2,002 research outputs found
Dielectric screening in extended systems using the self-consistent Sternheimer equation and localized basis sets
We develop a first-principles computational method for investigating the
dielectric screening in extended systems using the self-consistent Sternheimer
equation and localized non-orthogonal basis sets. Our approach does not require
the explicit calculation of unoccupied electronic states, only uses two-center
integrals, and has a theoretical scaling of order O(N^3). We demonstrate this
method by comparing our calculations for silicon, germanium, diamond, and LiCl
with reference planewaves calculations. We show that accuracy comparable to
planewaves calculations can be achieved via a systematic optimization of the
basis set.Comment: 6 pages, 3 figure
Simulation of neutrino and charged particle production and propagation in the atmosphere
A precise evaluation of the secondary particle production and propagation in
the atmosphere is very important for the atmospheric neutrino oscillation
studies. The issue is addressed with the extension of a previously developed
full 3-Dimensional Monte-Carlo simulation of particle generation and transport
in the atmosphere, to compute the flux of secondary protons, muons and
neutrinos. Recent balloon borne experiments have performed a set of accurate
flux measurements for different particle species at different altitudes in the
atmosphere, which can be used to test the calculations for the atmospheric
neutrino production, and constrain the underlying hadronic models. The
simulation results are reported and compared with the latest flux measurements.
It is shown that the level of precision reached by these experiments could be
used to constrain the nuclear models used in the simulation. The implication of
these results for the atmospheric neutrino flux calculation are discussed.Comment: 11 pages, 9 figure
Influence of pressure and temperature on key physicochemical properties of corn stover-derived biochar
This study focuses on analyzing the effect of both the peak temperature and pressure on the properties of biochar produced through slow pyrolysis of corn stover, which is a common agricultural waste that currently has little or no value. The pyrolysis experiments were carried out in a fixed-bed reactor at different peak temperatures (400, 525 and 650 °C) and absolute pressures (0.1, 0.85 and 1.6 MPa). The inert mass flow rate (at NTP conditions) was adjusted in each test to keep the gas residence time constant within the reactor. The as-received corn stover was pyrolyzed into a biochar without any physical pre-treatment as a way to reduce the operating costs. The properties of biochars showed that high peak temperature led to high fixed-carbon contents, high aromaticity and low molar H:C and O:C ratios; whereas a high pressure only resulted in a further decrease in the O:C ratio and a further increase in the fixed-carbon content. Increasing the operating pressure also resulted in a higher production of pyrolysis gas at the expense of water formation
Image states in metal clusters
The existence of image states in small clusters is shown, using a quantum-mechanical many-body approach. We present image state energies and wave functions for spherical jellium clusters up to 186 atoms, calculated in the GW approximation, where G is the Green's function and W is the dynamically screened Coulomb interaction, which by construction contains the dynamic long-range correlation effects that give rise to image effects. In addition, we find that image states are also subject to quantum confinement. To extrapolate our investigations to clusters in the mesoscopic size range, we propose a semiclassical model potential, which we test against our full GW results
Approximations for many-body Green's functions: insights from the fundamental equations
Several widely used methods for the calculation of band structures and photo
emission spectra, such as the GW approximation, rely on Many-Body Perturbation
Theory. They can be obtained by iterating a set of functional differential
equations relating the one-particle Green's function to its functional
derivative with respect to an external perturbing potential. In the present
work we apply a linear response expansion in order to obtain insights in
various approximations for Green's functions calculations. The expansion leads
to an effective screening, while keeping the effects of the interaction to all
orders. In order to study various aspects of the resulting equations we
discretize them, and retain only one point in space, spin, and time for all
variables. Within this one-point model we obtain an explicit solution for the
Green's function, which allows us to explore the structure of the general
family of solutions, and to determine the specific solution that corresponds to
the physical one. Moreover we analyze the performances of established
approaches like over the whole range of interaction strength, and we
explore alternative approximations. Finally we link certain approximations for
the exact solution to the corresponding manipulations for the differential
equation which produce them. This link is crucial in view of a generalization
of our findings to the real (multidimensional functional) case where only the
differential equation is known.Comment: 17 pages, 7 figure
Energy levels in polarization superlattices: a comparison of continuum strain models
A theoretical model for the energy levels in polarization superlattices is
presented. The model includes the effect of strain on the local
polarization-induced electric fields and the subsequent effect on the energy
levels. Two continuum strain models are contrasted. One is the standard strain
model derived from Hooke's law that is typically used to calculate energy
levels in polarization superlattices and quantum wells. The other is a
fully-coupled strain model derived from the thermodynamic equation of state for
piezoelectric materials. The latter is more complete and applicable to strongly
piezoelectric materials where corrections to the standard model are
significant. The underlying theory has been applied to AlGaN/GaN superlattices
and quantum wells. It is found that the fully-coupled strain model yields very
different electric fields from the standard model. The calculated intersubband
transition energies are shifted by approximately 5 -- 19 meV, depending on the
structure. Thus from a device standpoint, the effect of applying the
fully-coupled model produces a very measurable shift in the peak wavelength.
This result has implications for the design of AlGaN/GaN optical switches.Comment: Revtex
Precise Tight-binding Description of the Band Structure of MgB2
We present a careful recasting of first-principles band structure
calculations for MgB2 in a non-orthogonal sp-tight-binding (TB) basis. Our TB
results almost exactly reproduce our full potential linearized augmented plane
wave results for the energy bands, the densities of states and the total
energies. Our procedure generates transferable Slater-Koster parameters which
should be useful for other studies of this important material.Comment: REVTEX, 2 Encapsulated PostScript Figure
Anomalous Quasiparticle Lifetime in Graphite: Band Structure Effects
We report ab initio calculation of quasiparticle lifetimes in graphite, as
determined from the imaginary part of the self-energy operator within the GW
aproximation. The inverse lifetime in the energy range from 0.5 to 3.5 eV above
the Fermi level presents significant deviations from the quadratic behavior
naively expected from Fermi liquid theory. The deviations are explained in
terms of the unique features of the band structure of this material. We also
discuss the experimental results from different groups and make some
predictions for future experiments.Comment: 4 pages, 4 figures, submitted PR
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