18,083 research outputs found
Capacitance of Gated GaAs/AlGaAs Heterostructures Subject to In-plane Magnetic Fields
A detailed analysis of the capacitance of gated GaAs/AlGaAs heterostructures
is presented. The nonlinear dependence of the capacitance on the gate voltage
and in-plane magnetic field is discussed together with the capacitance quantum
steps connected with a population of higher 2D gas subbands. The results of
full self-consistent numerical calculations are compared to recent experimental
data.Comment: 4 pages, Revtex. 4 PostScript figures in an uuencoded compressed file
available upon request. Phys. Rev.B, in pres
Extreme 18O-enrichment in majorite constrains a crustal origin of transition zone diamonds
The fate of subducted oceanic lithosphere and its role in the planet-scale geochemical cycle is a key problem in solid Earth studies. Asthenospheric and transition zone minerals included in diamond have been interpreted as representing subducted oceanic crust based on inclusion REE patterns and strong 13C depletion of their host diamond (δ13C as low as -23 ‰). This view/explanation, however, has been challenged by alternative interpretations that variable carbon isotopic compositions either result from high temperature fractionation involving carbides, or reflect primordial, unhomogenised mantle reservoirs. Here, we present the first oxygen isotope analyses of inclusions in such ultradeep diamonds – majoritic garnets in diamond from Jagersfontein (South Africa). The oxygen isotope compositions provide unambiguous evidence for derivation of the inclusions from subducted crustal materials. The δ18OVSMOW values of the majorites range from +8.6 ‰ to +10.0 ‰, well outside that of ambient mantle (+5.5 ±0.4 ‰) and indicate that the protoliths were very heavily weathered at relatively low temperatures. When this information is combined with the broadly eclogitic composition of the majoritic garnets, a derivation from subducted sea-floor basalts is implied. Based on the association between the heavy oxygen and light carbon, the light carbon isotope composition cannot relate to deep mantle processes and is also ultimately derived from the crust
Digital flight control research
The results of studies which were undertaken to contribute to the design of digital flight control systems, particularly for transport aircraft are presented. In addition to the overall design considerations for a digital flight control system, the following topics are discussed in detail: (1) aircraft attitude reference system design, (2) the digital computer configuration, (3) the design of a typical digital autopilot for transport aircraft, and (4) a hybrid flight simulator
Broad Iron Emission from Gravitationally Lensed Quasars Observed by Chandra
Recent work has demonstrated the potential of gravitationally lensed quasars
to extend measurements of black hole spin out to high-redshift with the current
generation of X-ray observatories. Here we present an analysis of a large
sample of 27 lensed quasars in the redshift range 1.0<z<4.5 observed with
Chandra, utilizing over 1.6 Ms of total observing time, focusing on the
rest-frame iron K emission from these sources. Although the X-ray
signal-to-noise (S/N) currently available does not permit the detection of iron
emission from the inner accretion disk in individual cases in our sample, we
find significant structure in the stacked residuals. In addition to the narrow
core, seen almost ubiquitously in local AGN, we find evidence for an additional
underlying broad component from the inner accretion disk, with a clear red wing
to the emission profile. Based on simulations, we find the detection of this
broader component to be significant at greater than the 3-sigma level. This
implies that iron emission from the inner disk is relatively common in the
population of lensed quasars, and in turn further demonstrates that, with
additional observations, this population represents an opportunity to
significantly extend the sample of AGN spin measurements out to high-redshift.Comment: 5 pages, 2 figures, accepted for publication in Ap
Quark Effects in the Gluon Condensate Contribution to the Scalar Glueball Correlation Function
One-loop quark contributions to the dimension-four gluon condensate term in
the operator product expansion (OPE) of the scalar glueball correlation
function are calculated in the MS-bar scheme in the chiral limit of quark
flavours. The presence of quark effects is shown not to alter the cancellation
of infrared (IR) singularities in the gluon condensate OPE coefficients. The
dimension-four gluonic condensate term represents the leading power corrections
to the scalar glueball correlator and, therein, the one-loop logarithmic
contributions provide the most important condensate contribution to those QCD
sum-rules independent of the low-energy theorem (the subtracted sum-rules).Comment: latex2e, 6 pages, 7 figures embedded in latex fil
PSN13 REFINEMENT AND REDUCTION OF THE IMPACT OF PSORIASIS QUESTIONNAIRE: CLASSICAL TEST THEORY VS RASCH ANALYSIS
Primordial nucleosynthesis as a probe of fundamental physics parameters
We analyze the effect of variation of fundamental couplings and mass scales
on primordial nucleosynthesis in a systematic way. The first step establishes
the response of primordial element abundances to the variation of a large
number of nuclear physics parameters, including nuclear binding energies. We
find a strong influence of the n-p mass difference (for the 4He abundance), of
the nucleon mass (for deuterium) and of A=3,4,7 binding energies (for 3He, 6Li
and 7Li). A second step relates the nuclear parameters to the parameters of the
Standard Model of particle physics. The deuterium, and, above all, 7Li
abundances depend strongly on the average light quark mass hat{m} \equiv
(m_u+m_d)/2. We calculate the behaviour of abundances when variations of
fundamental parameters obey relations arising from grand unification. We also
discuss the possibility of a substantial shift in the lithium abundance while
the deuterium and 4He abundances are only weakly affected.Comment: v2: 34 pages, 2 figures, typo in last GUT scenario corrected, added
discussion and graph of nonlinear behaviour in GUT scenarios, added short
section discussing binding of dineutron and 8Be, refs added, conclusions
unaltered. Accepted for publication, Phys. Rev.
