277 research outputs found
Electronic band gap reduction and intense luminescence in Co and Mn ion-implanted SiO
Cobalt and manganese ions are implanted into SiO over a wide range of
concentrations. For low concentrations, the Co atoms occupy interstitial
locations, coordinated with oxygen, while metallic Co clusters form at higher
implantation concentrations. For all concentrations studied here, Mn ions
remain in interstitial locations and do not cluster. Using resonant x-ray
emission spectroscopy and Anderson impurity model calculations, we determine
the strength of the covalent interaction between the interstitial ions and the
SiO valence band, finding it comparable to Mn and Co monoxides. Further, we
find an increasing reduction in the SiO electronic band gap for increasing
implantation concentration, due primarily to the introduction of Mn- and
Co-derived conduction band states. We also observe a strong increase in a band
of x-ray stimulated luminescence at 2.75 eV after implantation, attributed to
oxygen deficient centers formed during implantation.Comment: 8 pages, 6 figure
Neutron - Mirror Neutron Oscillations: How Fast Might They Be?
We discuss the phenomenological implications of the neutron (n) oscillation
into the mirror neutron (n'), a hypothetical particle exactly degenerate in
mass with the neutron but sterile to normal matter. We show that the present
experimental data allow a maximal n-n' oscillation in vacuum with a
characteristic time much shorter than the neutron lifetime, in fact as
small as 1 sec. This phenomenon may manifest in neutron disappearance and
regeneration experiments perfectly accessible to present experimental
capabilities and may also have interesting astrophysical consequences, in
particular for the propagation of ultra high energy cosmic rays.Comment: 4 pages, 1 figure; revtex; matches paper published by P.R.
CPT-violating neutrino oscillations
We propose a simple phenomenological model predicting, through Lorentz
symmetry breaking, a CPT-violating asymmetry between particle and antiparticle
states in neutrino oscillations involving sterile neutrinos. Such a model is
able to explain the apparently observed anomalous excess of low-energy
\nu_e-like events, reported by the MiniBooNE collaboration, as well as the
non-observation of the corresponding anomalous excess of \ov{\nu}_e-like
events. The present model leads to very specific physical predictions in the
neutrino oscillations scenario, and account for the observed anomalies in terms
of only one CPT-Lorentz violation parameter of the order of the
Grand-Unification energy scale.Comment: revtex, 4 pages, no figure
Stability of boron-doped graphene/copper interface: DFT, XPS and OSEE studies
Two different types of boron-doped graphene/copper interfaces synthesized
using two different flow rates of Ar through the bubbler containing the boron
source were studied. X-ray photoelectron spectra (XPS) and optically stimulated
electron emission (OSEE) measurements have demonstrated that boron-doped
graphene coating provides a high corrosion resistivity of Cu-substrate with the
light traces of the oxidation of carbon cover. The density functional theory
calculations suggest that for the case of substitutional (graphitic)
boron-defect only the oxidation near boron impurity is energetically favorable
and creation of the vacancies that can induce the oxidation of copper substrate
is energetically unfavorable. In the case of non-graphitic boron defects
oxidation of the area, a nearby impurity is metastable that not only prevent
oxidation but makes boron-doped graphene. Modeling of oxygen reduction reaction
demonstrates high catalytic performance of these materials.Comment: 15 pages, 8 figures, to appear in Appl. Surf. Sc
Enhanced clustering tendency of Cu-impurities with a number of oxygen vacancies in heavy carbon-loaded TiO2 - the bulk and surface morphologies
The over threshold carbon-loadings (~50 at.%) of initial TiO2-hosts and
posterior Cu-sensitization (~7 at.%) was made using pulsed ion-implantation
technique in sequential mode with 1 hour vacuum-idle cycle between sequential
stages of embedding. The final Cx-TiO2:Cu samples were qualified using XPS
wide-scan elemental analysis, core-levels and valence band mappings. The
results obtained were discussed on the theoretic background employing
DFT-calculations. The combined XPS and DFT analysis allows to establish and
prove the final formula of the synthesized samples as Cx-TiO2:[Cu+][Cu2+] for
the bulk and Cx-TiO2:[Cu+][Cu0] for thin-films. It was demonstrated the in the
mode of heavy carbon-loadings the remaining majority of neutral C-C bonds
(sp3-type) is dominating and only a lack of embedded carbon is fabricating the
O-C=O clusters. No valence base-band width altering was established after
sequential carbon-copper modification of the atomic structure of initial
TiO2-hosts except the dominating majority of Cu 3s states after
Cu-sensitization. The crucial role of neutral carbon low-dimensional impurities
