624 research outputs found
Magnetism: the Driving Force of Order in CoPt. A First-Principles Study
CoPt or FePt equiatomic alloys order according to the tetragonal L10
structure which favors their strong magnetic anisotropy. Conversely magnetism
can influence chemical ordering. We present here {\it ab initio} calculations
of the stability of the L10 and L12 structures of Co-Pt alloys in their
paramagnetic and ferromagnetic states. They show that magnetism strongly
reinforces the ordering tendencies in this system. A simple tight-binding
analysis allows us to account for this behavior in terms of some pertinent
parameters
A Tight-Binding Grand Canonical Monte Carlo Study of the Catalytic Growth of Carbon Nanotubes
The nucleation of carbon nanotubes on small nickel clusters is studied using
a tight binding model coupled to grand canonical Monte Carlo simulations. This
technique closely follows the conditions of the synthesis of carbon nanotubes
by chemical vapor deposition. The possible formation of a carbon cap on the
catalyst particle is studied as a function of the carbon chemical potential,
for particles of different size, either crystalline or disordered. We show that
these parameters strongly influence the structure of the cap/particle interface
which in turn will have a strong effect on the control of the structure of the
nanotube. In particular, we discuss the presence of carbon on surface or in
subsurface layers
Nickel assisted healing of defective graphene
The healing of graphene grown from a metallic substrate is investigated using
tight-binding Monte Carlo simulations. At temperatures (ranging from 1000 to
2500 K), an isolated graphene sheet can anneal a large number of defects
suggesting that their healing are thermally activated. We show that in presence
of a nickel substrate we obtain a perfect graphene layer. The nickel-carbon
chemical bonds keep breaking and reforming around defected carbon zones,
providing a direct interaction, necessary for the healing. Thus, the action of
Ni atoms is found to play a key role in the reconstruction of the graphene
sheet by annealing defects
Study of phase stability of MnCr using the augmented space recursion based orbital peeling technique
In an earlier communication we have developed a recursion based approach to
the study of phase stability and transition of binary alloys. We had combined
the recursion method introduced by Haydock, Heine and Kelly and the our
augmented space approach with the orbital peeling technique proposed by Burke
to determine the small energy differences involved in the discussion of phase
stability. We extend that methodology for the study of MnCr alloys.Comment: 11 page
Long-range interactions between substitutional nitrogen dopants in graphene: electronic properties calculations
Being a true two-dimensional crystal, graphene has special properties. In
particular, a point-like defect in graphene may have effects in the long range.
This peculiarity questions the validity of using a supercell geometry in an
attempt to explore the properties of an isolated defect. Still, this approach
is often used in ab-initio electronic structure calculations, for instance. How
does this approach converge with the size of the supercell is generally not
tackled for the obvious reason of keeping the computational load to an
affordable level. The present paper addresses the problem of substitutional
nitrogen doping of graphene. DFT calculations have been performed for 9x9 and
10x10 supercells. Although these calculations correspond to N concentrations
that differ by about 10%, the local densities of states on and around the
defects are found to depend significantly on the supercell size. Fitting the
DFT results by a tight-binding Hamiltonian makes it possible to explore the
effects of a random distribution of the substitutional N atoms, in the case of
finite concentrations, and to approach the case of an isolated impurity when
the concentration vanishes. The tight-binding Hamiltonian is used to calculate
the STM image of graphene around an isolated N atom. STM images are also
calculated for graphene doped with 0.5 % concentration of nitrogen. The results
are discussed in the light of recent experimental data and the conclusions of
the calculations are extended to other point defects in graphene
Exciton interference in hexagonal boron nitride
In this letter we report a thorough analysis of the exciton dispersion in
bulk hexagonal boron nitride. We solve the ab initio GW Bethe-Salpeter equation
at finite , and we compare our results with
recent high-accuracy electron energy loss data. Simulations reproduce the
measured dispersion and the variation of the peak intensity. We focus on the
evolution of the intensity, and we demonstrate that the excitonic peak is
formed by the superposition of two groups of transitions that we call and
from the k-points involved in the transitions. These two groups
contribute to the peak intensity with opposite signs, each damping the
contributions of the other. The variations in number and amplitude of these
transitions determine the changes in intensity of the peak. Our results
contribute to the understanding of electronic excitations in this systems along
the direction, which is the relevant direction for spectroscopic
measurements. They also unveil the non-trivial relation between valley physics
and excitonic dispersion in h--BN, opening the possibility to tune excitonic
effects by playing with the interference between transitions. Furthermore, this
study introduces analysis tools and a methodology that are completely general.
They suggest a way to regroup independent-particle transitions which could
permit a deeper understanding of excitonic properties in any system
Magnetocrystalline anisotropy energy of Fe, Fe slabs and nanoclusters: a detailed local analysis within a tight-binding model
We report tight-binding (TB) calculations of magnetocrystalline anisotropy
energy (MAE) of Iron slabs and nanoclusters with a particuler focus on local
analysis. After clarifying various concepts and formulations for the
determination of MAE, we apply our realistic TB model to the analysis of the
magnetic anisotropy of Fe, Fe slabs and of two large Fe clusters
with and facets only: a truncated pyramid and a truncated
bipyramid containg 620 and 1096 atoms, respectively. It is shown that the MAE
of slabs originates mainly from outer layers, a small contribution from the
bulk gives rise, however, to an oscillatory behavior for large thicknesses.
Interestingly, the MAE of the nanoclusters considered is almost solely due to
facets and the base perimeter of the pyramid. We believe that this fact
could be used to efficiently control the anisotropy of Iron nanoparticles and
could also have consequences on their spin dynamics
Understanding the nucleation mechanisms of Carbon Nanotubes in catalytic Chemical Vapor Deposition
The nucleation of carbon caps on small nickel clusters is studied using a
tight binding model coupled to grand canonical Monte Carlo simulations. It
takes place in a well defined carbon chemical potential range, when a critical
concentration of surface carbon atoms is reached. The solubility of carbon in
the outermost Ni layers, that depends on the initial, crystalline or
disordered, state of the catalyst and on the thermodynamic conditions, is
therefore a key quantity to control the nucleation
Size dependent phase diagrams of Nickel-Carbon nanoparticles
The carbon rich phase diagrams of nickel-carbon nanoparticles, relevant to
catalysis and catalytic chemical vapor deposition synthesis of carbon
nanotubes, are calculated for system sizes up to about 3 nanometers (807 Ni
atoms). A tight binding model for interatomic interactions drives the Grand
Canonical Monte Carlo simulations used to locate solid, core/shell and liquid
stability domains, as a function of size, temperature and carbon chemical
potential or concentration. Melting is favored by carbon incorporation from the
nanoparticle surface, resulting in a strong relative lowering of the eutectic
temperature and a phase diagram topology different from the bulk one. This
should be taken into account in our understanding of the nanotube growth
mechanisms
- …