746 research outputs found
Matter-positronium interaction: An exact diagonalization study of the He atom - positronium system
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 (Ps) 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.Comment: 18 pages, 8 figure
Influence of Ga/N ratio on morphology, vacancies, and electrical transport in GaN grown by molecular beam epitaxy at high temperature
The effect of Ga/N flux ratio on surface morphology, incorporation of point defects and electrical transport properties of GaN films grown by plasma-assisted molecular beam epitaxy in a recently developed high-temperature growth regime was investigated. The homoepitaxial (0001) GaN films grown at ∼780–790 °C showed smoothest morphologies near the cross-over between N-rich and Ga-rich growth(0.75<Ga/N<1.1) contrasting previous observations for low-temperature growth. The higher-quality growth near Ga/N∼1 resulted from lower thermal decomposition rates and was corroborated by slightly lower Ga vacancy concentrations [VGa], lower unintentional oxygen incorporation, and improved electron mobilities. The consistently low [VGa], i.e., ∼10exp16 cm−3 for all films attribute further to the significant benefits of the high-temperature growth regime.Peer reviewe
No niin -partikkeliketjun käyttö yhdistyskokouksen puheenjohtajan topiikinvaihtovuoroissa
Tiivistelmä. Käsittelen kandidaatintutkielmassani no niin -partikkeliketjua kokouskeskustelussa puheenjohtajan käyttämänä. Tarkastelen kyseistä partikkeliketjua keskustelunanalyyttisesti. Tarkoituksenani on selvittää, millaisissa ympäristöissä no niin -partikkeliketju esiintyy suhteessa topiikinvaihdoksiin, ja onko partikkeliketjulla jokin funktio. Olen käyttänyt tutkimuksen aineistona videoitua kokoustilannetta ja siitä luotua litteraattia. Videoaineisto ja litteraatti ovat valittu Kikosa-kokoelmasta.
Aineistosta löytyneet tapaukset liittyivät pääsääntöisesti topiikinvaihdoksiin, kun 6 tapauksesta 5 esiintyi topiikinvaihdoksissa hieman eri ympäristöissä. Joissakin tapauksissa no niin -partikkeliketju esiintyy topiikinvaihdosta signaloivissa vuoroissa, kun taas muissa tapauksissa no niin -partikkeliketju esiintyy välittömästi topiikin vaihtavassa vuorossa. Tapauksista 1 on sellainen, jonka ei voi täysin tunnistaa signaloivan topiikinvaihtoa. Tämä johtuu siitä, että no niin -partikkeliketjun sisältävä vuoro on kohdistettu vain yhdelle kokouksen jäsenelle
Spin dependent electron transport through a magnetic resonant tunneling diode
Electron transport properties in nanostructures can be modeled, for example,
by using the semiclassical Wigner formalism or the quantum mechanical Green's
functions formalism. We compare the performance and the results of these
methods in the case of magnetic resonant-tunneling diodes. We have implemented
the two methods within the self-consistent spin-density-functional theory. Our
numerical implementation of the Wigner formalism is based on the
finite-difference scheme whereas for the Green's function formalism the
finite-element method is used. As a specific application, we consider the
device studied by Slobodskyy et all. [Phys. Rev. Lett. 90, 246601 (2003)] and
analyze their experimental results. The Wigner and Green's functions formalisms
give similar electron densities and potentials but, surprisingly, the former
method requires much more computer resources in order to obtain numerically
accurate results for currents. Both of the formalisms can successfully be used
to model magnetic resonant tunneling diode structures.Comment: 13 pages and 12 figure
Influence of V/III molar ratio on the formation of In vacancies in InN grown by metal-organic vapor-phase epitaxy
We have applied a slow positron beam to study InN samples grown by metal-organic vapor-phase epitaxy with different V/III molar ratios (3300–24 000) and at different growth temperatures (550–625°C). Indium vacancies were identified in samples grown at V/III ratios below 4000. Their concentration is in the 10exp17cm−3 range. No strong dependence of vacancy concentration on the molar ratio was observed. At low V/III ratios, however, In droplets and vacancy clusters are formed near the substrate interface. The elevated growth temperature enhances the In vacancy formation, possibly due to limited sticking of In on the growth surface close to the decomposition temperature.Peer reviewe
Compensating vacancy defects in Sn- and Mg-doped In 2O3
MBE-grown Sn- and Mg-doped epitaxial In2O3 thin-film samples with varying doping concentrations have been measured using positron Doppler spectroscopy and compared to a bulk crystal reference. Samples were subjected to oxygen or vacuum annealing and the effect on vacancy type defects was studied. Results indicate that after oxygen annealing the samples are dominated by cation vacancies, the concentration of which changes with the amount of doping. In highly Sn-doped In2O3, however, these vacancies are not the main compensating acceptor. Vacuum annealing increases the size of vacancies in all samples, possibly by clustering them with oxygen vacancies.Peer reviewe
Direct observations of the vacancy and its annealing in germanium
Weakly n-type doped germanium has been irradiated with protons up to a fluence of 3×10 exp 14 cm exp −2 at 35 K and 100 K in a unique experimental setup. Positron annihilation measurements show a defect lifetime component of 272±4 ps at 35 K in in situ positron lifetime measurements after irradiation at 100 K. This is identified as the positron lifetime in a germanium monovacancy. Annealing experiments in the temperature interval 35–300 K reveal two annealing stages. The first at 100 K is tentatively associated with the annealing of the Frenkel pair, the second at 200 K with the annealing of the monovacancy. Above 200 K it is observed that mobile neutral monovacancies form divacancies, with a positron lifetime of 315 ps.Peer reviewe
In vacancies in InN grown by plasma-assisted molecular beam epitaxy
The authors have applied positron annihilation spectroscopy to study the
effect of different growth conditions on vacancy formation in In- and N-polar
InN grown by plasma-assisted molecular beam epitaxy. The results suggest that
the structural quality of the material and limited diffusion of surface adatoms
during growth dictate the In vacancy formation in low electron-density undoped
epitaxial InN, while growth conditions and thermodynamics have a less important
role, contrary to what is observed in, e.g., GaN. Further, the results imply
that in high quality InN, the electron mobility is likely limited not by
ionized point defect scattering, but rather by threading dislocations.Comment: 15 pages, 2 figure
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