703 research outputs found
Titania/alumina bilayer gate insulators for InGaAs metal-oxide-semiconductor devices
We describe the electrical properties of atomic layer deposited TiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> bilayer gate oxides which simultaneously achieve high gate capacitance density and low gate leakage current density. Crystallization of the initially amorphous TiO<sub>2</sub> film contributes to a significant accumulation capacitance increase (âŒ33%) observed after a forming gas anneal at 400â°C. The bilayer dielectrics reduce gate leakage current density by approximately one order of magnitude at flatband compared to Al<sub>2</sub>O<sub>3</sub> single layer of comparable capacitance equivalent thickness. The conduction band offset of TiO<sub>2</sub> relative to InGaAs is 0.6âeV, contributing to the ability of the stacked dielectric to suppress gate leakage conduction
Thermal conductivity of ions in a neutron star envelope
We analyze the thermal conductivity of ions (equivalent to the conductivity
of phonons in crystalline matter) in a neutron star envelope.
We calculate the ion/phonon thermal conductivity in a crystal of atomic
nuclei using variational formalism and performing momentum-space integration by
Monte Carlo method. We take into account phonon-phonon and phonon-electron
scattering mechanisms and show that phonon-electron scattering dominates at not
too low densities. We extract the ion thermal conductivity in ion liquid or gas
from literature.
Numerical values of the ion/phonon conductivity are approximated by
analytical expressions, valid for T>10^5 K and 10^5 g cm^-3 < \rho < 10^14 g
cm^-3. Typical magnetic fields B~10^12 G in neutron star envelopes do not
affect this conductivity although they strongly reduce the electron thermal
conductivity across the magnetic field. The ion thermal conductivity remains
much smaller than the electron conductivity along the magnetic field. However,
in the outer neutron star envelope it can be larger than the electron
conductivity across the field, that is important for heat transport across
magnetic field lines in cooling neutron stars. The ion conductivity can greatly
reduce the anisotropy of heat conduction in outer envelopes of magnetized
neutron stars.Comment: 12 pages, 5 figures; to appear in MNRA
Emission Spectra of Fallback Disks Around Young Neutron Stars
The nature of the energy source powering anomalous X-ray pulsars is
uncertain. Proposed scenarios involve either an ultramagnetized neutron star,
or accretion onto a neutron star. We consider the accretion model proposed
recently by Chatterjee, Hernquist & Narayan, in which a disk is fed by fallback
material following a supernova. We compute the optical, infrared, and
submillimeter emission expected from such a disk, including both viscous
dissipation and reradiation of X-ray flux impinging on the disk from the
pulsar. We find that it is possible with current instruments to put serious
constraints on this and on other accretion models of AXPs. Fallback disks could
also be found around isolated radio pulsars and we compute the corresponding
spectra. We show that the excess emission in the R and I bands observed for the
pulsar PSR 0656+14 is broadly consistent with emission from a disk.Comment: 12 pages, 1 table, 4 figures, submitted to Ap
Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
We survey the current situation regarding chemical modelling of the synthesis
of molecules in the interstellar medium. The present state of knowledge
concerning the rate coefficients and their uncertainties for the major
gas-phase processes -- ion-neutral reactions, neutral-neutral reactions,
radiative association, and dissociative recombination -- is reviewed. Emphasis
is placed on those reactions that have been identified, by sensitivity
analyses, as 'crucial' in determining the predicted abundances of the species
observed in the interstellar medium. These sensitivity analyses have been
carried out for gas-phase models of three representative, molecule-rich,
astronomical sources: the cold dense molecular clouds TMC-1 and L134N, and the
expanding circumstellar envelope IRC +10216. Our review has led to the proposal
of new values and uncertainties for the rate coefficients of many of the key
reactions. The impact of these new data on the predicted abundances in TMC-1
and L134N is reported. Interstellar dust particles also influence the observed
abundances of molecules in the interstellar medium. Their role is included in
gas-grain, as distinct from gas-phase only, models. We review the methods for
incorporating both accretion onto, and reactions on, the surfaces of grains in
such models, as well as describing some recent experimental efforts to simulate
and examine relevant processes in the laboratory. These efforts include
experiments on the surface-catalysed recombination of hydrogen atoms, on
chemical processing on and in the ices that are known to exist on the surface
of interstellar grains, and on desorption processes, which may enable species
formed on grains to return to the gas-phase.Comment: Accepted for publication in Space Science Review
Spins, Electromagnetic Moments, and Isomers of 107-129Cd
The neutron-rich isotopes of cadmium up to the N=82 shell closure have been
investigated by high-resolution laser spectroscopy. Deep-UV excitation at 214.5
nm and radioactive-beam bunching provided the required experimental
sensitivity. Long-lived isomers are observed in 127Cd and 129Cd for the first
time. One essential feature of the spherical shell model is unambiguously
confirmed by a linear increase of the 11/2- quadrupole moments. Remarkably,
this mechanism is found to act well beyond the h11/2 shell
A hierarchical model for aging
We present a one dimensional model for diffusion on a hierarchical tree
structure. It is shown that this model exhibits aging phenomena although no
disorder is present. The origin of aging in this model is therefore the
hierarchical structure of phase space.Comment: 10 pages LaTeX, 4 postscript-figures include
Drifting subpulses and inner acceleration regions in radio pulsars
The classical vacuum gap model of Ruderman & Sutherland, in which
spark-associated subbeams of subpulse emission circulate around the magnetic
axis due to the EB drift, provides a natural and plausible physical mechanism
of the subpulse drift phenomenon. Recent progress in the analysis of drifting
subpulses in pulsars has provided a strong support to this model by revealing a
number of subbeams circulating around the magnetic axis in a manner compatible
with theoretical predictions. However, a more detailed analysis revealed that
the circulation speed in a pure vacuum gap is too high when compared with
observations. Moreover, some pulsars demonstrate significant time variations of
the drift rate, including a change of the apparent drift direction, which is
obviously inconsistent with the EB drift scenario in a pure vacuum gap. We
resolved these discrepancies by considering a partial flow of iron ions from
the positively charged polar cap, coexisting with the production of outflowing
electron-positron plasmas. By fitting the observationally deduced drift-rates
to the theoretical values, we managed to estimate polar cap surface
temperatures in a number of pulsars. The estimated surface temperatures
correspond to a small charge depletion of the order of a few percent of the
corotational charge density. We also argue that if the thermionic electron
outflow from the surface of a negatively charged polar cap is slightly below
the Goldreich-Julian density, then the resulting small charge depletion will
have similar consequences as in the case of the ions outflow. We thus believe
that the sparking discharge of a partially shielded acceleration potential drop
occurs in all pulsars, with both positively (``pulsars'') and negatively
(``anti-pulsars'') charged polar caps
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