows for a fast update of a large number of tetrahedral elements. The speed-up is obtained by the use of a specific preconditioner that is updated at low frequency. The preconditioning enables an optimized computation of both large deformations and precise contact response. Moreover, homogeneous and inhomogeneous tissues are simulated with the same accuracy. Finally, we illustrate our method in a simulation of one step in a cataract surgery procedure, which require to handle contacts with non homogeneous objects precisely
First-principles study of As interstitials in GaAs: Convergence, relaxation, and formation energy
Convergence of density-functional supercell calculations for defect formation
energies, charge transition levels, localized defect state properties, and
defect atomic structure and relaxation is investigated using the arsenic split
interstitial in GaAs as an example. Supercells containing up to 217 atoms and a
variety of {\bf k}-space sampling schemes are considered. It is shown that a
good description of the localized defect state dispersion and charge state
transition levels requires at least a 217-atom supercell, although the defect
structure and atomic relaxations can be well converged in a 65-atom cell.
Formation energies are calculated for the As split interstitial, Ga vacancy,
and As antisite defects in GaAs, taking into account the dependence upon
chemical potential and Fermi energy. It is found that equilibrium
concentrations of As interstitials will be much lower than equilibrium
concentrations of As antisites in As-rich, -type or semi-insulating GaAs.Comment: 10 pages, 5 figure
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
Nature of Sonoluminescence: Noble Gas Radiation Excited by Hot Electrons in "Cold" Water
We show that strong electric fields occurring in water near the surface of
collapsing gas bubbles because of the flexoelectric effect can provoke dynamic
electric breakdown in a micron-size region near the bubble and consider the
scenario of the SBSL. The scenario is: (i) at the last stage of incomplete
collapse of the bubble the gradient of pressure in water near the bubble
surface has such a value and sign that the electric field arising from the
flexoelectric effect exceeds the threshold field of the dynamic electrical
breakdown of water and is directed to the bubble center; (ii) mobile electrons
are generated because of thermal ionization of water molecules near the bubble
surface; (iii) these electrons are accelerated in ''cold'' water by the strong
electric fields; (iv) these hot electrons transfer noble gas atoms dissolved in
water to high-energy excited states and optical transitions between these
states produce SBSL UV flashes in the trasparency window of water; (v) the
breakdown can be repeated several times and the power and duration of the UV
flash are determined by the multiplicity of the breakdowns. The SBSL spectrum
is found to resemble a black-body spectrum where temperature is given by the
effective temperature of the hot electrons. The pulse energy and some other
characteristics of the SBSL are found to be in agreement with the experimental
data when realistic estimations are made.Comment: 11 pages (RevTex), 1 figure (.ps
Active membranes:3D printing of elastic fibre patterns on pre-stretched textiles
There has been a steady growth, over several decades, in the deployment of fabrics in architectural applications; both in terms of quantity and variety of application. More recently 3D printing and additive manufacturing have added to the palette of technologies that designers in architecture and related disciplines can call upon. Here we report on research that brings those two technologies together - the development of active membrane elements and structures. We show how these active membranes have been achieved by laminating 3D printed elasto-plastic fibres onto pre-stretched textile membranes. We report on a set of experiments involving one-, two- and multi-directional geometric arrangements that take TPU 95 and Polypropylene filaments and apply them to lycra textile sheets, to form active composite panels. The process involves a parametrised design, actualized through a particular fabrication process. Our findings document the investigation into mapping between the initial two-dimensional geometries and their resulting three-dimensional doubly-curved forms, as well as accomplishments and products of the resulting, partly serendipitous, design process
Self-consistent solution of Kohn-Sham equations for infinitely extended systems with inhomogeneous electron gas
The density functional approach in the Kohn-Sham approximation is widely used
to study properties of many-electron systems. Due to the nonlinearity of the
Kohn-Sham equations, the general self-consistence searching method involves
iterations with alternate solving of the Poisson and Schr\"{o}dinger equations.
One of problems of such an approach is that the charge distribution renewed by
means of the Schr\"{o}dinger equation solution does not conform to boundary
conditions of Poisson equation for Coulomb potential. The resulting instability
or even divergence of iterations manifests itself most appreciably in the case
of infinitely extended systems. The published attempts to deal with this
problem are reduced in fact to abandoning the original iterative method and
replacing it with some approximate calculation scheme, which is usually
semi-empirical and does not permit to evaluate the extent of deviation from the
exact solution. In this work, we realize the iterative scheme of solving the
Kohn-Sham equations for extended systems with inhomogeneous electron gas, which
is based on eliminating the long-range character of Coulomb interaction as the
cause of tight coupling between charge distribution and boundary conditions.
The suggested algorithm is employed to calculate energy spectrum,
self-consistent potential, and electrostatic capacitance of the semi-infinite
degenerate electron gas bounded by infinitely high barrier, as well as the work
function and surface energy of simple metals in the jellium model. The
difference between self-consistent Hartree solutions and those taking into
account the exchange-correlation interaction is analyzed. The case study of the
metal-semiconductor tunnel contact shows this method being applied to an
infinitely extended system where the steady-state current can flow.Comment: 38 pages, 9 figures, to be published in ZhETF (J. Exp. Theor. Phys.
Electronic and structural properties of vacancies on and below the GaP(110) surface
We have performed total-energy density-functional calculations using
first-principles pseudopotentials to determine the atomic and electronic
structure of neutral surface and subsurface vacancies at the GaP(110) surface.
The cation as well as the anion surface vacancy show a pronounced inward
relaxation of the three nearest neighbor atoms towards the vacancy while the
surface point-group symmetry is maintained. For both types of vacancies we find
a singly occupied level at mid gap. Subsurface vacancies below the second layer
display essentially the same properties as bulk defects. Our results for
vacancies in the second layer show features not observed for either surface or
bulk vacancies: Large relaxations occur and both defects are unstable against
the formation of antisite vacancy complexes. Simulating scanning tunneling
microscope pictures of the different vacancies we find excellent agreement with
experimental data for the surface vacancies and predict the signatures of
subsurface vacancies.Comment: 10 pages, 6 figures, Submitted to Phys. Rev. B, Other related
publications can be found at http://www.rz-berlin.mpg.de/th/paper.htm
Physics and chemistry of hydrogen in the vacancies of semiconductors
Hydrogen is well known to cause electrical passivation of lattice vacancies in semiconductors. This effect follows from the chemical passivation of the dangling bonds. Recently it was found that H in the carbon vacancy of SiC forms a three-center bond with two silicon neighbors in the vacancy, and gives rise to a new electrically active state. In this paper we examine hydrogen in the anion vacancies of BN, AlN, and GaN. We find that three-center bonding of H is quite common and follows clear trends in terms of the second-neighbor distance in the lattice, the typical (two-center) hydrogen-host-atom bond length, the electronegativity difference between host atoms and hydrogen, as well as the charge state of the vacancy. Three-center bonding limits the number of H atoms a nitrogen vacancy can capture to two, and prevents electric passivation in GaAs as well
- …