1,722 research outputs found
Defect distribution in a-plane GaN on Al2O3
The authors studied the structural and point defect distributions of hydride vapor phase epitaxial GaN film grown in the [11−20] a direction on (1−102) r-plane sapphire with metal-organic vapor phase deposited a-GaN template using transmission electron microscopy, secondary ion mass spectrometry, and positron annihilation spectroscopy. Grown-in extended and point defects show constant behavior as a function of thickness, contrary to the strong nonuniform defect distribution observed in GaN grown along the [0001] direction. The observed differences are explained by orientation-dependent and kinetics related defect incorporation.Peer reviewe
Chemical Self-Enrichment of HII Regions by the Wolf-Rayet Phase of an 85 Msun star
It is clear from stellar evolution and from observations of WR stars that
massive stars are releasing metal-enriched gas through their stellar winds in
the Wolf-Rayet phase. Although HII region spectra serve as diagnostics to
determine the present-day chemical composition of the interstellar medium, it
is far from being understood to what extent the HII gas is already contaminated
by chemically processed stellar wind. Therefore, we analyzed our models of
radiative and wind bubbles of an isolated 85 Msun star with solar metallicity
(Kr\"oger et al. 2006) with respect to the chemical enrichment of the
circumstellar HII region. Plausibly, the hot stellar wind bubble (SWB) is
enriched with 14N during the WN phase and even much higher with 12C and 16O
during the WC phase of the star. During the short period that the 85 Msun star
spends in the WC stage enriched SWB material mixes with warm HII gas of solar
abundances and thus enhances the metallicity in the HII region. However, at the
end of the stellar lifetime the mass ratios of the traced elements N and O in
the warm ionized gas are insignificantly higher than solar, whereas an
enrichment of 22 % above solar is found for C. Important issues from the
presented study comprise a steeper radial gradient of C than O and a decreasing
effect of self-enrichment for metal-poor galaxies.Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Adsorption structure of glycine on TiO2(1 1 0): a photoelectron diffraction determination
High-resolution core-level photoemission and scanned-energy mode photoelectron diffraction (PhD) of the O 1s and N 1s states have been used to investigate the interaction of glycine with the rutile TiO2(1 1 0) surface. Whilst there is clear evidence for the presence of the zwitterion View the MathML sourceCH2COO− with multilayer deposition, at low coverage only the deprotonated glycinate species, NH2CH2COO is present. Multiple-scattering simulations of the O 1s PhD data show the glycinate is bonded to the surface through the two carboxylate O atoms which occupy near-atop sites above the five-fold-coordinated surface Ti atoms, with a Ti–O bondlength of 2.12 ± 0.06 Å. Atomic hydrogen arising from the deprotonation is coadsorbed to form hydroxyl species at the bridging oxygen sites with an associated Ti–O bondlength of 2.01 ± 0.03 Å. Absence of any significant PhD modulations of the N 1s emission is consistent with the amino N atom not being involved in the surface bonding, unlike the case of glycinate on Cu(1 1 0) and Cu(1 0 0)
Role of elastic scattering in electron dynamics at ordered alkali overlayers on Cu(111)
Scanning tunneling spectroscopy of p(2x2) Cs and Na ordered overlayers on
Cu(111) reveals similar line widths of quasi two-dimensional quantum well
states despite largely different binding energies. Detailed calculations show
that 50% of the line widths are due to electron-phonon scattering while
inelastic electron-electron scattering is negligible. A frequently ignored
mechanism for ordered structures, i.e., enhanced elastic scattering due to
Brillouin zone back folding, contributes the remaining width.Comment: 4 pages, 2 figures, 1 tabl
Tuning the properties of complex transparent conducting oxides: role of crystal symmetry, chemical composition and carrier generation
The electronic properties of single- and multi-cation transparent conducting
oxides (TCOs) are investigated using first-principles density functional
approach. A detailed comparison of the electronic band structure of
stoichiometric and oxygen deficient InO, - and
-GaO, rock salt and wurtzite ZnO, and layered InGaZnO
reveals the role of the following factors which govern the transport and
optical properties of these TCO materials: (i) the crystal symmetry of the
oxides, including both the oxygen coordination and the long-range structural
anisotropy; (ii) the electronic configuration of the cation(s), specifically,
the type of orbital(s) -- , or -- which form the conduction band;
and (iii) the strength of the hybridization between the cation's states and the
p-states of the neighboring oxygen atoms. The results not only explain the
experimentally observed trends in the electrical conductivity in the
single-cation TCO, but also demonstrate that multicomponent oxides may offer a
way to overcome the electron localization bottleneck which limits the charge
transport in wide-bandgap main-group metal oxides. Further, the advantages of
aliovalent substitutional doping -- an alternative route to generate carriers
in a TCO host -- are outlined based on the electronic band structure
calculations of Sn, Ga, Ti and Zr-doped InGaZnO. We show that the
transition metal dopants offer a possibility to improve conductivity without
compromising the optical transmittance
Pattern formation from consistent dynamical closures of uniaxial nematic liquid crystals
Pattern formation in uniaxial polymeric liquid crystals is studied for
different dynamic closure approximations. Using the principles of mesoscopic
non-equilibrium thermodynamics in a mean-field approach, we derive a
Fokker-Planck equation for the single-particle non-homogeneous distribution
function of particle orientations and the evolution equations for the second
and fourth order orientational tensor parameters. Afterwards, two dynamic
closure approximations are discussed, one of them considering the relaxation of
the fourth order orientational parameter and leading to a novel expression for
the free-energy like function in terms of the scalar order parameter.
Considering the evolution equation of the density of the system and values of
the interaction parameter for which isotropic and nematic phases coexist, our
analysis predicts that patterns and traveling waves can be produced in
lyotropic uniaxial nematics even in the absence of external driving.Comment: 34 pages, 7 figure
Improved lattice QCD with quarks: the 2 dimensional case
QCD in two dimensions is investigated using the improved fermionic lattice
Hamiltonian proposed by Luo, Chen, Xu, and Jiang. We show that the improved
theory leads to a significant reduction of the finite lattice spacing errors.
The quark condensate and the mass of lightest quark and anti-quark bound state
in the strong coupling phase (different from t'Hooft phase) are computed. We
find agreement between our results and the analytical ones in the continuum.Comment: LaTeX file (including text + 10 figures
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