234 research outputs found
Exploring approximations to the GW self-energy ionic gradients
The accuracy of the many-body perturbation theory GW formalism to calculate
electron-phonon coupling matrix elements has been recently demonstrated in the
case of a few important systems. However, the related computational costs are
high and thus represent strong limitations to its widespread application. In
the present study, we explore two less demanding alternatives for the
calculation of electron-phonon coupling matrix elements on the many-body
perturbation theory level. Namely, we test the accuracy of the static
Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the
constant screening approach, where variations of the screened Coulomb potential
W upon small changes of the atomic positions along the vibrational eigenmodes
are neglected. We find this latter approximation to be the most reliable,
whereas the static COHSEX ansatz leads to substantial errors. Our conclusions
are validated in a few paradigmatic cases: diamond, graphene and the C60
fullerene. These findings open the way for combining the present many-body
perturbation approach with efficient linear-response theories
Short to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: a Bethe-Salpeter study
We study using the Bethe-Salpeter formalism the excitation energies of the
zincbacteriochlorinbacteriochlorin dyad, a paradigmatic photosynthetic complex.
In great contrast with standard timedependent density functional theory
calculations with (semi)local kernels, charge transfer excitations are
correctly located above the intramolecular Q-bands transitions found to be in
excellent agreement with experiment. Further, the asymptotic Coulomb behavior
towards the true quasiparticle gap for charge transfer excitations at long
distance is correctly reproduced, showing that the present scheme allows to
study with the same accuracy intramolecular and charge transfer excitations at
various spatial range and screening environment without any adjustable
parameter.Comment: 5 pages, 2 figures, 1 tabl
Self-similar impulsive capillary waves on a ligament
We study the short-time dynamics of a liquid ligament, held between two solid
cylinders, when one is impulsively accelerated along its axis. A set of
one-dimensional equations in the slender-slope approximation is used to
describe the dynamics, including surface tension and viscous effects. An exact
self-similar solution to the linearized equations is successfully compared to
experiments made with millimetric ligaments. Another non-linear self-similar
solution of the full set of equations is found numerically. Both the linear and
non-linear solutions show that the axial depth at which the liquid is affected
by the motion of the cylinder scales like . The non-linear solution
presents the peculiar feature that there exists a maximum driving velocity
above which the solution disappears, a phenomenon probably related to
the de-pinning of the contact line observed in experiments for large pulling
velocities
A decade of DNA-hybrid catalysis: from innovation to comprehension
crosscheck: This document is CrossCheck deposited identifier: Michael Smietana (ORCID) identifier: Stellios Arseniyadis (ORCID) identifier: Stellios Arseniyadis (ResearcherID) copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal history: Received 22 January 2017; Accepted 23 April 2017; Accepted Manuscript published 25 April 2017; Advance Article published 9 May 2017We would like to thank the Agence Nationale de la Recherche for funding – the NCiS project (ANR-2010-JCJC-715-1) and the D-CYSIV project (ANR-2015-CE29-0021-01
Astrometric positions for 18 irregular satellites of giant planets from 23 years of observations
The irregular satellites of the giant planets are believed to have been
captured during the evolution of the solar system. Knowing their physical
parameters, such as size, density, and albedo is important for constraining
where they came from and how they were captured. The best way to obtain these
parameters are observations in situ by spacecrafts or from stellar occultations
by the objects. Both techniques demand that the orbits are well known. We aimed
to obtain good astrometric positions of irregular satellites to improve their
orbits and ephemeris. We identified and reduced observations of several
irregular satellites from three databases containing more than 8000 images
obtained between 1992 and 2014 at three sites (Observat\'orio do Pico dos Dias,
Observatoire de Haute-Provence, and European Southern Observatory - La Silla).
We used the software PRAIA (Platform for Reduction of Astronomical Images
Automatically) to make the astrometric reduction of the CCD frames. The UCAC4
catalog represented the International Celestial Reference System in the
reductions. Identification of the satellites in the frames was done through
their ephemerides as determined from the SPICE/NAIF kernels. Some procedures
were followed to overcome missing or incomplete information (coordinates,
date), mostly for the older images. We managed to obtain more than 6000
positions for 18 irregular satellites: 12 of Jupiter, 4 of Saturn, 1 of Uranus
(Sycorax), and 1 of Neptune (Nereid). For some satellites the number of
obtained positions is more than 50\% of what was used in earlier orbital
numerical integrations. Comparison of our positions with recent JPL ephemeris
suggests there are systematic errors in the orbits for some of the irregular
satellites. The most evident case was an error in the inclination of Carme.Comment: 9 pages, with 3 being online materia
Accurate strain measurements in highly strained Ge microbridges
Ge under high strain is predicted to become a direct bandgap semiconductor.
Very large deformations can be introduced using microbridge devices. However,
at the microscale, strain values are commonly deduced from Raman spectroscopy
using empirical linear models only established up to 1.2% for uniaxial stress.
In this work, we calibrate the Raman-strain relation at higher strain using
synchrotron based microdiffraction. The Ge microbridges show unprecedented high
tensile strain up to 4.9 % corresponding to an unexpected 9.9 cm-1 Raman shift.
We demonstrate experimentally and theoretically that the Raman strain relation
is not linear and we provide a more accurate expression.Comment: 10 pages, 4 figure
Metallic behaviour in SOI quantum wells with strong intervalley scattering
Supplementary code for the calculation of WL with intervalley scattering available at the publisher's siteInternational audienceThe fundamental properties of valleys are recently attracting growing attention due to electrons in new and topical materials possessing this degree-of-freedom and recent proposals for val-leytronics devices. In silicon MOSFETs, the interest has a longer history since the valley degree of freedom had been identified as a key parameter in the observation of the controversial " metallic behaviour " in two dimensions. However, while it has been recently demonstrated that lifting valley degeneracy can destroy the metallic behaviour, little is known about the role of intervalley scattering. Here, we show that the metallic behaviour can be observed in the presence of strong interval-ley scattering in silicon on insulator (SOI) quantum wells. Analysis of the conductivity in terms of quantum corrections reveals that interactions are much stronger in SOI than in conventional MOSFETs, leading to the metallic behaviour despite the strong intervalley scattering. The prospect of manipulating the valley degree of freedom in materials like AlAs, 1 silicon 2–4 graphene
Термодинамический расчет диаграммы плавкости системы Mg—MgO—B при давлении 2 ГПа
В рамках модельных представлений феноменологической термодинамики выполнен расчет фазовых равновесий с участием жидкой фазы в тройной системе Mg—MgO—B при давлении 2 ГПа. Диаграмма плавкости системы характеризуется наличием пяти нонвариантных четырехфазных равновесий, три из которых перитектические, а два — эвтектические. Область первичной кристаллизации диборида магния MgB₂ расположена вблизи двойной системы Mg—B и выклинивается по мере увеличения концентрации кислорода, уступая в точке перитектического равновесия L + MgB₂ ↔ MgO + MgB₄ (1345 K, 68 % (ат.) В, 10 % (ат.) О) областям кристаллизации MgO и MgB₄
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