The evidence of quasi-free positronium state in GiPS-AMOC spectra of glycerol

We present the results of processing of Age-Momentum Correlation (AMOC) spectra that were measured for glycerol by the Gamma-induced positron spectroscopy (GiPS) facility. Our research has shown that the shape of experimental s(t) curve cannot be explained without introduction of the intermediate state of positronium (Ps), called quasi-free Ps. This state yields the wide Doppler line near zero lifetimes. We discuss the possible properties of this intermediate Ps state from the viewpoint of developed model. The amount of annihilation events produced by quasi-free Ps is estimated to be less than 5% of total annihilations. In the proposed model, quasi-free Ps serves as a precursor for trapped Ps of para- and ortho-states

Vacancy complexes in nonequilibrium germanium-tin semiconductors

Understanding the nature and behavior of vacancy-like defects in epitaxial GeSn metastable alloys is crucial to elucidate the structural and optoelectronic properties of these emerging semiconductors. The formation of vacancies and their complexes is expected to be promoted by the relatively low substrate temperature required for the epitaxial growth of GeSn layers with Sn contents significantly above the equilibrium solubility of 1 at.%. These defects can impact both the microstructure and charge carrier lifetime. Herein, to identify the vacancy-related complexes and probe their evolution as a function of Sn content, depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to investigate GeSn epitaxial layers with Sn content in the 6.5-13.0 at.% range. The samples were grown by chemical vapor deposition method at temperatures between 300 and 330 {\deg}C. Regardless of the Sn content, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, which confirmed the presence of open volume defects. The measured average positron lifetimes were the highest (380-395 ps) in the region near the surface and monotonically decrease across the analyzed thickness, but remain above 350 ps. All GeSn layers exhibit lifetimes that are 85 to 110 ps higher than the Ge reference layers. Surprisingly, these lifetimes were found to decrease as Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were found to also contain a small fraction of vacancy clusters, which become less important as Sn content increases

Vacancy cluster in ZnO films grown by pulsed laser deposition

© 2019, The Author(s). Undoped and Ga-doped ZnO films were grown on c-sapphire using pulsed laser deposition (PLD) at the substrate temperature of 600 °C. Positron annihilation spectroscopy study (PAS) shows that the dominant V Zn -related defect in the as-grown undoped ZnO grown with relative low oxygen pressure P(O 2 ) is a vacancy cluster (most likely a V Zn -nV O complex with n = 2, 3) rather than the isolated V Zn which has a lower formation energy. Annealing these samples at 900 °C induces out-diffusion of Zn from the ZnO film into the sapphire creating the V Zn  at the film/sapphire interface, which favors the formation of vacancy cluster containing relatively more V Zn . Increasing the P(O 2 ) during growth also lead to the formation of the vacancy cluster with relatively more V Zn . For Ga-doped ZnO films, the oxygen pressure during growth has significant influence on the electron concentration and the microstructure of the V Zn -related defect. Green luminescence (GL) and yellow luminescence (YL) were identified in the cathodoluminescence study (CL) study, and both emission bands were quenched after hydrogen plasma treatment. The origin of the GL is discussed

Nuclear deformation and neutron excess as competing effects for pygmy dipole strength

The electromagnetic dipole strength below the neutron-separation energy has been studied for the xenon isotopes with mass numbers A = 124, 128, 132, and 134 in nuclear resonance fluorescence experiments using the ELBE bremsstrahlung facility at Helmholtz-Zentrum Dresden-Rossendorf and the HIgS facility at Triangle Universities Nuclear Laboratory Durham. The systematic study gained new information about the influence of the neutron excess as well as of nuclear deformation on the strength in the region of the pygmy dipole resonance. The results are compared with those obtained for the chain of molybdenum isotopes and with predictions of a random-phase approximation in a deformed basis. It turned out that the effect of nuclear deformation plays a minor role compared with the one caused by neutron excess. A global parametrization of the strength in terms of neutron and proton numbers allowed us to derive a formula capable of predicting the summed E1 strengths in the pygmy region for a wide mass range of nuclides.Comment: 5 pages, subimtted to Physical Review Letter

