37 research outputs found
A variational polaron self-interaction corrected total-energy functional for charge excitations in wide-band gap insulators
We conduct a detailed investigation of the polaron self-interaction (pSI)
error in standard approximations to the exchange-correlation (XC) functional
within density-functional theory (DFT). The pSI leads to delocalization error
in the polaron wave function and energy, as calculated from the Kohn-Sham (KS)
potential in the native charge state of the polaron. This constitutes the
origin of the systematic failure of DFT to describe polaron formation in band
insulators. It is shown that the delocalization error in these systems is,
however, largely absent in the KS potential of the closed-shell neutral charge
state. This leads to a modification of the DFT total-energy functional that
corrects the pSI in the XC functional. The resulting pSIC-DFT method
constitutes an accurate parameter-free {\it ab initio} methodology for
calculating polaron properties in insulators at a computational cost that is
orders of magnitude smaller than hybrid XC functionals. Unlike approaches that
rely on parametrized localized potentials such as DFT+, the pSIC-DFT method
properly captures both site and bond-centered polaron configurations. This is
demonstrated by studying formation and migration of self-trapped holes in
alkali halides (bond-centered) as well as self-trapped electrons in an
elpasolite compound (site-centered). The pSIC-DFT approach consistently
reproduces the results obtained by hybrid XC functionals parametrized by
DFT+ calculations. Finally, we generalize the pSIC approach to hybrid
functionals, and show that in stark contrast to conventional hybrid
calculations of polaron energies, the pSIC-hybrid method is insensitive to the
parametrization of the hybrid XC functional. On this basis, we further
rationalize the success of the pSIC-DFT approach.Comment: 10 pages, 7 figure
A first-principles study of co-doping in lanthanum bromide
Co-doping of Ce-doped LaBr with Ba, Ca, or Sr improves the energy
resolution that can be achieved by radiation detectors based on these
materials. Here, we present a mechanism that rationalizes of this enhancement
that on the basis of first principles electronic structure calculations and
point defect thermodynamics. It is shown that incorporation of Sr creates
neutral -Sr complexes that can temporarily trap
electrons. As a result, Auger quenching of free carriers is reduced, allowing
for a more linear, albeit slower, scintillation light yield response.
Experimental Stokes shifts can be related to different
Ce-Sr- triple complex configurations.
Co-doping with other alkaline as well as alkaline earth metals is considered as
well. Alkaline elements are found to have extremely small solubilities on the
order of 0.1 ppm and below at 1000 K. Among the alkaline earth metals the
lighter dopant atoms prefer interstitial-like positions and create strong
scattering centers, which has a detrimental impact on carrier mobilities. Only
the heavier alkaline earth elements combine matching ionic radii with
sufficiently high solubilities. This provides a rationale for the experimental
finding that improved scintillator performance is exclusively achieved using
Sr, Ca, or Ba. The present mechanism demonstrates that co-doping of wide gap
materials can provide an efficient means for managing charge carrier
populations under out-of-equilibrium conditions. In the present case dopants
are introduced that manipulate not only the concentrations but the electronic
properties of intrinsic defects without introducing additional gap levels. This
leads to the availability of shallow electron traps that can temporarily
localize charge carriers, effectively deactivating carrier-carrier
recombination channels. The principles of this ... [continued]Comment: 13 pages, 10 figures, accepted for publication in the Physical Review
Efficacy of the DFT+U formalism for modeling hole polarons in perovskite oxides
We investigate the formation of self-trapped holes (STH) in three
prototypical perovskites (SrTiO3, BaTiO3, PbTiO3) using a combination of
density functional theory (DFT) calculations with local potentials and hybrid
functionals. First we construct a local correction potential for polaronic
configurations in SrTiO3 that is applied via the DFT+U method and matches the
forces from hybrid calculations. We then use the DFT+U potential to search the
configuration space and locate the lowest energy STH configuration. It is
demonstrated that both the DFT+U potential and the hybrid functional yield a
piece-wise linear dependence of the total energy on the occupation of the STH
level suggesting that self-interaction effects have been properly removed. The
DFT+U model is found to be transferable to BaTiO3 and PbTiO3, and formation
energies from DFT+U and hybrid calculations are in close agreement for all
three materials. STH formation is found to be energetically favorable in SrTiO3
and BaTiO3 but not in PbTiO3, which can be rationalized by considering the
alignment of the valence band edges on an absolute energy scale. In the case of
PbTiO3 the strong coupling between Pb 6s and O 2p states lifts the valence band
minimum (VBM) compared to SrTiO3 and BaTiO3. This reduces the separation
between VBM and STH level and renders the STH configuration metastable with
respect to delocalization (band hole state). We expect that the present
approach can be adapted to study STH formation also oxides with different
crystal structures and chemical composition.Comment: 7 pages, 6 figure
Thermodynamic and mechanical properties of copper precipitates in alpha-iron from atomistic simulations
The Fe-Cu system has attracted much attention over the last several decades
due to its technological importance as a model alloy for Cu steels. In spite of
these efforts several aspects of its phase diagram remain unexplained. Here we
use atomistic simulations to characterize the polymorphic phase diagram of Cu
precipitates in body-centered cubic (BCC) Fe and establish a consistent link
between their thermodynamic and mechanical properties in terms of thermal
stability, shape, and strength. The size at which Cu precipitates transform
from BCC to a close-packed 9R structure is found to be strongly temperature
dependent, ranging from approximately 4 nm in diameter (~2,700 atoms) at 200 K
to about 8 nm (~22,800 atoms) at 700 K. These numbers are in very good
agreement with the interpretation of experimental data given Monzen et al.
