1,018 research outputs found
Modulation of neutral interstellar He, Ne, O in the heliosphere. Survival probabilities and abundances at IBEX
Direct sampling of neutral interstellar (NIS) atoms by the Interstellar
Boundary Explorer (IBEX) can potentially provide a complementary method for
studying element abundances in the Local Interstellar Cloud and processes in
the heliosphere interface.}{We set the stage for abundance-aimed in-depth
analysis of measurements of NIS He, Ne, and O by IBEX and determine systematic
differences between abundances derived from various calculation methods and
their uncertainties.}{Using a model of ionization rates of the NIS species in
the heliosphere, based on independent measurements of the solar wind and solar
EUV radiation, we develop a time-dependent method of calculating the survival
probabilities of NIS atoms from the termination shock (TS) of the solar wind to
IBEX. With them, we calculate densities of these species along the Earth's
orbit and simulate the fluxes of NIS species as observed by IBEX. We study
pairwise ratios of survival probabilities, densities and fluxes of NIS species
at IBEX to calculate correction factors for inferring the abundances at
TS.}{The analytic method to calculate the survival probabilities gives
acceptable results only for He and Ne during low solar activity. For the
remaining portions of the solar cycle, and at all times for O, a fully time
dependent model should be used. Electron impact ionization is surprisingly
important for NIS O. Interpreting the IBEX observations using the time
dependent model yields the LIC Ne/O abundance of . The uncertainty
is mostly due to uncertainties in the ionization rates and in the NIS gas flow
vector.}{The Ne/He, O/He and Ne/O ratios for survival probabilities, local
densities, and fluxes scaled to TS systematically differ and thus an analysis
based only on survival probabilities or densities is not recommended, except
the Ne/O abundance for observations at low solar activity.Comment: Astronomy & Astrophysics, in press. Language and editing corrections
implemente
Influence of roughness on ZDDP tribofilm formation in boundary lubricated fretting
Influence of initial surface topography on tribofilm formation in ZDDP lubricated contact was analysed. A small displacement fretting tests with sinusoidal motion were carried out in classical sphere/plane configuration. A range of surfaces with different initial roughness were prepared by milling and grinding processes. Tests were carried out using variable displacement method where amplitude of imposed displacement was gradually increased after every 1000 cycles from 2 to 30 µm. The surfaces after tribological tests were measured by interferometric profiler. Main findings confirm that initial roughness has a significant influence on antiwear tribofilm formation in boundary lubricated contact. Tribofilm form faster and require less energy to activate in case of rough surface obtained by milling process than in case of smooth grinded surface. However, in contact lubricated by ZDDP additive a significant transfer of material occurred from plane to sphere specimen
Very high two-dimensional hole gas mobilities in strained silicon germanium
We report on the growth by solid source MBE and characterization of remote doped Si/SiGe/Si two-dimensional hole gas structures. It has been found that by reducing the Ge composition to <=13% and limiting the thickness of the alloy layer, growth temperatures can be increased up to 950 °C for these structures while maintaining good structural integrity and planar interfaces. Record mobilities of 19 820 cm2 V−1 s−1 at 7 K were obtained in normal structures. Our calculations suggest that alloy scattering is not important in these structures and that interface roughness and interface charge scattering limit the low temperature mobilities
Elemental boron doping behavior in silicon molecular beam epitaxy
Boron-doped Si epilayers were grown by molecular beam epitaxy (MBE) using an elemental boron source, at levels up to 2×1020 cm−3, to elucidate profile control and electrical activation over the growth temperature range 450–900 °C. Precipitation and surface segregation effects were observed at doping levels of 2×1020 cm−3 for growth temperatures above 600 °C. At growth temperatures below 600 °C, excellent profile control was achieved with complete electrical activation at concentrations of 2×1020 cm−3, corresponding to the optimal MBE growth conditions for a range of Si/SixGe1−x heterostructures
Growth studies on Si0.8Ge0.2 channel two-dimensional hole gases
We report a study of the influences of MBE conditions on the low-temperature mobilities of Si/Si0.8Ge0.2 2DHG structures. A significant dependence of 2DHG mobility on growth temperature is observed with the maximum mobility of 3640 cm2 V−1 s−1 at 5.4 K being achieved at the relatively high-growth temperature of 640 °C. This dependence is associated with a reduction in interface charge density. Studies on lower mobility samples show that Cu contamination can be reduced both by growth interruptions and by modifications to the Ge source; this reduction produces improvements in the low-temperature mobilities. We suggest that interface charge deriving from residual metal contamination is currently limiting the 4-K mobility
Evidence for quantum confinement in the photoluminescence of porous Si and SiGe
We have used anodization techniques to process porous surface regions in p-type Czochralski Si and in p-type Si0.85Ge0.15 epitaxial layers grown by molecular beam epitaxy. The SiGe layers were unrelaxed before processing. We have observed strong near-infrared and visible light emission from both systems. Analysis of the radiative and nonradiative recombination processes indicate that the emission is consistent with the decay of excitons localized in structures of one or zero dimensions
The downwind hemisphere of the heliosphere: Eight years of IBEX-Lo observations
We present a comprehensive study of energetic neutral atoms (ENAs) of 10 eV
to 2.5 keV from the downwind hemisphere of the heliosphere. These ENAs are
believed to originate mostly from pickup protons and solar wind protons in the
inner heliosheath. This study includes all low-energy observations made with
the Interstellar Boundary Explorer over the first 8 years. Since the protons
around 0.1 keV dominate the plasma pressure in the inner heliosheath in
downwind direction, these ENA observations offer the unique opportunity to
constrain the plasma properties and dimensions of the heliosheath where no
in-situ observations are available.
We first derive energy spectra of ENA intensities averaged over time for 49
macropixels covering the entire downwind hemisphere. The results confirm
previous studies regarding integral intensities and the roll-over around 0.1
keV energy. With the expanded dataset we now find that ENA intensities at 0.2
and 0.1 keV seem to anti-correlate with solar activity. We then derive the
product of total plasma pressure and emission thickness of protons in the
heliosheath to estimate lower limits on the thickness of the inner heliosheath.
The temporally averaged ENA intensities support a rather spherical shape of the
termination shock and a heliosheath thickness between 150 and 210 au for most
regions of the downwind hemisphere. Around the nominal downwind direction of
76{\deg} ecliptic longitude, the heliosheath is at least 280 au thick. There,
the neutral hydrogen density seems to be depleted compared to upwind directions
by roughly a factor of 2.Comment: Preprint of article in The Astrophysical Journa
Concentration of atomic hydrogen diffused into silicon in the temperature range 900–1300 °C
Boron-doped Czochralski silicon samples with [B]~1017 cm−3 have been heated at various temperatures in the range 800–1300 °C in an atmosphere of hydrogen and then quenched. The concentration of [H-B] pairs was measured by infrared localized vibrational mode spectroscopy. It was concluded that the solubility of atomic hydrogen is greater than [Hs] = 5.6 × 1018 exp( − 0.95 eV/kT)cm−3 at the temperatures investigated
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