12,088 research outputs found
Assessing the performance of the Random Phase Approximation for exchange and superexchange coupling constants in magnetic crystalline solids
The Random Phase Approximation (RPA) for total energies has previously been
shown to provide a qualitatively correct description of static correlation in
molecular systems, where density functional theory (DFT) with local functionals
are bound to fail. This immediately poses the question of whether the RPA is
also able to capture the correct physics of strongly correlated solids such as
Mott insulators. Due to strong electron localization, magnetic interactions in
such systems are dominated by superexchange, which in the simplest picture can
be regarded as the analogue of static correlation for molecules. In the present
work we investigate the performance of the RPA for evaluating both
superexchange and direct exchange interactions in the magnetic solids NiO, MnO,
Na3Cu2SbO6, Sr2CuO3, Sr2CuTeO6, and a monolayer of CrI3, which are chosen to
represent a broad variety of magnetic interactions. It is found that the RPA
can accurately correct the large errors introduced by Hartree-Fock -
independent of the input orbitals used for the perturbative expansion. However,
in most cases, accuracies similar to RPA can be obtained with DFT+U, which is
significantly simpler from a computational point of view.Comment: 9 page
Calculating critical temperatures for ferromagnetic order in two-dimensional materials
Magnetic order in two-dimensional (2D) materials is intimately coupled to
magnetic anisotropy (MA) since the Mermin-Wagner theorem implies that
rotational symmetry cannot be spontaneously broken at finite temperatures in
2D. Large MA thus comprises a key ingredient in the search for magnetic 2D
materials that retains the magnetic order above room temperature. Magnetic
interactions are typically modeled in terms of Heisenberg models and the
temperature dependence on magnetic properties can be obtained with the Random
Phase Approximation (RPA), which treats magnon interactions at the mean-field
level. In the present work we show that large MA gives rise to strong
magnon-magnon interactions that leads to a drastic failure of the RPA. We then
demonstrate that classical Monte Carlo (MC) simulations correctly describe the
critical temperatures in the large MA limit and agree with RPA when the MA
becomes small. A fit of the MC results leads to a simple expression for the
critical temperatures as a function of MA and exchange coupling constants,
which significantly simplifies the theoretical search for new 2D magnetic
materials with high critical temperatures. The expression is tested on a
monolayer of CrI, which were recently observed to exhibit ferromagnetic
order below 45 K and we find excellent agreement with the experimental value.Comment: 8 pages, 6 figure
Valley Hall effect in disordered monolayer MoS2 from first principles
Electrons in certain two-dimensional crystals possess a pseudospin degree of
freedom associated with the existence of two inequivalent valleys in the
Brillouin zone. If, as in monolayer MoS2, inversion symmetry is broken and
time-reversal symmetry is present, equal and opposite amounts of k-space Berry
curvature accumulate in each of the two valleys. This is conveniently
quantified by the integral of the Berry curvature over a single valley - the
valley Hall conductivity. We generalize this definition to include
contributions from disorder described with the supercell approach, by mapping
("unfolding") the Berry curvature from the folded Brillouin zone of the
disordered supercell onto the normal Brillouin zone of the pristine crystal,
and then averaging over several realizations of disorder. We use this scheme to
study from first-principles the effect of sulfur vacancies on the valley Hall
conductivity of monolayer MoS2. In dirty samples the intrinsic valley Hall
conductivity receives gating-dependent corrections that are only weakly
dependent on the impurity concentration, consistent with side-jump scattering
and the unfolded Berry curvature can be interpreted as a k-space resolved
side-jump. At low impurity concentrations skew scattering dominates, leading to
a divergent valley Hall conductivity in the clean limit. The implications for
the recently-observed photoinduced anomalous Hall effect are discussed.Comment: 13 page
Vibrationally Mediated Control of Single Electron Transmission in Weakly Coupled Molecule-Metal Junctions
We propose a mechanism which allows one to control the transmission of single
electrons through a molecular junction. The principle utilizes the emergence of
transmission sidebands when molecular vibrational modes are coupled to the
electronic state mediating the transmission. We will show that if a
molecule-metal junction is biased just below a molecular resonance one may
induce the transmission of a single electron by externally exciting a
vibrational mode of the molecule. The analysis is quite general but requires
that the molecular orbital does not hybridize strongly with the metallic
states. As an example we perform a density functional theory (DFT) analysis of
a benzene molecule between two Au(111) contacts and show that exciting a
particular vibrational mode can give rise to transmission of a single electro
Origin of power laws for reactions at metal surfaces mediated by hot electrons
A wide range of experiments have established that certain chemical reactions
at metal surfaces can be driven by multiple hot electron mediated excitations
of adsorbates. A high transient density of hot electrons is obtained by means
of femtosecond laser pulses and a characteristic feature of such experiments is
the emergence of a power law dependence of the reaction yield on the laser
fluence . We propose a model of multiple inelastic scattering by hot
electrons, which reproduces this power law and the experimentally found
exponents of several experiments. All parameters are calculated within Density
Functional Theory and the Delta Self-Consistent Field method. With a simplified
assumption, the power law becomes exact and we obtain a simple and very useful
physical interpretation of the exponent , which represents the number of
adsorbate vibrational states participating in the reaction
Memory effects in non-adiabatic molecular dynamics at metal surfaces
We study the effect of temporal correlation in a Langevin equation describing
non-adiabatic dynamics at metal surfaces. For a harmonic oscillator the
Langevin equation preserves the quantum dynamics exactly and it is demonstrated
that memory effects are needed in order to conserve the ground state energy of
the oscillator. We then compare the result of Langevin dynamics in a harmonic
potential with a perturbative master equation approach and show that the
Langevin equation gives a better description in the non-perturbative range of
high temperatures and large friction. Unlike the master equation, this approach
is readily extended to anharmonic potentials. Using density functional theory
we calculate representative Langevin trajectories for associative desorption of
N from Ru(0001) and find that memory effects lowers the dissipation of
energy. Finally, we propose an ab-initio scheme to calculate the temporal
correlation function and dynamical friction within density functional theory
Vole spatial distribution and dispersal in European organic and conventional farming systems
North European landscapes are highly dominated by agriculture, where small biotopes, e.g. meadows, uncultivated grassland, hedge rows, field boundaries, surroundings of water ponds, only comprise a low percentage. In recent years organic farming has expanded in acreage due to customers increased awareness regarding pesticide and fertilizer use and biodiversity conservation. However, organic farming has changed from an extensive production with small fields, low mechanical impact and high crop diversity towards larger fields, intensive mechanical treatment, lower weed densities and lower field diversity. Still, organic farms could play an important, role in the agricultural landscape as refuges for some small mammal species.
