Theory of positrons in solids and on solid surfaces

Puska, Martti J.; Nieminen, Risto M.

doi:10.1103/revmodphys.66.841

Cited by 1,024 publications

(717 citation statements)

References 257 publications

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“…5 Here we also demonstrate the effect of the PAW transformation on the PS wave functions by showing also results calculated using only the PS wave functions of the PAW method. In the calculation of the electronic structures we employ the LDA exchange-correlation energy.…”

Section: A Testing the Paw Methodsmentioning

confidence: 99%

Modeling the momentum distributions of annihilating electron-positron pairs in solids

2006

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Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material. Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement ͑electron-positron correlation͒ schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancytype defect on the ionic relaxations.

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Section: A Testing the Paw Methodsmentioning

confidence: 99%

Modeling the momentum distributions of annihilating electron-positron pairs in solids

2006

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“…Here, we do not take into account that a part of the positron beam is backscattered from the target and thus does not contribute to the signal. Typically, this fraction lies in the order of 10% for a 1-keV positron beam and seems to decrease with decreasing beam energy [16]. For method 2, the beam was collected for 30 s and the count rate in the 511 keV annihilation peak was recorded.…”

Section: Beam Intensitymentioning

confidence: 99%

Characterization of the NEPOMUC primary and remoderated positron beams at different energies

Stanja

Hergenhahn

Niemann

et al. 2016

Nuclear Instruments and Methods in Physics Research Section A:

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We report on the characterization of the positron beam provided at the open beam port of the NEPOMUC facility at the Heinz Maier-Leibnitz Zentrum (MLZ) Garching. The absolute positron flux of the primary beam at 400 eV and 1 keV kinetic energy and of the remoderated beam at 5, 12 and 22 eV were determined. Energy-dependent intensities in the range of (1-5)·10 8 e + /s and (2-6)·10 7 e + /s have been observed for the primary and remoderated beam, respectively. We attribute the significant losses for the primary beam, in comparison with the expected value, to the non-adiabatic positron guiding in the beam line. We also measured the longitudinal energy distribution of the remoderated beam, yielding an energy spread below 3.3 eV. The mean transverse energy of the remoderated beam, determined from measurements in different final magnetic fields, was found to be below 1.3 eV. These results are likely to apply to the NEPOMUC beam delivered to other user stations.

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“…11 The trapping and annihilation processes are described by a set of kinetic equations. They relate the rate of change of density of positrons in a determined state to the trapping, detrapping, and annihilation rates at all the states of the positron.…”

Section: Positron Trapping At Defectsmentioning

confidence: 99%

Positron annihilation lifetime spectroscopy of ZnO bulk samples

et al. 2007

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In order to gain a further insight into the knowledge of point defects of ZnO, positron annihilation lifetime spectroscopy was performed on bulk samples annealed under different atmospheres. The samples were characterized at temperatures ranging from 10 to 500 K. Due to difficulties in the conventional fitting of the lifetime spectra caused by the low intensity of the defect signals, we have used an alternative method as a solution to overcome these difficulties and resolve all the lifetime components present in the spectra. Two different vacancy-type defects are identified in the samples: Zn vacancy complexes ͑V Zn -X͒ and vacancy clusters consisting of up to five missing Zn-O pairs. In addition to the vacancies, we observe negative-ion-type defects, which are tentatively attributed to intrinsic defects in the Zn sublattice. The effect of the annealing on the observed defects is discussed. The concentrations of the V Zn -X complexes and negative-ion-type defects are in the 0.2-2 ppm range, while the cluster concentrations are 1-2 orders of magnitude lower.

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Theory of positrons in solids and on solid surfaces

Cited by 1,024 publications

References 257 publications

Modeling the momentum distributions of annihilating electron-positron pairs in solids

Modeling the momentum distributions of annihilating electron-positron pairs in solids

Characterization of the NEPOMUC primary and remoderated positron beams at different energies

Positron annihilation lifetime spectroscopy of ZnO bulk samples

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