[Phil. Mag. A 80, 711 (2000)]. The strong temperature dependence originates
from the entropic stabilization of BCC Cu, which is mechanically unstable as a
bulk phase. While at high temperatures the transition exhibits first-order
characteristics, the hysteresis, and thus the nucleation barrier, vanish at
temperatures below approximately 300\,K. This behavior is explained in terms of
the mutual cancellation of the energy differences between core and shell
(wetting layer) regions of BCC and 9R nanoprecipitates, respectively. The
proposed mechanism is not specific for the Fe--Cu system but could generally be
observed in immiscible systems, whenever the minority component is unstable in
the lattice structure of the host matrix. Finally, we also study the
interaction of precipitates with screw dislocations as a function of both
structure and orientation. The results provide a coherent picture of
precipitate strength that unifies previous calculations and experimental
observations.Comment: 15 pages, 15 figure
The Urbach tail in silica glass from first principles
We present density-functional theory calculations of the optical absorption
spectra of silica glass for temperatures up to 2400 K. The calculated spectra
exhibit exponential tails near the fundamental absorption edge that follow the
Urbach rule, in good agreement with experiments. We also discuss the accuracy
of our results by comparing to hybrid exchange correlation functionals. By
deriving a simple relationship between the exponential tails of the absorption
coefficient and the electronic density-of-states, we establish a direct link
between the photoemission and the absorption spectra near the absorption edge.
This relationship is subsequently employed to determine the lower bound to the
Urbach frequency regime. Most interestingly, in this frequency interval, the
optical absorption is Poisson distributed with very large statistical
fluctuations. Finally, We determine the upper bound to the Urbach frequency
regime by identifying the frequency at which transition to Poisson distribution
takes place.Comment: 5 pages, 3 figure
A scalable parallel Monte Carlo algorithm for atomistic simulations of precipitation in alloys
We present an extension of the semi-grandcanonical (SGC) ensemble that we
refer to as the variance-constrained semi-grandcanonical (VC-SGC) ensemble. It
allows for transmutation Monte Carlo simulations of multicomponent systems in
multiphase regions of the phase diagram and lends itself to scalable
simulations on massively parallel platforms. By combining transmutation moves
with molecular dynamics steps structural relaxations and thermal vibrations in
realistic alloys can be taken into account. In this way, we construct a robust
and efficient simulation technique that is ideally suited for large-scale
simulations of precipitation in multicomponent systems in the presence of
structural disorder. To illustrate the algorithm introduced in this work, we
study the precipitation of Cu in nanocrystalline Fe.Comment: 12 pages; 10 figure
Origin of resolution enhancement by co-doping of scintillators: Insight from electronic structure calculations
It was recently shown that the energy resolution of Ce-doped LaBr
scintillator radiation detectors can be crucially improved by co-doping with
Sr, Ca, or Ba. Here we outline a mechanism for this enhancement on the basis of
electronic structure calculations. We show that (i) Br vacancies are the
primary electron traps during the initial stage of thermalization of hot
carriers, prior to hole capture by Ce dopants; (ii) isolated Br vacancies are
associated with deep levels; (iii) Sr doping increases the Br vacancy
concentration by several orders of magnitude; (iv) binds
to resulting in a stable neutral complex; and (v) association
with Sr causes the deep vacancy level to move toward the conduction band edge.
The latter is essential for reducing the effective carrier density available
for Auger quenching during thermalization of hot carriers. Subsequent
de-trapping of electrons from complexes then
can activate Ce dopants that have previously captured a hole leading to
luminescence. This mechanism implies an overall reduction of Auger quenching of
free carriers, which is expected to improve the linearity of the photon light
yield with respect to the energy of incident electron or photon