We studied the responses of populations to habitat patches of different size and different surrounding management strategies (ecological and conventional farming). Studies were performed at two localities in Denmark, Kalø Estate in Eastern Jutland and the Bjerringbro area in Central Jutland. The sampling sites were represented by cultivated grassland habitat, small biotopes within cultivated fields and hedgerows between fields in rotation.
Small mammal species assemblages were low in numbers in cultural farmland, and, on a property basis, not significantly different between organic and conventional farms. Very few species and individuals were present in the field matrix, and the small biotopes were by far the most important source of species richness. Species density was positively correlated with the size of the habitat, and, generally, more voles were found in organic habitat patches than in conventional ones. More field voles were found in organic grassland and more bank voles (Myodes glareolus) in organic hedge rows than in conventional ones. Telemetry studies of field voles showed low rates of dispersal and low colonization rates of the more or less isolated small biotopes at the time of year with no vegetation cover in the surrounding fields. We found no significant correlations between distance to nearest stepping stones/dispersal corridors and small mammal densities or species composition. In agricultural areas landscape structure influences the small mammal species living in this fragmented habitat matrix. The value of organic farms in respect to small mammal biodiversity depends mainly upon the number and area of small biotopes, and only to a minor degree upon the management of the fields. This is presumably related to a more dense and diverse vegetation cover, due to a lack of pesticide and fertilizer treatment in the organically managed small biotopes
Extending the random-phase approximation for electronic correlation energies: The renormalized adiabatic local density approximation
The adiabatic connection fluctuation-dissipation theorem with the random
phase approximation (RPA) has recently been applied with success to obtain
correlation energies of a variety of chemical and solid state systems. The main
merit of this approach is the improved description of dispersive forces while
chemical bond strengths and absolute correlation energies are systematically
underestimated. In this work we extend the RPA by including a parameter-free
renormalized version of the adiabatic local density (ALDA) exchange-correlation
kernel. The renormalization consists of a (local) truncation of the ALDA kernel
for wave vectors , which is found to yield excellent results for the
homogeneous electron gas. In addition, the kernel significantly improves both
the absolute correlation energies and atomization energies of small molecules
over RPA and ALDA. The renormalization can be straightforwardly applied to
other adiabatic local kernels.Comment: 5 page
Static correlation beyond the random phase approximation: Dissociating H2 with the Bethe-Salpeter equation and time-dependent GW
We investigate various approximations to the correlation energy of a H
molecule in the dissociation limit, where the ground state is poorly described
by a single Slater determinant. The correlation energies are derived from the
density response function and it is shown that response functions derived from
Hedin's equations (Random Phase Approximation (RPA), Time-dependent
Hartree-Fock (TDHF), Bethe-Salpeter equation (BSE), and Time-Dependent GW
(TDGW)) all reproduce the correct dissociation limit. We also show that the BSE
improves the correlation energies obtained within RPA and TDHF significantly
for intermediate binding distances. A Hubbard model for the dimer allow us to
obtain exact analytical results for the various approximations, which is
readily compared with the exact diagonalization of the model. Moreover, the
model is shown to reproduce all the qualitative results from the \textit{ab
initio} calculations and confirms that BSE greatly improves the RPA and TDHF
results despite the fact that the BSE excitation spectrum breaks down in the
dissociation limit. In contrast, Second Order Screened Exchange (SOSEX) gives a
poor description of the dissociation limit, which can be attributed to the fact
that it cannot be derived from an irreducible response function
Hot electron mediated desorption rates calculated from excited state potential energy surfaces
We present a model for Desorption Induce by (Multiple) Electronic Transitions
(DIET/DIMET) based on potential energy surfaces calculated with the Delta
Self-Consistent Field extension of Density Functional Theory. We calculate
potential energy surfaces of CO and NO molecules adsorbed on various transition
metal surfaces, and show that classical nuclear dynamics does not suffice for
propagation in the excited state. We present a simple Hamiltonian describing
the system, with parameters obtained from the excited state potential energy
surface, and show that this model can describe desorption dynamics in both the
DIET and DIMET regime, and reproduce the power law behavior observed
experimentally. We observe that the internal stretch degree of freedom in the
molecules is crucial for the energy transfer between the hot electrons and the
molecule when the coupling to the surface is strong.Comment: Typos corrected. Comment on thermal ensemble Green function added in
appendix
- …