116 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
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
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
Fidelity and coherence measures from interference
By utilizing single particle interferometry, the fidelity or coherence of a pair of quantum states is identified with their capacity for interference. We consider processes acting on the internal degree of freedom (e.g., spin or polarization) of the interfering particle, preparing it in states ρA or ρB in the respective path of the interferometer. The maximal visibility depends on the choice of interferometer, as well as the locality or nonlocality of the preparations, but otherwise depends only on the states ρA and ρB and not the individual preparation processes themselves. This allows us to define interferometric measures which probe locality and correlation properties of spatially or temporally separated processes, and can be used to differentiate between processes that cannot be distinguished by direct process tomography using only the internal state of the particle
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
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