The impact of Mn nonstoichiometry on the oxygen mass transport properties of La0.8Sr0.2MnyO3±δ thin films

Oxygen mass transport in perovskite oxides is relevant for a variety of energy and information technologies. In oxide thin films, cation nonstoichiometry is often found but its impact on the oxygen transport properties is not well understood. Here, we used oxygen isotope exchange depth profile technique coupled with secondary ion mass spectrometry to study oxygen mass transport and the defect compensation mechanism of Mn-deficient La0.8Sr0.2Mn (y) O-3 +/-delta epitaxial thin films. Oxygen diffusivity and surface exchange coefficients were observed to be consistent with literature measurements and to be independent on the degree of Mn deficiency in the layers. Defect chemistry modeling, together with a collection of different experimental techniques, suggests that the Mn-deficiency is mainly compensated by the formation of La-x(Mn) antisite defects. The results highlight the importance of antisite defects in perovskite thin films for mitigating cationic nonstoichiometry effects on oxygen mass transport properties

Electric and magnetic dipole strength in Zn 66

The dipole strength of the nuclide Zn66 was studied in photon-scattering experiments using bremsstrahlung produced with electron beams of energies of 7.5 and 13.4 MeV at the γELBE facility as well as using quasimonoenergetic and linearly polarized photon beams of 30 energies within the range of 4.3 to 9.9 MeV at the HIγS facility. A total of 128 J=1 states were identified, among them 9 with 1+ and 86 with 1- assignments. The quasicontinuum of unresolved transitions was included in the analysis of the spectra and the intensities of branching transitions were estimated on the basis of simulations of statistical γ-ray cascades. As a result, the photoabsorption cross section up to the neutron-separation energy was determined and compared with predictions of the statistical reaction model. The experimental M1 strengths from resolved 1+ states are compared with results of large-scale shell-model calculations

In Situ Heating Positron Annihilation Lifetime Spectroscopy Experiments on an Al Mg Alloy

The binding between vacancies and Mg atoms in an aluminum solid solution is not fully understood but essential for understanding its role in age hardening of many Al alloys. After annealing and quenching, Mg prevents the loss of excess vacancies during natural ageing and forms complexes containing one, possibly two, vacancies, and various Mg atoms. By heating the alloy after natural ageing, these complexes are dissolved, i.e., natural ageing is reverted. This reversion process is studied by in amp; 8201;situ positron annihilation lifetime spectroscopy utilizing the very high count rate at the accelerator driven facility ELBE. Positron spectra are continuously acquired during heating at rates between 3 and 50 amp; 8201;K amp; 8201;min amp; 8722;1. After correcting for the contributions of the oxidized surface and decomposing spectra into components, the process can be followed in detail and is found to take place in distinct stages first, the number of vacancy Mg complexes is reduced and then the liberated vacancies agglomerate into clusters that eventually dissolve at even higher temperature

Vacancy-Hydrogen Dynamics in Samples during Low Temperature Baking

The recent discovery of a modified low temperature baking process established an increased accelerating gradient of TESLA shaped cavities through reduction of surface losses. A possible explanation for the performance gain is the suppression of lossy nanohydrides via defect trapping, with vacancy-hydrogen(v+nH) complexes forming at the lower temperatures. Utilizing Doppler broadening Positron Annihilation Spectroscopy, Positron Annihilation Lifetime Spectroscopy and Nuclear Reaction Analysis, samples made from European XFEL niobium sheets and cavity cut-outs were investigated. The evolution of vacancies, hydrogen and their interaction at different temperature levels have been studied during in-situ and ex-situ annealing and in-situ cooldowns. Measurements of niobium samples and a correlation between RF, material properties, and v+nH distribution in cavity cut-outs have been carried out