63 research outputs found
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
Electronic stopping and proton dynamics in InP, GaP, and InGaP from first principles
The phosphide-based III-V semiconductors InP, GaP, and InGaP
are promising materials for solar panels in outer space and radioisotope
batteries, for which lifetime is a major issue. In order to understand high
radiation tolerance of these materials and improve it further, it is necessary
to describe the early stages of radiation damage on fast time and short length
scales. In particular, the influence of atomic ordering, as observed e.g. in
InGaP, on electronic stopping is unknown.We use real-time
time-dependent density functional theory and the adiabatic local density
approximation to simulate electronic stopping of protons in InP, GaP, and the
CuAu-I ordered phase of InGaP across a large kinetic energy
range.These results are compared to SRIM and we investigate the dependence on
the channel of the projectile through the target.We show that stopping can be
enhanced or reduced in InGaP and explain this using the
electron-density distribution. By comparing Ehrenfest and Born-Oppenheimer
molecular dynamics, we illustrate the intricate dynamics of a proton on a
channeling trajectory
Exciting Imperfection: Real-structure effects in magnesium-, cadmium-, and zinc-oxide
Computational condensed-matter physics, as a branch of modern solid-state physics, comprises the field of ab-initio calculations. Nowadays, challenging parameter-free studies can be carried out that deal with the many-body aspects due to the involved electron-electron interaction. Theoretical-spectroscopy techniques provide insight into electronic excitations, paving the way towards computer-aided materials design, e.g., for photovoltaics. In this thesis the transparent conductive oxides MgO, ZnO, and CdO are investigated; they are important materials for transparent-oxide electronics.
Initially, a hybrid functional is used to model exchange and correlation. Subsequently, quasiparticle energies are calculated using Hedin's GW approximation of the electronic self energy. This leads to band structures, densities of states, spin-orbit splittings, effective band masses, and natural band discontinuities. Solving a Bethe-Salpeter equation for the optical polarization function yields the complex dielectric function including excitonic and local-field effects. All underlying atomic geometries result from density-functional theory based on local or semi-local approximations to exchange and correlation.
Furthermore, this thesis deals with imperfections that affect the electronic and optical properties. The influence of uniaxial and biaxial strain on the band structure of ZnO is explored. Iso- and heterostructural alloys are investigated, taking the sample-preparation conditions into account via a cluster-expansion method. Exciton binding energies for the defect-related optical-absorption peaks are calculated for the oxygen vacancy in MgO. Effects due to free electrons in heavily doped ZnO are incorporated into the description of the optical properties. This allows to study the Mahan exciton as well as the inter-conduction-band absorption in this system and explains why a Mott transition of the exciton is hard to observe. The agreement with experimental results is reassuring
Effect of dynamical screening in the Bethe-Salpeter framework: Excitons in crystalline naphthalene
Solving the Bethe-Salpeter equation (BSE) for the optical polarization
functions is a first principles means to model optical properties of materials
including excitonic effects. One almost ubiquitously used approximation
neglects the frequency dependence of the screened electron-hole interaction.
This is commonly justified by the large difference in magnitude of electronic
plasma frequency and exciton binding energy. We incorporated dynamical effects
into the screening of the electron-hole interaction in the BSE using two
different approximations as well as exact diagonalization of the exciton
Hamiltonian. We compare these approaches for a naphthalene organic crystal, for
which the difference between exciton binding energy and plasma frequency is
only about a factor of ten. Our results show that in this case, corrections due
to dynamical screening are about 15\,\% of the exciton binding energy. We
analyze the effect of screening dynamics on optical absorption across the
visible spectral range and use our data to establish an \emph{effective}
screening model as a computationally efficient approach to approximate
dynamical effects in complex materials in the future.Comment: 11 pages main text, 5 figures main text, 9 pages supplemental, 6
figures supplementa
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