6,653 research outputs found
On some geometric features of the Kramer interior solution for a rotating perfect fluid
Geometric features (including convexity properties) of an exact interior
gravitational field due to a self-gravitating axisymmetric body of perfect
fluid in stationary, rigid rotation are studied. In spite of the seemingly
non-Newtonian features of the bounding surface for some rotation rates, we
show, by means of a detailed analysis of the three-dimensional spatial
geodesics, that the standard Newtonian convexity properties do hold. A central
role is played by a family of geodesics that are introduced here, and provide a
generalization of the Newtonian straight lines parallel to the axis of
rotation.Comment: LaTeX, 15 pages with 4 Poscript figures. To be published in Classical
and Quantum Gravit
Adsorption of cobalt on graphene: Electron correlation effects from a quantum chemical perspective
In this work, we investigate the adsorption of a single cobalt atom (Co) on
graphene by means of the complete active space self-consistent field approach,
additionally corrected by the second-order perturbation theory. The local
structure of graphene is modeled by a planar hydrocarbon cluster
(CH). Systematic treatment of the electron correlations and the
possibility to study excited states allow us to reproduce the potential energy
curves for different electronic configurations of Co. We find that upon
approaching the surface, the ground-state configuration of Co undergoes several
transitions, giving rise to two stable states. The first corresponds to the
physisorption of the adatom in the high-spin ()
configuration, while the second results from the chemical bonding formed by
strong orbital hybridization, leading to the low-spin () state.
Due to the instability of the configuration, the adsorption energy of Co
is small in both cases and does not exceed 0.35 eV. We analyze the obtained
results in terms of a simple model Hamiltonian that involves Coulomb repulsion
() and exchange coupling () parameters for the 3 shell of Co, which we
estimate from first-principles calculations. We show that while the exchange
interaction remains constant upon adsorption ( eV), the Coulomb
repulsion significantly reduces for decreasing distances (from 5.3 to
2.60.2 eV). The screening of favors higher occupations of the 3
shell and thus is largely responsible for the interconfigurational transitions
of Co. Finally, we discuss the limitations of the approaches that are based on
density functional theory with respect to transition metal atoms on graphene,
and we conclude that a proper account of the electron correlations is crucial
for the description of adsorption in such systems.Comment: 12 pages, 6 figures, 2 table
Interfacial interactions between local defects in amorphous SiO and supported graphene
We present a density functional study of graphene adhesion on a realistic
SiO surface taking into account van der Waals (vdW) interactions. The
SiO substrate is modeled at the local scale by using two main types of
surface defects, typical for amorphous silica: the oxygen dangling bond and
three-coordinated silicon. The results show that the nature of adhesion between
graphene and its substrate is qualitatively dependent on the surface defect
type. In particular, the interaction between graphene and silicon-terminated
SiO originates exclusively from the vdW interaction, whereas the
oxygen-terminated surface provides additional ionic contribution to the binding
arising from interfacial charge transfer (-type doping of graphene). Strong
doping contrast for the different surface terminations provides a mechanism for
the charge inhomogeneity of graphene on amorphous SiO observed in
experiments. We found that independent of the considered surface morphologies,
the typical electronic structure of graphene in the vicinity of the Dirac point
remains unaltered in contact with the SiO substrate, which points to the
absence of the covalent interactions between graphene and amorphous silica. The
case of hydrogen-passivated SiO surfaces is also examined. In this
situation, the binding with graphene is practically independent of the type of
surface defects and arises, as expected, from the vdW interactions. Finally,
the interface distances obtained are shown to be in good agreement with recent
experimental studies.Comment: 10 pages, 4 figure
Dynamical singlets and correlation-assisted Peierls transition in VO2
A theory of the metal-insulator transition in vanadium dioxide from the
high-temperature rutile to the low- temperature monoclinic phase is proposed on
the basis of cluster dynamical mean field theory, in conjunction with the
density functional scheme. The interplay of strong electronic Coulomb
interactions and structural distortions, in particular the dimerization of
vanadium atoms in the low temperature phase, plays a crucial role. We find that
VO2 is not a conventional Mott insulator, but that the formation of dynamical
V-V singlet pairs due to strong Coulomb correlations is necessary to trigger
the opening of a Peierls gap.Comment: 5 page
Graphene adhesion on mica: Role of surface morphology
We investigate theoretically the adhesion and electronic properties of
graphene on a muscovite mica surface using the density functional theory (DFT)
with van der Waals (vdW) interactions taken into account (the vdW-DF approach).
We found that irregularities in the local structure of cleaved mica surface
provide different mechanisms for the mica-graphene binding. By assuming
electroneutrality for both surfaces, the binding is mainly of vdW nature,
barely exceeding thermal energy per carbon atom at room temperature. In
contrast, if potassium atoms are non uniformly distributed on mica, the
different regions of the surface give rise to - or -type doping of
graphene. In turn, an additional interaction arises between the surfaces,
significantly increasing the adhesion. For each case the electronic states of
graphene remain unaltered by the adhesion. It is expected, however, that the
Fermi level of graphene supported on realistic mica could be shifted relative
to the Dirac point due to asymmetry in the charge doping. Obtained variations
of the distance between graphene and mica for different regions of the surface
are found to be consistent with recent atomic force microscopy experiments. A
relative flatness of mica and the absence of interlayer covalent bonding in the
mica-graphene system make this pair a promising candidate for practical use.Comment: 6 pages, 3 figure
Adsorption of diatomic halogen molecules on graphene: A van der Waals density functional study
The adsorption of fluorine, chlorine, bromine, and iodine diatomic molecules
on graphene has been investigated using density functional theory with taking
into account nonlocal correlation effects by means of vdW-DF approach. It is
shown that the van der Waals interaction plays a crucial role in the formation
of chemical bonding between graphene and halogen molecules, and is therefore
important for a proper description of adsorption in this system. In-plane
orientation of the molecules has been found to be more stable than the
orientation perpendicular to the graphene layer. In the cases of F, Br
and I we also found an ionic contribution to the binding energy, slowly
vanishing with distance. Analysis of the electronic structure shows that ionic
interaction arises due to the charge transfer from graphene to the molecules.
Furthermore, we found that the increase of impurity concentration leads to the
conduction band formation in graphene due to interaction between halogen
molecules. In addition, graphite intercalation by halogen molecules has been
investigated. In the presence of halogen molecules the binding between graphite
layers becomes significantly weaker, which is in accordance with the results of
recent experiments on sonochemical exfoliation of intercalated graphite.Comment: Submitted to PR
Controlling the Kondo Effect in CoCu_n Clusters Atom by Atom
Clusters containing a single magnetic impurity were investigated by scanning
tunneling microscopy, spectroscopy, and ab initio electronic structure
calculations. The Kondo temperature of a Co atom embedded in Cu clusters on
Cu(111) exhibits a non-monotonic variation with the cluster size. Calculations
model the experimental observations and demonstrate the importance of the local
and anisotropic electronic structure for correlation effects in small clusters.Comment: 4 pages, 4 figure
- …