2,448 research outputs found
Non-local Coulomb interactions and metal-insulator transition in TiO: a cluster LDA+DMFT approach
We present an ab initio quantum theory of the metal-insulator transition in
TiO. The recently developed cluster LDA+DMFT scheme is applied to
describe the many-body features of this compound. The conventional single site
DMFT cannot reproduce a low temperature insulating phase for any reasonable
values of the Coulomb interaction. We show that the non-local Coulomb
interactions and the strong chemical bonding within Ti-Ti pair is the origin of
the small gap insulating ground state of TiO
Correlation effects in electronic structure of PuCoGa5
We report on results of the first realistic electronic structure calculations
of the Pu-based PuCoGa5 superconductor based on the dynamical mean field
theory. We find that dynamical correlations due to the local Coulomb
interaction between Pu f-electrons lead to substantial modification of the
electronic structure with a narrow peak being formed in vicinity of the Fermi
energy, in agreement with the experimental photoemission spectra, and in
contrast with the recent calculations within the LDA+U method, where only
static electronic correlations have been included. Both Pu and Co contribute in
equal footing to the narrow peak on the density of states at the Fermi level,
the Co partial density of states being prominently affected by electronic
correlations on the Pu sites. The k-resolved spectral density is calculated and
the theoretical spectral function resolved extended Van Hove singularity near
the Fermi energy. This singularity may lead to enchancement of the magnetic
susceptebility and favour d-wave superconductivity
Excitonic Instability and Pseudogap Formation in Nodal Line Semimetal ZrSiS
Electron correlation effects are studied in ZrSiS using a combination of
first-principles and model approaches. We show that basic electronic properties
of ZrSiS can be described within a two-dimensional lattice model of two nested
square lattices. High degree of electron-hole symmetry characteristic for ZrSiS
is one of the key features of this model. Having determined model parameters
from first-principles calculations, we then explicitly take electron-electron
interactions into account and show that at moderately low temperatures ZrSiS
exhibits excitonic instability, leading to the formation of a pseudogap in the
electronic spectrum. The results can be understood in terms of
Coulomb-interaction-assisted pairing of electrons and holes reminiscent to that
of an excitonic insulator. Our finding allows us to provide a physical
interpretation to the unusual mass enhancement of charge carriers in ZrSiS
recently observed experimentally.Comment: 6 pages, 4 figures. Final versio
Probing of valley polarization in graphene via optical second-harmonic generation
Valley polarization in graphene breaks inversion symmetry and therefore leads
to second-harmonic generation. We present a complete theory of this effect
within a single-particle approximation. It is shown that this may be a
sensitive tool to measure the valley polarization created, e.g., by polarized
light and, thus, can be used for a development of ultrafast valleytronics in
graphene.Comment: 5 pages, 3 figure
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
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
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