Electron-positron Car-Parrinello methods: Self-consistent treatment of charge densities and ionic relaxations

Computational study of positron annihilation parameters for cation mono-vacancies and vacancy complexes in nitride semiconductor alloys

We calculate positron annihilation parameters, namely the S and W parameters from the Doppler broadening spectroscopy and the positron lifetime [Formula: see text], for defect-free states as well as cation mono-vacancies and vacancy complexes in nitride semiconductor alloys Al_0.5Ga_0.5N, In_0.5Ga_0.5N and Al_0.5In_0.5N. The obtained distributions of these parameters differ from compound to compound. Especially, the S-W relation for In_0.5Ga_0.5N is very different from that for Al_0.5Ga_0.5N. For the cation mono-vacancies, introducing local structural parameters, their correlations with S, W and [Formula: see text] are investigated. The S and [Formula: see text] variations are well described with the size distributions of the vacancies while the W variation is related to the presence of localized d electrons. For the vacancy complexes as well as the cation mono-vacancies, multiple-linear-regression models to describe S, W and [Formula: see text] are successfully constructed using the local structural parameters as descriptors. The S-W and S-[Formula: see text] relations are also compared with those for AlN, GaN and InN.

Positron lifetimes of bare and hydrogenated zirconium vacancies in cubic yttria-stabilized zirconia: an ab initio study

Recent studies by positron-annihilation spectroscopy (PAS) in single-crystal and nanostructured yttria-stabilized zirconia (YSZ) revealed extensive positron trapping at vacancy-type point defects and other structural imperfections. The present work reports first principles calculations of formation energies and positron lifetimes of Zr vacancies in 10.3 mol% cubic YSZ. The lifetime calculations are based on two-component density-functional theory within the local-density approximation for the electron-positron correlation energy. Gradient-correction effects were also examined. Zr monovacancies were found to be potent trapping sites for positrons with large binding energies and lifetimes in excess of 220 ps. A strong dependency of the lifetimes on the Zr site and local coordination number was observed, a consequence of the structural disorder of the stabilized lattice. In contrast, oxygen vacancies did not act as attractive centers for positrons, either as single defects or as nearest neighbors to seven-fold coordinated Zr vacancies. Defect association of the Zr vacancies with hydrogen led to stable ([Formula: see text]-H) complexes with hydrogen bound to oxygen ions which are nearest neighbors to the vacancies. The negatively-charged ([Formula: see text]-H) defects were also found to trap positrons with corresponding lifetimes strongly dependent upon the hydrogen content within the complex.

Time-Dependent Multicomponent Density Functional Theory for Coupled Electron-Positron Dynamics

Electron-positron interactions have been utilized in various fields of science. Here we develop time-dependent multicomponent density functional theory to study the coupled electron-positron dynamics from first principles. We prove that there are coupled time-dependent single-particle equations that can provide the electron and positron density dynamics, and derive the formally exact expression for their effective potentials. Introducing the adiabatic local density approximation to time-dependent electron-positron correlation, we apply the theory to the dynamics of a positronic lithium hydride molecule under a laser field. We demonstrate the significance of the coupling between electronic and positronic motion by revealing the complex positron detachment mechanism and the suppression of electronic resonant excitation by the screening effect of the positron.

Evaluation of the concentration of point defects in GaN

Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linearly with the concentration of related defects only up to 1016 cm-3. At higher concentrations, the PL intensity associated with individual defects tends to saturate, and accordingly, does not directly correlate with the concentration of defects. For this reason, SIMS analysis, with relatively high detection limits, may not be helpful for classifying unidentified point defects in GaN. Additionally, we highlight challenges in correlating defects identified by PL with those by PAS and DLTS methods.

First-principles calculations of momentum distributions of annihilating electron-positron pairs in defects in UO2

We performed first-principles calculations of the momentum distributions of annihilating electron-positron pairs in vacancies in uranium dioxide. Full atomic relaxation effects (due to both electronic and positronic forces) were taken into account and self-consistent two-component density functional theory schemes were used. We present one-dimensional momentum distributions (Doppler-broadened annihilation radiation line shapes) along with line-shape parameters S and W. We studied the effect of the charge state of the defect on the Doppler spectra. The effect of krypton incorporation in the vacancy was also considered and it was shown that it should be possible to observe the fission gas incorporation in defects in UO₂ using positron annihilation spectroscopy. We suggest that the Doppler broadening measurements can be especially useful for studying impurities and dopants in UO₂ and of mixed actinide oxides.

Positron annihilation spectroscopy of vacancy-related defects in CdTe:Cl and CdZnTe:Ge at different stoichiometry deviations

Positron annihilation spectroscopy (PAS) was used to examine the effect of defined Cd-rich and Te-rich annealing on point defects in Cl-doped CdTe and Ge-doped CdZnTe semi-insulating single crystals. The as-grown crystals contain open-volume defects connected with Cd vacancies . It was found that the Cd vacancies agglomerate into clusters coupled with Cl in CdTe:Cl, and in CdZnTe:Ge they are coupled with Ge donors. While annealing in Cd pressure reduces of the density, subsequent annealing in Te pressure restores . The CdTe:Cl contains negatively-charged shallow traps interpreted as Rydberg states of A-centres and representing the major positron trapping sites at low temperature. Positrons confined in the shallow traps exhibit lifetime, which is shorter than the CdTe bulk lifetime. Interpretation of the PAS data was successfully combined with electrical resistivity, Hall effect measurements and chemical analysis, and allowed us to determine the principal point defect densities.

Self-energy correction to momentum-density distribution of positron-electron pairs

Positron two-dimensional angular correlation of annihilation radiation (2D ACAR), i.e., the 2D projection of the electron momentum densities sampled by positron, in Si is employed to verify the prediction of the density functional theory within the local-density approximation (LDA). Carefully conducted test shows that the LDA introduces small but definite discrepancies to the 2D-ACAR anisotropies. Self-energy calculation using the GW method indicates that density-fluctuation contributes anisotropic momentum-density correction and thus improves the agreement between theory and experiment. These results provide valuable annotations to the arguments concerning the accuracy and validity of the LDA and GW schemes.