1,234 research outputs found
Effect of long-range structural corrugations on magnetotransport properties of phosphorene in tilted magnetic field
Rippling is an inherent quality of two-dimensional materials playing an
important role in determining their properties. Here, we study the effect of
structural corrugations on the electronic and transport properties of monolayer
black phosphorus (phosphorene) in the presence of tilted magnetic field. We
follow a perturbative approach to obtain analytical corrections to the spectrum
of Landau levels induced by a long-wavelength corrugation potential. We show
that surface corrugations have a non-negligible effect on the electronic
spectrum of phosphorene in tilted magnetic field. Particularly, the Landau
levels are shown to exhibit deviations from the linear field dependence. The
observed effect become especially pronounced at large tilt angles and
corrugation amplitudes. Magnetotransport properties are further examined in the
low temperature regime taking into account impurity scattering. We calculate
magnetic field dependence of the longitudinal and Hall resistivities and find
that the nonlinear effects reflecting the corrugation might be observed even in
moderate fields (\mbox{ T})
An ab-initio study of the electron-phonon coupling within a Cr(001)-surface
It is experimentally well established that the Cr(001)-surface exhibits a
sharp resonance around the Fermi level. However, there is no consensus about
its physical origin. It is proposed to be either due to a single particle dz2
surface state renormalised by electron-phonon coupling or the orbital Kondo
effect involving the degenerate dxz/dyz states. In this work we examine the
electron-phonon coupling of the Cr(001)-surface by means of ab-initio
calculations in the form of density functional perturbation theory. More
precisely, the electron-phonon mass-enhancement factor of the surface layer is
investigated for the 3d states. For the majority and minority spin dz2 surface
states we find values of 0.19 and 0.16. We show that these calculated
electron-phonon mass-enhancement factors are not in agreement with the
experimental data even if we use realistic values for the temperature range and
surface Debye frequency for the fit of the experimental data. More precisely,
then experimentally an electron-phonon mass-enhancement factor of 0.70~0.10 is
obtained, which is not in agreement with our calculated values of 0.19 and
0.16. Therefore, we conclude that the experimentally observed resonance at the
Cr(001)-surface is not due to polaronic effects, but due to electron-electron
correlation effects
Toward a realistic description of multilayer black phosphorus: from approximation to large-scale tight-binding simulations
We provide a tight-binding model parametrization for black phosphorus (BP)
with an arbitrary number of layers. The model is derived from partially
self-consistent approach, where the screened Coulomb interaction
is calculated within the random phase approximation on the basis of density
functional theory. We thoroughly validate the model by performing a series of
benchmark calculations, and determine the limits of its applicability. The
application of the model to the calculations of electronic and optical
properties of multilayer BP demonstrates good quantitative agreement with
\emph{ab initio} results in a wide energy range. We also show that the proposed
model can be easily extended for the case of external fields, yielding the
results consistent with those obtained from first principles. The model is
expected to be suitable for a variety of realistic problems related to the
electronic properties of multilayer BP including different kinds of disorder,
external fields, and many-body effects.Comment: 10 pages, 9 figures, 2 tables (final version, minor changes
Exchange interactions in transition metal oxides: The role of oxygen spin polarization
Magnetism of transition metal (TM) oxides is usually described in terms of
the Heisenberg model, with orientation-independent interactions between the
spins. However, the applicability of such a model is not fully justified for TM
oxides because spin polarization of oxygen is usually ignored. In the
conventional model based on the Anderson principle, oxygen effects are
considered as a property of the TM ion and only TM interactions are relevant.
Here, we perform a systematic comparison between two approaches for spin
polarization on oxygen in typical TM oxides. To this end, we calculate the
exchange interactions in NiO, MnO, and hematite (Fe2O3) for different magnetic
configurations using the magnetic force theorem. We consider the full spin
Hamiltonian including oxygen sites, and also derive an effective model where
the spin polarization on oxygen renormalizes the exchange interactions between
TM sites. Surprisingly, the exchange interactions in NiO depend on the magnetic
state if spin polarization on oxygen is neglected, resulting in non-Heisenberg
behavior. In contrast, the inclusion of spin polarization in NiO makes the
Heisenberg model more applicable. Just the opposite, MnO behaves as a
Heisenberg magnet when oxygen spin polarization is neglected, but shows strong
non-Heisenberg effects when spin polarization on oxygen is included. In
hematite, both models result in non-Heisenberg behavior. General applicability
of the magnetic force theorem as well as the Heisenberg model to TM oxides is
discussed.Comment: 19 pages, 2 figure
Polaronic effects in monolayer black phosphorus on polar substrates
We investigate the effect of charge carrier interaction with surface optical
phonons on the band properties of monolayer black phosphorus induced by polar
substrates. We develop an analytical method based on the Lee-Low-Pines theory
to calculate the spectrum of Fr\"ohlich type continuum Hamiltonian in the
long-wavelength limit. We examine the modification of a band gap and
renormalization of effective masses due to the substrate-related polaronic
effect. Our results show that an energy gap in supported monolayer black
phosphorus is enlarged depending on a particular substrate and the interlayer
distance, . Among the substrate considered, the largest gap broadening at \AA{} is observed for the AlO substrate, which is found to
be meV. Carrier-phonon coupling also renormalizes the effective
masses which is more pronounced along the zigzag direction. Anisotropy of the
effective masses becomes stronger by the influence of the polaronic effect
corresponding to direction-dependent carrier-phonon coupling. We conclude that
substrate phonons have a non-negligible effect on the static band properties of
monolayer black phosphorus, which may be further exploited in its experimental
and theoretical studies
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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