as the precursors for the new phases growth was shown for Cu-sensitized Cx-TiO2
intermediate-state hosts.Comment: 27 pages, 7 figures, accepted to Solid State Science
Octahedral conversion of a-SiO2-host matrix by pulsed ion implantation
This is the abstract. The results of measurements of X-ray photoelectron
spectra (XPS) of a-SiO2-host material after pulsed implantation with [Mn+] and
[Co+, Mn+]-ions as well as DFT-calculations are presented. The low-energy shift
is found in XPS Si 2p and O 1s core-levels of single [Mn+] and dual [Co+, Mn+]
pulsed ion-implanted a-SiO2 (E = 30 keV, D = 2*10^17 cm^-2) with respect to
those of untreated a-SiO2.The similar changes are found in XPS Si 2p and O 1s
of stishovite compared to those of quartz. This means that the pulsed
ion-implantation induces the local high pressure effect which leads to an
appearance of SiO6-structural units in alpha-SiO2 host, forming
"stishovite-like" local atomic structure. This process can be described within
electronic bonding transition from the four-fold "quartz-like" to six-fold
"stishovite-like" high-pressure phase in SiO2 host-matrix. It is found that
such octahedral conversion depends on the fluence and starts with doses higher
than D = 3*10^16 cm^-2.Comment: 15 pages, 6 figures, 1 table, accepted in phys. stat. solidi (b
Relativistic Magnetic Monopole Flux Constraints from RICE
We report an upper limit on the flux of relativistic monopoles based on the
non-observation of in-ice showers by the Radio Ice Cherenkov Experiment (RICE)
at the South Pole. We obtain a 95% C.L. limit of order 10^{-18}/(cm^2-s-sr) for
intermediate mass monopoles of 10^7<gamma<10^{12} at the anticipated energy
E=10^{16} GeV. This bound is over an order of magnitude stronger than all
previously published experimental limits for this range of boost parameters
gamma, and exceeds two orders of magnitude improvement over most of the range.
We review the physics of radio detection, describe a Monte Carlo simulation
including continuous and stochastic energy losses, and compare to previous
experimental limits.Comment: 16 pages, 6 figures. Accepted for publication in Phys. Rev. D. Minor
revisions, including expanded discussion of monopole energy uncertaint
Ultra high energy cosmic rays: clustering, GUT scale and neutrino masses
The clustering of ultra high energy (above 5\cdot 10^{19} eV) cosmic rays
(UHECR) suggests that they might be emitted by compact sources. We present a
statistical analysis on the source density based on the multiplicities. The
propagation of UHECR protons is studied in detail. The UHECR spectrum is
consistent with the decay of GUT scale particles and/or with the Z-burst. The
predicted GUT mass is m_X=10^b GeV, where b=14.6_{-1.7}^{+1.6}. Our neutrino
mass prediction depends on the origin of the power part of the spectrum:
m_\nu=2.75^{+1.28}_{-0.97} eV for halo and 0.26^{+0.20}_{-0.14} eV for
extragalactic (EG) origin.Comment: 4 pages, 5 figures. Talk given at TAUP 2001, September 8-12, 2001 in
Gran Sass
A possible origin of superconducting currents in cosmic strings
The scattering and capture of right-handed neutrinos by an Abelian cosmic
string in the SO(10) grand unification model are considered. The scattering
cross-section of neutrinos per unit length due to the interaction with the
gauge and Higgs fields of the string is much larger in its scaling regime than
in the friction one because of the larger infrared cutoff of the former.The
probability of capture in a zero mode of the string accompanied by the emission
of a gauge or Higgs boson shows a resonant peak for neutrino momentum of the
order of its mass. Considering the decrease of number of strings per unit
comoving volume in the scaling epoch the cosmological consequences of the
superconducting strings formed in this regime will be much smaller than those
which could be produced already in the friction one.Comment: 14 pages Latex, 4 figues/ep
Electronic structure and photoluminescence properties of Zn-ion implanted silica glass before and after thermal annealing
The results of XPS core-level and valence band measurements, photoluminescence spectra of a-SiO2 implanted by Zn-ions (E = 30 keV, D = 1 · 1017 cm− 2) and Density Functional Theory calculations of electronic structure as well as formation energies of structural defects in silica glass induced by Zn-ion implantation are presented. Both theory and experiment show that it is energetically more favorable for implanted zinc ions to occupy the interstitial positions instead of cation substitution. As a result, the Zn-ions embedded to interstitials, form chemical bonds with the surrounding oxygen atoms, fabrication ZnO-like nanoparticles and oxygen-deficient SiOx matrix. The subsequent thermal annealing at 900 °C (1 h) strongly reduces the amount of ZnO nanoparticles and induces the formation of secondary α-Zn2SiO4 phase which markedly enhances the green emission. © 201
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