We investigate the modeling of positronium (Ps) states and their pick-off annihilation trapped at open volumes pockets in condensed molecular matter. Our starting point is the interacting many-body system of Ps and a He atom because it is the smalles
t entity that can mimic the energy gap between the highest occupied and lowest unoccupied molecular orbitals of molecules and yet the the many-body structure of the HePs system can be calculated accurately enough. The exact-diagonalization solution of the HePs system enables us to construct a pair-wise full-correlation single-particle potential for the Ps-He interaction and the total potential in solids is obtained as a superposition of the pair-wise potentials. We study in detail Ps states and their pick-off annihilation rates in voids inside solid He and analyse experimental results for Ps-induced voids in liquid He obtaining the radii of the voids. More importantly, we generalize our conclusions by testing the validity of the Tao-Eldrup model, widely used to analyse ortho-Ps annihilation measurements for voids in molecular matter, against our theoretical results for the solid He. Moreover, we discuss the influence of the partial charges of polar molecules and the strength of the van der Waals interaction on the pick-off annihilation rate.
In this work we define single-particle potentials for a positron and a positronium atom interacting with light atoms (H, He, Li and Be) by inverting a single-particle Schrodinger equation. For this purpose, we use accurate energies and positron densi
ties obtained from the many-body wavefunction of the corresponding positronic systems. The introduced potentials describe the exact correlations for the calculated systems including the formation of a positronium atom. We show that the scattering lengths and the low-energy s-wave phase shifts from accurate many-body calculations are well accounted for by the introduced potential. We also calculate self-consistent two-component density-functional theory positron potentials and densities for the bound positronic systems within the local density approximation. They are in a very good agreement with the many-body results, provided that the finite-positron-density electron-positron correlation potential is used, and they can also describe systems comprising a positronium atom. We argue that the introduced single-particle positron potentials defined for single molecules are transferable to the condensed phase when the inter-molecular interactions are weak. When this condition is fulfilled, the total positron potential can be constructed in a good approximation as the superposition of the molecular potentials.
Acceptor-type defects in highly n-type InN are probed using positron annihilation spectroscopy. Results are compared to Hall effect measurements and calculated electron mobilities. Based on this, self-compensation in n-type InN is studied and the mic
roscopic origin of compensating and scattering centers in irradiated and Si-doped InN is discussed. We find significant compensation through negatively charged indium vacancy complexes as well as additional acceptor-type defects with no or small effective open volume, which act as scattering centers in highly n-type InN samples.
We present the studies of magnetic properties of 2 MeV 4He+-irradiated GaN grown by metal-organic chemical-vapor deposition. Particle irradiation allowed controllable introduction of Ga-vacancy in the samples. The magnetic moments with concentrations
changing between 4.3...8.3x10^17 cm^-3 showing superparamagnetic blocking at room temperature are observed. The appearance of clear hysteresis curve at T = 5 K with coercive field of about H_C = 270 Oe suggests that the formation of more complex Ga vacancy related defects is promoted with increasing Ga vacancy content. The small concentration of the observed magnetically-active defects with respect to the total Ga- vacancy concentration suggests that the presence of the oxygen/hydrogen-related vacancy complexes is the source of the observed magnetic moments.
The many-body system comprising a He nucleus, three electrons, and a positron has been studied using the exact diagonalization technique. The purpose has been to clarify to which extent the system can be considered as a distinguishable positronium (P
s) atom interacting with a He atom and, thereby, to pave the way to a practical atomistic modeling of Ps states and annihilation in matter. The maximum value of the distance between the positron and the nucleus is constrained and the Ps atom at different distances from the nucleus is identified from the electron and positron densities, as well as from the electron-positron distance and center-of-mass distributions. The polarization of the Ps atom increases as its distance from the nucleus decreases. A depletion of the He electron density, particularly large at low density values, has been observed. The ortho-Ps pick-off annihilation rate calculated as the overlap of the positron and the free He electron densities has to be corrected for the observed depletion, specially at large pores/voids.
