4,095 research outputs found
Effect of dimensionality on the charge-density-wave in few-layers 2H-NbSe
We investigate the charge density wave (CDW) instability in single and double
layers, as well as in the bulk 2H-NbSe. We demonstrate that the density
functional theory correctly describes the metallic CDW state in the bulk
2H-NbSe. We predict that both mono- and bilayer NbSe undergo a CDW
instability. However, while in the bulk the instability occurs at a momentum
, in free-standing layers it
occurs at . Furthermore, while
in the bulk the CDW leads to a metallic state, in a monolayer the ground state
becomes semimetallic, in agreement with recent experimental data. We elucidate
the key role that an enhancement of the electron-phonon matrix element at
plays in forming the CDW ground state.Comment: 4 pages 5 figure
All-electron magnetic response with pseudopotentials: NMR chemical shifts
A theory for the ab initio calculation of all-electron NMR chemical shifts in
insulators using pseudopotentials is presented. It is formulated for both
finite and infinitely periodic systems and is based on an extension to the
Projector Augmented Wave approach of Bloechl [P. E. Bloechl, Phys. Rev. B 50,
17953 (1994)] and the method of Mauri et al [F. Mauri, B.G. Pfrommer, and S.G.
Louie, Phys. Rev. Lett. 77, 5300 (1996)]. The theory is successfully validated
for molecules by comparison with a selection of quantum chemical results, and
in periodic systems by comparison with plane-wave all-electron results for
diamond.Comment: 25 pages, 4 tables, submitted to Physical Review
MODELLING OF POWDER FLOW IN ROTATIONAL MOULDING
Rotational moulding is a widely used technological process to obtain hollow plastic articles, in which polymer powders melt within a rotating mould. The first step in modelling the melting process is to analyse the kinematics of the powder in the rotating system. To this goal, a series of experimental observations was performed on a rotating cylinder partially filled with a powder (Sand X, average size 460 microns) with known physical characteristics (such as angles of repose and angles of approach). A phenomenological model was then developed, based on the assumption that the powder behaves as a Bingham-plastic fluid, obtaining theoretical predictions which were in good agreement with the experimental measurements
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
Large scale quantum simulations: C_60 impacts on a semiconducting surface
We present tight binding molecular dynamics simulations of C_60 collisions on
the reconstructed diamond(111) surface, carried out with an O(N) method and
with cells containing 1140 atoms. The results of our simulations are in very
good agreement with experiments performed under the same impact conditions.
Furthermore our calculations provide a detailed characterization of the
microscopic processes occuring during the collision, and allow the
identification of three impact regimes, as a function of the fullerene incident
energy. Finally, the study of the reactivity between the cluster and the
surface gives insight into the deposition mechanisms of C_60 on semiconducting
substrates
Acceleration Schemes for Ab-Initio Molecular Dynamics and Electronic Structure Calculations
We study the convergence and the stability of fictitious dynamical methods
for electrons. First, we show that a particular damped second-order dynamics
has a much faster rate of convergence to the ground-state than first-order
steepest descent algorithms while retaining their numerical cost per time step.
Our damped dynamics has efficiency comparable to that of conjugate gradient
methods in typical electronic minimization problems. Then, we analyse the
factors that limit the size of the integration time step in approaches based on
plane-wave expansions. The maximum allowed time step is dictated by the highest
frequency components of the fictitious electronic dynamics. These can result
either from the large wavevector components of the kinetic energy or from the
small wavevector components of the Coulomb potential giving rise to the so
called {\it charge sloshing} problem. We show how to eliminate large wavevector
instabilities by adopting a preconditioning scheme that is implemented here for
the first-time in the context of Car-Parrinello ab-initio molecular dynamics
simulations of the ionic motion. We also show how to solve the charge-sloshing
problem when this is present. We substantiate our theoretical analysis with
numerical tests on a number of different silicon and carbon systems having both
insulating and metallic character.Comment: RevTex, 9 figures available upon request, to appear in Phys. Rev.
Adiabatic and non-adiabatic phonon dispersion in a Wannier function approach
We develop a first-principles scheme to calculate adiabatic and non-adiabatic
phonon frequencies in the full Brillouin zone. The method relies on the
variational properties of a force-constants functional with respect to the
first-order perturbation of the electronic charge density and on the
localization of the deformation potential in the Wannier function basis. This
allows for calculation of phonon dispersion curves free from convergence issues
related to Brillouin zone sampling. In addition our approach justify the use of
the static screened potential in the calculation of the phonon linewidth due to
decay in electron-hole pairs. We apply the method to the calculation of the
phonon dispersion and electron-phonon coupling in MgB and CaC. In both
compounds we demonstrate the occurrence of several Kohn anomalies, absent in
previous calculations, that are manifest only after careful electron and phonon
momentum integration. In MgB, the presence of Kohn anomalies on the
E branches improves the agreement with measured phonon spectra and
affects the position of the main peak in the Eliashberg function. In CaC we
show that the non-adiabatic effects on in-plane carbon vibrations are not
localized at zone center but are sizable throughout the full Brillouin zone.
Our method opens new perspectives in large-scale first-principles calculations
of dynamical properties and electron-phonon interaction.Comment: 18 pages, 8 figure
Spin torque, tunnel-current spin polarization and magnetoresistance in MgO magnetic tunnel junctions
We examine the spin torque (ST) response of magnetic tunnel junctions (MTJs)
with ultra-thin MgO tunnel barrier layers to investigate the relationship
between the spin-transfer torque and the tunnel magnetoresistance (TMR) under
finite bias. We find that the spin torque per unit current exerted on the free
layer decreases by less than 10% over a bias range where the TMR decreases by
over 40%. We examine the implications of this result for various spin-polarized
tunneling models and find that it is consistent with magnetic-state-dependent
effective tunnel decay lengths.Comment: 4 pages, 3 figure
Ab-initio theory of NMR chemical shifts in solids and liquids
We present a theory for the ab-initio computation of NMR chemical shifts
(sigma) in condensed matter systems, using periodic boundary conditions. Our
approach can be applied to periodic systems such as crystals, surfaces, or
polymers and, with a super-cell technique, to non-periodic systems such as
amorphous materials, liquids, or solids with defects. We have computed the
hydrogen sigma for a set of free molecules, for an ionic crystal, LiH, and for
a H-bonded crystal, HF, using density functional theory in the local density
approximation. The results are in excellent agreement with experimental data.Comment: to appear in Physical Review Letter
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