628 research outputs found
Van der Waals Density Functional for General Geometries
A scheme within density functional theory is proposed that provides a
practical way to generalize to unrestricted geometries the method applied with
some success to layered geometries [H. Rydberg, et al., Phys. Rev. Lett. 91,
126402 (2003)]. It includes van der Waals forces in a seamless fashion. By
expansion to second order in a carefully chosen quantity contained in the long
range part of the correlation functional, the nonlocal correlations are
expressed in terms of a density-density interaction formula. It contains a
relatively simple parametrized kernel, with parameters determined by the local
density and its gradient. The proposed functional is applied to rare gas and
benzene dimers, where it is shown to give a realistic description.Comment: 4 pages, 4 figure
Nature and strength of bonding in a crystal of semiconducting nanotubes: van der Waals density functional calculations and analytical results
The dispersive interaction between nanotubes is investigated through ab
initio theory calculations and in an analytical approximation. A van der Waals
density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)] is used to
determine and compare the binding of a pair of nanotubes as well as in a
nanotube crystal. To analyze the interaction and determine the importance of
morphology, we furthermore compare results of our ab initio calculations with a
simple analytical result that we obtain for a pair of well-separated nanotubes.
In contrast to traditional density functional theory calculations, the vdW-DF
study predicts an intertube vdW bonding with a strength that is consistent with
recent observations for the interlayer binding in graphitics. It also produce a
nanotube wall-to-wall separation which is in very good agreement with
experiments. Moreover, we find that the vdW-DF result for the nanotube-crystal
binding energy can be approximated by a sum of nanotube-pair interactions when
these are calculated in vdW-DF. This observation suggests a framework for an
efficient implementation of quantum-physical modeling of the CNT bundling in
more general nanotube bundles, including nanotube yarn and rope structures.Comment: 10 pages, 4 figure
Enhanced dispersion interaction between quasi-one dimensional conducting collinear structures
Recent investigations have highlighted the failure of a sum of terms
to represent the dispersion interaction in parallel metallic, anisotropic,
linear or planar nanostructures [J. F. Dobson, A. White, and A. Rubio, Phys.
Rev. Lett. 96, 073201 (2006) and references therein]. By applying a simple
coupled plasmon approach and using electron hydrodynamics, we numerically
evaluate the dispersion (non-contact van der Waals) interaction between two
conducting wires in a collinear pointing configuration. This case is compared
to that of two insulating wires in an identical geometry, where the dispersion
interaction is modelled both within a pairwise summation framework, and by
adding a pinning potential to our theory leading to a standard oscillator-type
model of insulating dielectric behavior. Our results provide a further example
of enhanced dispersion interaction between two conducting nanosystems compared
to the case of two insulating ones. Unlike our previous work, this calculation
explores a region of relatively close coupling where, although the electronic
clouds do not overlap, we are still far from the asymptotic region where a
single power law describes the dispersion energy. We find that strong
differences in dispersion attraction between metallic and semiconducting /
insulating cases persist into this non-asymptotic region. While our theory will
need to be supplemented with additional short-ranged terms when the electronic
clouds overlap, it does not suffer from the short-distance divergence exhibited
by purely asymptotic theories, and gives a natural saturation of the dispersion
energy as the wires come into contact.Comment: 10 pages, 5 figures. Added new extended numerical calculations, new
figures, extra references and heavily revised tex
FoldIndex©: a simple tool to predict whether a given protein sequence is intrinsically unfolded
Summary: An easy-to-use, versatile and freely available graphic web server, FoldIndex© is described: it predicts if a given protein sequence is intrinsically unfolded implementing the algorithm of Uversky and co-workers, which is based on the average residue hydrophobicity and net charge of the sequence. FoldIndex© has an error rate comparable to that of more sophisticated fold prediction methods. Sliding windows permit identification of large regions within a protein that possess folding propensities different from those of the whole protein. Availability: FoldIndex© can be accessed at http://bioportal.weizmann.ac.il/fldbin/findex Contact: [email protected] Supplementary information: http://www.weizmann.ac.il/sb/faculty_pages/Sussman/papers/suppl/Prilusky_200
Atomic Supersymmetry, Rydberg Wave Packets, and Radial Squeezed States
We study radial wave packets produced by short-pulsed laser fields acting on
Rydberg atoms, using analytical tools from supersymmetry-based quantum-defect
theory. We begin with a time-dependent perturbative calculation for
alkali-metal atoms, incorporating the atomic-excitation process. This provides
insight into the general wave packet behavior and demonstrates agreement with
