Quasi-particle spectra, absorption spectra, and excitonic properties of sodium iodide and strontium iodide from many-body perturbation theory

We investigate the basic quantum mechanical processes behind non-proportional response of scintillators to incident radiation responsible for reduced resolution. For this purpose, we conduct a comparative first principles study of quasiparticle spectra on the basis of the $G_0W_0$ approximation as well as absorption spectra and excitonic properties by solving the Bethe-Salpeter equation for two important systems, NaI and SrI$_2$. The former is a standard scintillator material with well-documented non-proportionality while the latter has recently been found to exhibit a very proportional response. We predict band gaps for NaI and SrI$_2$ of 5.5 and 5.2 eV, respectively, in good agreement with experiment. Furthermore, we obtain binding energies for the groundstate excitons of 216 meV for NaI and 195$\pm$25 meV for SrI$_2$. We analyze the degree of exciton anisotropy and spatial extent by means of a coarse-grained electron-hole pair-correlation function. Thereby, it is shown that the excitons in NaI differ strongly from those in SrI$_2$ in terms of structure and symmetry, even if their binding energies are similar. Furthermore, we show that quite unexpectedly the spatial extents of the highly anisotropic low-energy excitons in SrI$_2$ in fact exceed those in NaI by a factor of two to three in terms of the full width at half maxima of the electron-hole pair-correlation function.Comment: 10 pages, 9 figure

A first-principles study of helium storage in oxides and at oxide--iron interfaces

Density-functional theory calculations based on conventional as well as hybrid exchange-correlation functionals have been carried out to study the properties of helium in various oxides (Al2O3, TiO2, Y2O3, YAP, YAG, YAM, MgO, CaO, BaO, SrO) as well as at oxide-iron interfaces. Helium interstitials in bulk oxides are shown to be energetically more favorable than substitutional helium, yet helium binds to existing vacancies. The solubility of He in oxides is systematically higher than in iron and scales with the free volume at the interstitial site nearly independently of the chemical composition of the oxide. In most oxides He migration is significantly slower and He--He binding is much weaker than in iron. To quantify the solubility of helium at oxide-iron interfaces two prototypical systems are considered (Fe|MgO, Fe|FeO|MgO). In both cases the He solubility is markedly enhanced in the interface compared to either of the bulk phases. The results of the calculations allow to construct a schematic energy landscape for He interstitials in iron. The implications of these results are discussed in the context of helium sequestration in oxide dispersion strengthened steels, including the effects of interfaces and lattice strain.Comment: 13 pages, 10 figures, 4 table

Are there stable long-range ordered Fe(1-x)Cr(x) compounds?

The heat of formation of Fe-Cr alloys undergoes an anomalous change of sign at small Cr concentrations. This observation raises the question whether there are intermetallic phases present in this composition range. Here we report the discovery of several long-range ordered structures that represent ground state phases at zero Kelvin. In particular we have identified a structure at 3.7% Cr with an embedding energy which is 49 meV/Cr atom below the solid solution. This implies there is an effective long-range attractive interaction between Cr atoms. We propose that the structures found in this study complete the low temperature-low Cr region of the phase diagram.Comment: 3 pages, 2 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$_3$ 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) $\text{Sr}_\text{La}$ binds to $V_\text{Br}$ 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 $\text{Sr}_\text{La}-V_\text{La}$ 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

icet - A Python library for constructing and sampling alloy cluster expansions

Alloy cluster expansions (CEs) provide an accurate and computationally efficient mapping of the potential energy surface of multi-component systems that enables comprehensive sampling of the many-dimensional configuration space. Here, we introduce \textsc{icet}, a flexible, extensible, and computationally efficient software package for the construction and sampling of CEs. \textsc{icet} is largely written in Python for easy integration in comprehensive workflows, including first-principles calculations for the generation of reference data and machine learning libraries for training and validation. The package enables training using a variety of linear regression algorithms with and without regularization, Bayesian regression, feature selection, and cross-validation. It also provides complementary functionality for structure enumeration and mapping as well as data management and analysis. Potential applications are illustrated by two examples, including the computation of the phase diagram of a prototypical metallic alloy and the analysis of chemical ordering in an inorganic semiconductor.Comment: 10 page

Investigation of the chemical vicinity of crystal defects in ion-irradiated Mg and AZ31 with coincident Doppler broadening spectroscopy

