6,213 research outputs found
A new route towards uniformly functionalized single-layer graphene
It is shown, by DFT calculations, that the uniform functionalization of upper
layer of graphite by hydrogen or fluorine does not change essentially its
bonding energy with the underlying layers, whereas the functionalization by
phenyl groups decreases the bonding energy by a factor of approximately ten.
This means that the functionalized monolayer in the latter case can be easily
separated by mild sonication. According to our computational results, such
layers can be cleaned up to pure graphene, as well as functionalized further up
to 25% coverage, without essential difficulties. The energy gap within the
interval from 0.5 to 3 eV can be obtained by such one-side funtionalization
using different chemical species.Comment: 15 pages, 3 figures, to appear in J. Phys. D: Applied Physic
Modelling of epitaxial graphene functionalization
A new model for graphene, epitaxially grown on silicon carbide is proposed.
Density functional theory modelling of epitaxial graphene functionalization by
hydrogen, fluorine and phenyl groups has been performed with hydrogen and
fluorine showing a high probability of cluster formation in high adatom
concentration. It has also been shown that the clusterization of fluorine
adatoms provides midgap states in formation due to significant flat distortion
of graphene. The functionalization of epitaxial graphene using larger species
(methyl and phenyl groups) renders cluster formation impossible, due to the
steric effect and results in uniform coverage with the energy gap opening.Comment: 15 pages, 4 figures, to appear in Nanotechnolog
Determination of the magnetic anisotropy axes of single-molecule magnets
Simple methods are presented allowing the determination of the magnetic
anisotropy axes of a crystal of a single-molecule magnet (SMM). These methods
are used to determine an upper bound of the easy axis tilts in a standard
Mn12-Ac crystal. The values obtained in the present study are significately
smaller than those reported in recent high frequency electron paramagnetic
resonance (HF-EPR) studies which suggest distributions of hard-axes tilts.Comment: 10 pages, 6 figure
The nature of the low energy band of the Fenna-Matthews-Olson complex: vibronic signatures
Based entirely upon actual experimental observations on electron-phonon
coupling, we develop a theoretical framework to show that the lowest energy
band of the Fenna- Matthews-Olson (FMO) complex exhibits observable features
due to the quantum nature of the vibrational manifolds present in its
chromophores. The study of linear spectra provides us with the basis to
understand the dynamical features arising from the vibronic structure in
non-linear spectra in a progressive fashion, starting from a microscopic model
to finally performing an inhomogenous average. We show that the discreteness of
the vibronic structure can be witnessed by probing the diagonal peaks of the
non-linear spectra by means of a relative phase shift in the waiting time
resolved signal. Moreover, we demonstrate the photon-echo and non-rephasing
paths are sensitive to different harmonics in the vibrational manifold when
static disorder is taken into account. Supported by analytical and numerical
calculations, we show that nondiagonal resonances in the 2D spectra in the
waiting time, further capture the discreteness of vibrations through a
modulation of the amplitude without any effect in the signal intrinsic
frequency. This fact generates a signal that is highly sensitive to
correlations in the static disorder of the excitonic energy albeit protected
against dephasing due to inhomogeneities of the vibrational ensemble.Comment: 14 pages, 6 figure
Toward detailed prominence seismology - I. Computing accurate 2.5D magnetohydrodynamic equilibria
Context. Prominence seismology exploits our knowledge of the linear
eigenoscillations for representative magnetohydro- dynamic models of filaments.
To date, highly idealized models for prominences have been used, especially
with respect to the overall magnetic configurations.
Aims. We initiate a more systematic survey of filament wave modes, where we
consider full multi-dimensional models with twisted magnetic fields
representative of the surrounding magnetic flux rope. This requires the ability
to compute accurate 2.5 dimensional magnetohydrodynamic equilibria that balance
Lorentz forces, gravity, and pressure gradients, while containing density
enhancements (static or in motion).
Methods. The governing extended Grad-Shafranov equation is discussed, along
with an analytic prediction for circular flux ropes for the Shafranov shift of
the central magnetic axis due to gravity. Numerical equilibria are computed
with a finite element-based code, demonstrating fourth order accuracy on an
explicitly known, non-trivial test case.
Results. The code is then used to construct more realistic prominence
equilibria, for all three possible choices of a free flux-function. We quantify
the influence of gravity, and generate cool condensations in hot cavities, as
well as multi- layered prominences.
Conclusions. The internal flux rope equilibria computed here have the
prerequisite numerical accuracy to allow a yet more advanced analysis of the
complete spectrum of linear magnetohydrodynamic perturbations, as will be
demonstrated in the companion paper.Comment: Accepted by Astronomy & Astrophysics, 15 pages, 15 figure
Non-hexagonal-ring defects and structures induced by strain in graphene and in functionalized graphene
We perform {\textit ab initio} calculations for the strain-induced formation
of non-hexagonal-ring defects in graphene, graphane (planar CH), and graphenol
(planar COH). We find that the simplest of such topological defects, the
Stone-Wales defect, acts as a seed for strain-induced dissociation and
multiplication of topological defects. Through the application of inhomogeneous
deformations to graphene, graphane and graphenol with initially small
concentrations of pentagonal and heptagonal rings, we obtain several novel
stable structures that possess, at the same time, large concentrations of
non-hexagonal rings (from fourfold to elevenfold) and small formation energies
Magnetism and half-metallicity at the O surfaces of ceramic oxides
The occurence of spin-polarization at ZrO, AlO and MgO
surfaces is proved by means of \textit{ab-initio} calculations within the
density functional theory. Large spin moments, as high as 1.56 , develop
at O-ended polar terminations, transforming the non-magnetic insulator into a
half-metal. The magnetic moments mainly reside in the surface oxygen atoms and
their origin is related to the existence of holes of well-defined spin
polarization at the valence band of the ionic oxide. The direct relation
between magnetization and local loss of donor charge makes possible to extend
the magnetization mechanism beyond surface properties
Electronic structure interpolation via atomic orbitals
We present an efficient scheme for accurate electronic structure
interpolations based on the systematically improvable optimized atomic
orbitals. The atomic orbitals are generated by minimizing the spillage value
between the atomic basis calculations and the converged plane wave basis
calculations on some coarse -point grid. They are then used to calculate the
band structure of the full Brillouin zone using the linear combination of
atomic orbitals (LCAO) algorithms. We find that usually 16 -- 25 orbitals per
atom can give an accuracy of about 10 meV compared to the full {\it ab initio}
calculations. The current scheme has several advantages over the existing
interpolation schemes. The scheme is easy to implement and robust which works
equally well for metallic systems and systems with complex band structures.
Furthermore, the atomic orbitals have much better transferability than the
Shirley's basis and Wannier functions, which is very useful for the
perturbation calculations
Quantum phase interference (Berry phase) in single-molecule magnets of Mn12
Magnetization measurements of a molecular clusters Mn12 with a spin ground
state of S = 10 show resonance tunneling at avoided energy level crossings. The
observed oscillations of the tunnel probability as a function of the magnetic
field applied along the hard anisotropy axis are due to topological quantum
phase interference of two tunnel paths of opposite windings. Mn12 is therefore
the second molecular clusters presenting quantum phase interference.Comment: 3 pages, 4 figures, MMM'01 conference (12-16 Nov.
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