2,143 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
Nonlinear acoustic waves in channels with variable cross sections
The point symmetry group is studied for the generalized Webster-type equation
describing non-linear acoustic waves in lossy channels with variable cross
sections. It is shown that, for certain types of cross section profiles, the
admitted symmetry group is extended and the invariant solutions corresponding
to these profiles are obtained. Approximate analytic solutions to the
generalized Webster equation are derived for channels with smoothly varying
cross sections and arbitrary initial conditions.Comment: Revtex4, 10 pages, 2 figure. This is an enlarged contribution to
Acoustical Physics, 2012, v.58, No.3, p.269-276 with modest stylistic
corrections introduced mainly in the Introduction and References. Several
typos were also correcte
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
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
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