621,297 research outputs found
Minimal graphene thickness for wear protection of diamond
We show by means of molecular dynamics simulations that graphene is an
excellent coating for diamond. The transformation of diamond to amorphous
carbon while sliding under pressure can be prevented by having at least two
graphene layers between the diamond slabs, making this combination of materials
suitable for new coatings and micro- and nanoelectromechanical devices. Grain
boundaries, vacancies and adatoms on the diamond surface do not change this
picture whereas reactive adsorbates between the graphene layers may have
detrimental effects. Our findings can be explained by the properties of layered
materials where the weak interlayer bonding evolves to a strong interlayer
repulsion under pressure
CKVul: evolving nebula and three curious background stars
We analyse the remnants of CK Vul (Nova Vul 1670) using optical imaging and
spectroscopy. The imaging, obtained between 1991 and 2010, spans 5.6% of the
life-time of the nebula. The flux of the nebula decreased during the last 2
decades. The central source still maintains the ionization of the innermost
part of the nebula, but recombination proceeds in more distant parts of the
nebula. Surprisingly, we discovered two stars located within 10 arcsec of the
expansion centre of the radio emission that are characterized by pronounced
long term variations and one star with high proper motion. The high proper
motion star is a foreground object, and the two variable stars are background
objects. The photometric variations of two variables are induced by a dusty
cloud ejected by CK Vul and passing through the line of sight to those stars.
The cloud leaves strong lithium absorption in the spectra of the stars. We
discuss the nature of the object in terms of recent observations.Comment: Published in MNRAS, available at
http://mnras.oxfordjournals.org/cgi/content/abstract/stt426
The development of a mapping tool for the evaluation of building systems for future climate scenarios on European scale
The paper presents a tool for the mapping of the performance of building
systems on European scale for different (future) time periods. The tool is to
use for users and be applicable for different building systems. Users should
also be able to use a broad range of climate parameters to assess the influence
of climate change on these climatic parameters. Also should the calculation
time be reasonable short. The mapping tool is developed in MATLAB, which can be
used by other users for their own studies.Comment: 21 pages, 24 figures, pre-conferenc
The shortcomings of semi-local and hybrid functionals: what we can learn from surface science studies
A study of the adsorption of CO on late 4d and transition metal (111)
surfaces (Ru, Rh, Pd, Ag, Os, Ir, and Pt) considering atop and hollow site
adsorption is presented. The applied functionals include the gradient corrected
PBE and BLYP functional, and the corresponding hybrid Hartree-Fock density
functionals HSE and B3LYP. We find that PBE based hybrid functionals
(specifically HSE) yield, with the exception of Pt, the correct site order on
all considered metals, but they also considerably overestimate the adsorption
energies compared to experiment. On the other hand, the semi-local BLYP
functional and the corresponding hybrid functional B3LYP yield very
satisfactory adsorption energies and the correct adsorption site for all
surfaces. We are thus faced with a Procrustean problem: the B3LYP and BLYP
functionals seem to be the overall best choice for describing adsorption on
metal surfaces, but they simultaneously fail to account well for the properties
of the metal, vastly overestimating the equilibrium volume and underestimating
the atomization energies. Setting out from these observations, general
conclusions are drawn on the relative merits and drawbacks of various
semi-local and hybrid functionals. The discussion includes a revised version of
the PBE functional specifically optimized for bulk properties and surface
energies (PBEsol), a revised version of the PBE functional specifically
optimized to predict accurate adsorption energies (rPBE), as well as the
aforementioned BLYP functional. We conclude that no semi-local functional is
capable to describe all aspects properly, and including non-local exchange also
only improves some, but worsens other properties.Comment: 12 pages, 6 figures; to be published in New Journal of Physic
BES with FEM: Building Energy Simulation using Finite Element Methods
An overall objective of energy efficiency in the built environment is to
improve building and systems performances in terms of durability, comfort and
economics. In order to predict, improve and meet a certain set of performance
requirements related to the indoor climate of buildings and the associated
energy demand, building energy simulation (BES) tools are indispensable. Due to
the rapid development of FEM software and the Multiphysics approaches, it
should possible to build and simulate full 3D models of buildings regarding the
energy demand. The paper presents a methodology for performing building energy
simulation with Comsol. The method was applied to an international test box
experiment. The results showed an almost perfect agreement between the used BES
model and Comsol. These preliminary results confirm the great opportunities to
use FEM related software for building energy performance simulation.Comment: 5 pages, 6 figures, Proceedings of the 2012 COMSOL Conference in
Mila
BCFW recursion for TMD parton scattering
We investigate the application of the BCFW recursion relation to scattering
amplitudes with one off-shell particle in a Yang-Mills theory with fermions. We
provide a set of conditions of applicability of the BCFW recursion, stressing
some important differences with respect to the pure on-shell case. We show how
the formulas for Maximally-Helicity-Violating (MHV) configurations with any
number of partons, which are well known in the fully on-shell case, are
generalized to this kinematic regime. We also derive analytic expressions for
all the helicity configurations of the 5-point color-stripped tree-level
amplitudes for any of the partons being off the mass shell.Comment: Some typos in text and formulas correcte
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