conventional theory. We then obtain an alternative analytical description of a
radial wave packet as a member of a particular family of squeezed states, which
we call radial squeezed states. By construction, these have close to minimum
uncertainty in the radial coordinates during the first pass through the outer
apsidal point. The properties of radial squeezed states are investigated, and
they are shown to provide a description of certain aspects of Rydberg atoms
excited by short-pulsed laser fields. We derive expressions for the time
evolution and the autocorrelation of the radial squeezed states, and we study
numerically and analytically their behavior in several alkali-metal atoms. Full
and fractional revivals are observed. Comparisons show agreement with other
theoretical results and with experiment.Comment: published in Physical Review
Semiconducting Monolayer Materials as a Tunable Platform for Excitonic Solar Cells
The recent advent of two-dimensional monolayer materials with tunable
optoelectronic properties and high carrier mobility offers renewed
opportunities for efficient, ultra-thin excitonic solar cells alternative to
those based on conjugated polymer and small molecule donors. Using
first-principles density functional theory and many-body calculations, we
demonstrate that monolayers of hexagonal BN and graphene (CBN) combined with
commonly used acceptors such as PCBM fullerene or semiconducting carbon
nanotubes can provide excitonic solar cells with tunable absorber gap,
donor-acceptor interface band alignment, and power conversion efficiency, as
well as novel device architectures. For the case of CBN-PCBM devices, we
predict the limit of power conversion efficiencies to be in the 10 - 20% range
depending on the CBN monolayer structure. Our results demonstrate the
possibility of using monolayer materials in tunable, efficient, polymer-free
thin-film solar cells in which unexplored exciton and carrier transport regimes
are at play.Comment: 7 pages, 5 figure
Unified Treatment of Asymptotic van der Waals Forces
In a framework for long-range density-functional theory we present a unified
full-field treatment of the asymptotic van der Waals interaction for atoms,
molecules, surfaces, and other objects. The only input needed consists of the
electron densities of the interacting fragments and the static polarizability
or the static image plane, which can be easily evaluated in a ground-state
density-functional calculation for each fragment. Results for separated atoms,
molecules, and for atoms/molecules outside surfaces are in agreement with those
of other, more elaborate, calculations.Comment: 6 pages, 5 figure
Mechanical and Electronic Properties of MoS Nanoribbons and Their Defects
We present our study on atomic, electronic, magnetic and phonon properties of
one dimensional honeycomb structure of molybdenum disulfide (MoS) using
first-principles plane wave method. Calculated phonon frequencies of bare
armchair nanoribbon reveal the fourth acoustic branch and indicate the
stability. Force constant and in-plane stiffness calculated in the harmonic
elastic deformation range signify that the MoS nanoribbons are stiff quasi
one dimensional structures, but not as strong as graphene and BN nanoribbons.
Bare MoS armchair nanoribbons are nonmagnetic, direct band gap
semiconductors. Bare zigzag MoS nanoribbons become half-metallic as a
result of the (2x1) reconstruction of edge atoms and are semiconductor for
minority spins, but metallic for the majority spins. Their magnetic moments and
spin-polarizations at the Fermi level are reduced as a result of the
passivation of edge atoms by hydrogen. The functionalization of MoS
nanoribbons by adatom adsorption and vacancy defect creation are also studied.
The nonmagnetic armchair nanoribbons attain net magnetic moment depending on
where the foreign atoms are adsorbed and what kind of vacancy defect is
created. The magnetization of zigzag nanoribbons due to the edge states is
suppressed in the presence of vacancy defects.Comment: 11 pages, 5 figures, first submitted at November 23th, 200
FoldIndex: a simple tool to predict whether a given protein sequence is intrinsically unfolded
An easy-to-use, versatile and freely available graphic web server, FoldIndex is described: it predicts if a given protein sequence is intrinsically unfolded implementing the algorithm of Uversky and co-workers, which is based on the average residue hydrophobicity and net charge of the sequence. FoldIndex has an error rate comparable to that of more sophisticated fold prediction methods. Sliding windows permit identification of large regions within a protein that possess folding propensities different from those of the whole protein
Many-body GW calculations of ground-state properties: Quasi-2D electron systems and van der Waals forces
We present GW many-body results for ground-state properties of two simple but very distinct families of inhomogeneous systems in which traditional implementations of density-functional theory (DFT) fail drastically. The GW approach gives notably better results than the well-known random-phase approximation, at a similar computational cost. These results establish GW as a superior alternative to standard DFT schemes without the expensive numerical effort required by quantum Monte Carlo simulations
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