Wide-field mid-infrared and millimetre imaging of the high-redshift radio galaxy, 4C41.17
We present deep 350- and 1200-micron imaging of the region around 4C41.17 --
one of the most distant (z = 3.792) and luminous known radio galaxies --
obtained with the Submillimeter High Angular Resolution Camera (SHARC-II) and
the Max Planck Millimeter Bolometer Array (MAMBO). The radio galaxy is robustly
detected at 350- and 1200-micron, as are two nearby 850-micron-selected
galaxies; a third 850-micron source is detected at 350-micron and coincides
with a ~ 2-sigma feature in the 1200-micron map. Further away from the radio
galaxy an additional nine sources are detected at 1200-micron, bringing the
total number of detected (sub)millimeter selected galaxies (SMGs) in this field
to 14. Using radio images from the Very Large Array (VLA) and Spitzer
mid-infrared (mid-IR) data, we find statistically robust radio and/or 24-micron
counterparts to eight of the 14 SMGs in the field around 4C41.17. Follow-up
spectroscopy with Keck/LRIS has yielded redshifts for three of the eight
robustly identified SMGs, placing them in the redshift range 0.5 < z < 2.7,
i.e. well below that of 4C41.17. We infer photometric redshifts for a further
four sources using their 1.6-micron (rest-frame) stellar feature as probed by
the IRAC bands; only one of them is likely to be at the same redshift as
4C41.17. Thus at least four, and as many as seven, of the SMGs within the
4C41.17 field are physically unrelated to the radio galaxy. With the redshift
information at hand we are able to constrain the observed over-densities of
SMGs within radial bins stretching to R=50 and 100" (~ 0.4 and ~ 0.8Mpc at z ~
3.8) from the radio galaxy to ~ 5x and ~ 2x that of the field, dropping off to
the background value at R=150". [Abridged]Comment: 20 pages, 9 figures, accepted for publication in MNRA
Scaling laws of solar and stellar flares
In this study we compile for the first time comprehensive data sets of solar
and stellar flare parameters, including flare peak temperatures T_p, flare peak
volume emission measures EM_p, and flare durations t_f from both solar and
stellar data, as well as flare length scales L from solar data. Key results are
that both the solar and stellar data are consistent with a common scaling law
of EM_p ~ T_p^4.7, but the stellar flares exhibit ~250 times higher emission
measures (at the same flare peak temperature). For solar flares we observe also
systematic trends for the flare length scale L(T_p) ~ T_p^0.9 and the flare
duration t_F(T_p) ~ T_p^0.9 as a function of the flare peak temperature. Using
the theoretical RTV scaling law and the fractal volume scaling observed for
solar flares, i.e., V(L) ~ L^2.4, we predict a scaling law of EM_p ~ T_p^4.3,
which is consistent with observations, and a scaling law for electron densities
in flare loops, n_p ~ T_p^2/L ~ T_p^1.1. The RTV-predicted electron densities
were also found to be consistent with densities inferred from total emission
measures, n_p=(EM_p/q_V*V)^1/2, using volume filling factors of q_V=0.03-0.08
constrained by fractal dimensions measured in solar flares. Our results affect
also the determination of radiative and conductive cooling times, thermal
energies, and frequency distributions of solar and stellar flare energies.Comment: 9 Figs., (paper in press, The Astrophsycial Journal
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