Crystal defects in magnesium and magnesium based alloys like AZ31 are of major importance for the understanding of their macroscopic properties. We have investigated defects and their chemical surrounding in Mg and AZ31 on an atomic scale with Doppler broadening spectroscopy of the positron annihilation radiation. In these Doppler spectra the chemical information and the defect contribution have to be thoroughly separated. For this reason samples of annealed Mg were irradiated with Mg-ions in order to create exclusively defects. In addition Al- and Zn-ion irradiation on Mg-samples was performed in order to create samples with defects and impurity atoms. The ion irradiated area on the samples was investigated with laterally and depth resolved positron Doppler broadening spectroscopy (DBS) and compared with preceding SRIM-simulations of the vacancy distribution, which are in excellent agreement. The investigation of the chemical vicinity of crystal defects in AZ31 was performed with coincident Doppler broadening spectroscopy (CDBS) by comparing Mg-ion irradiated AZ31 with Mg-ion irradiated Mg. No formation of solute-vacancy complexes was found due to the ion irradiation, despite the high defect mobility.Comment: Submitted to Physical Review B on March 20 20076. Revised version submitted on September 28 2007. Accepted on October 19 200

Short-range order and precipitation in Fe-rich Fe-Cr alloys: Atomistic off-lattice Monte Carlo simulations

Short-range order (SRO) in Fe-rich Fe-Cr alloys is investigated by means of atomistic off-lattice Monte Carlo simulations in the semi-grand canonical ensemble using classical interatomic potentials. The SRO parameter defined by Cowley [Phys. Rev. B 77, 669 (1950)] is used to quantify the degree of ordering. In agreement with experiments a strong ordering tendency in the Cr distribution at low Cr concentrations (~< 5%) is observed, as manifested in negative values of the SRO parameters. For intermediate Cr concentrations (5% ~< c_Cr ~< 15%) the SRO parameter for the alpha-phase goes through a minimum, but at the solubility limit the alpha-phase still displays a rather strong SRO. In thermodynamic equilibrium for concentrations within the two-phase region the SRO parameter measured over the entire sample therefore comprises the contributions from both the alpha and alpha-prime phases. If both of these contributions are taken into account, it is possible to quantitatively reproduce the experimental results and interpret their physical implications. It is thereby shown that the inversion of the SRO observed experimentally is due to the formation of stable (supercritical) alpha-prime precipitates. It is not related to the loss of SRO in the alpha-phase or to the presence of unstable (subcritical) Cr precipitates in the alpha-phase.Comment: 9 pages, 8 figure

Inhomogeneity of the intrinsic magnetic field in superconducting YBa2Cu3OX compounds as revealed by rare-earth EPR-probe

X-band electron paramagnetic resonance on doped Er3+ and Yb3+ ions in Y0.99(Yb,Er)0.01Ba2Cu3OX compounds with different oxygen contents in the wide temperature range (4-120)K have been made. In the superconducting species, the strong dependencies of the linewidth and resonance line position from the sweep direction of the applied magnetic field are revealed at the temperatures significantly below TC. The possible origins of the observed hysteresis are analyzed. Applicability of the presented EPR approach to extract information about the dynamics of the flux-line lattice and critical state parameters (critical current density, magnetic penetration depth, and characteristic spatial scale of the inhomogeneity) is discussedComment: 17 pages, 5 Figures. Renewed versio

Contributions of point defects, chemical disorder, and thermal vibrations to electronic properties of Cd1-xZnxTe alloys

We present a first-principles study based on density functional theory of thermodynamic and electronic properties of the most important intrinsic defects in the semiconductor alloy Cd1-xZnxTe with x < 0.13. The alloy is represented by a set of supercells with disorder on the Cd/Zn sublattice. Defect formation energies as well as electronic and optical transition levels are analyzed as a function of composition. We show that defect formation energies increase with Zn content with the exception of the neutral Te vacancy. This behavior is qualitatively similar to but quantitatively rather different from the effect of volumetric strain on defect properties in pure CdTe. Finally, the relative carrier scattering strengths of point defects, alloy disorder, and phonons are obtained. It is demonstrated that for realistic defect concentrations, carrier mobilities are limited by phonon scattering for temperatures above approximately 150 K