7,772 research outputs found
Determination of resistance and trimming moment of planing water craft
This report presents a new theory which makes it possible to interpret the resistance and the trimming moment for any loading of a planing aircraft when these values are given for one load. This application of the new theory forms the basis of the present paper. Derivations of various conversion formulas are given as well as numerous examples
Coherent transport in Nb/delta-doped-GaAs hybrid microstructures
Coherent transport in Nb/GaAs superconductor-semiconductor microstructures is
presented. The structures fabrication procedure is based on delta-doped layers
grown by molecular-beam-epitaxy near the GaAs surface, followed by an As cap
layer to protect the active semiconductor layers during ex situ transfer. The
superconductor is then sputter deposited in situ after thermal desorption of
the protective layer. Two types of structures in particular will be discussed,
i.e., a reference junction and the engineered one that contains an additional
insulating AlGaAs barrier inserted during the growth in the semiconductor. This
latter configuration may give rise to controlled interference effects and
realizes the model introduced by de Gennes and Saint-James in 1963. While both
structures show reflectionless tunneling-dominated transport, only the
engineered junction shows additionally a low-temperature single marked
resonance peaks superimposed to the characteristic Andreev-dominated subgap
conductance. The analysis of coherent magnetotransport in both microstructures
is successfully performed within the random matrix theory of Andreev transport
and ballistic effects are included by directly solving the Bogoliubov-de Gennes
equations. The impact of junction morphology on reflectionless tunneling and
the application of the employed fabrication technique to the realization of
complex semiconductor-superconductor systems are furthermore discussed.Comment: 9 pages, 8 figures, invited review paper, to be published in Mod.
Phys. Lett.
Voltage modulated electro-luminescence spectroscopy and negative capacitance - the role of sub-bandgap states in light emitting devices
Voltage modulated electroluminescence spectra and low frequency ({\leq} 100
kHz) impedance characteristics of electroluminescent diodes are studied.
Voltage modulated light emission tracks the onset of observed negative
capacitance at a forward bias level for each modulation frequency. Active
participation of sub-bandgap defect states in minority carrier recombination
dynamics is sought to explain the results. Negative capacitance is understood
as a necessary dielectric response to compensate any irreversible transient
changes in the minority carrier reservoir due to radiative recombinations
mediated by slowly responding sub-bandgap defects. Experimentally measured
variations of the in-phase component of modulated electroluminescence spectra
with forward bias levels and modulation frequencies support the dynamic
influence of these states in the radiative recombination process. Predominant
negative sign of the in-phase component of voltage modulated
electroluminescence signal further confirms the bi-molecular nature of light
emission. We also discuss how these states can actually affect the net density
of minority carriers available for radiative recombination. Results indicate
that these sub-bandgap states can suppress external quantum efficiency of such
devices under high frequency operation commonly used in optical communication.Comment: 21 pages, 4 sets of figure
Soft thermal contributions to 3-loop gauge coupling
We analyze 3-loop contributions to the gauge coupling felt by ultrasoft
("magnetostatic") modes in hot Yang-Mills theory. So-called soft/hard terms,
originating from dimension-six operators within the soft effective theory, are
shown to cancel 1097/1098 of the IR divergence found in a recent determination
of the hard 3-loop contribution to the soft gauge coupling. The remaining
1/1098 originates from ultrasoft/hard contributions, induced by dimension-six
operators in the ultrasoft effective theory. Soft 3-loop contributions are
likewise computed, and are found to be IR divergent, rendering the ultrasoft
gauge coupling non-perturbative at relative order O(alphas^{3/2}). We elaborate
on the implications of these findings for effective theory studies of physical
observables in thermal QCD.Comment: 31 pages. v2: clarifications added, published versio
2MASS J05162881+2607387: A New Low-Mass Double-Lined Eclipsing Binary
We show that the star known as 2MASS J05162881+2607387 (hereafter J0516) is a
double-lined eclipsing binary with nearly identical low-mass components. The
spectroscopic elements derived from 18 spectra obtained with the High
Resolution Spectrograph on the Hobby-Eberly Telescope during the Fall of 2005
are K_1=88.45 +/- 0.48 km/s and K_2=90.43 +/- 0.60 km/s, resulting in a mass
ratio of$q=K_1/K_2 = 0.978 +/- 0.018 and minimum masses of M_1 sin^{3}i=0.775
+/- 0.016 solar masses and M_2 sin^{3}i=0.759 +/- 0.012 solar masses,
respectively. We have extensive differential photometry of J0516 obtained over
several nights between 2004 January-March (epoch 1) and 2004 October-2005
January plus 2006 January (epoch 2) using the 1m telescope at the Mount Laguna
Observatory. The source was roughly 0.1 mag brighter in all three bandpasses
during epoch 1 when compared to epoch 2. Also, phased light curves from epoch 1
show considerable out-of-eclipse variability, presumably due to bright spots on
one or both stars. In contrast, the phased light curves from epoch 2 show
little out-of-eclipse variability. The light curves from epoch 2 and the radial
velocity curves were analyzed using our ELC code with updated model atmospheres
for low-mass stars. We find the following: M_1=0.787 +/- 0.012 solar masses,
R_1=0.788 +/- 0.015 solar radii, M_2=0.770 +/- 0.009 solar masses, and
R_2=0.817 +/- 0.010 solar radii. The stars in J0516 have radii that are
significantly larger than model predictions for their masses, similar to what
is seen in a handful of other well-studied low-mass double-lined eclipsing
binaries. We compiled all recent mass and radius determinations from low-mass
binaries and determine an empirical mass-radius relation of the form R = 0.0324
+ 0.9343M + 0.0374M^2, where the quantities are in solar units.Comment: 16 pages, 10 figures (Figure 1 has degraded quality), to appear in
Ap
Solubilization method for isolation of photosynthetic mega- And super-complexes from conifer thylakoids
Photosynthesis is the main process by which sunlight is harvested and converted into chemical energy and has been a focal point of fundamental research in plant biology for decades. In higher plants, the process takes place in the thylakoid membranes where the two photosystems (PSI and PSII) are located. In the past few decades, the evolution of biophysical and biochemical techniques allowed detailed studies of the thylakoid organization and the interaction between protein complexes and cofactors. These studies have mainly focused on model plants, such as Arabidopsis, pea, spinach, and tobacco, which are grown in climate chambers even though significant differences between indoor and outdoor growth conditions are present. In this manuscript, we present a new mild-solubilization procedure for use with “fragile” samples such as thylakoids from conifers growing outdoors. Here, the solubilization protocol is optimized with two detergents in two species, namely Norway spruce (Picea abies) and Scots pine (Pinus sylvestris). We have optimized the isolation and characterization of PSI and PSII multimeric mega- and super-complexes in a close-to-native condition by Blue-Native gel electrophoresis. Eventually, our protocol will not only help in the characterization of photosynthetic complexes from conifers but also in understanding winter adaptation
Cygnus X-2: the Descendant of an Intermediate-Mass X-Ray Binary
The X-ray binary Cygnus X-2 (Cyg X-2) has recently been shown to contain a
secondary that is much more luminous and hotter than is appropriate for a
low-mass subgiant. We present detailed binary-evolution calculations which
demonstrate that the present evolutionary state of Cyg X-2 can be understood if
the secondary had an initial mass of around 3.5 M_sun and started to transfer
mass near the end of its main-sequence phase (or, somewhat less likely, just
after leaving the main sequence). Most of the mass of the secondary must have
been ejected from the system during an earlier rapid mass-transfer phase. In
the present phase, the secondary has a mass of around 0.5 M_sun with a
non-degenerate helium core. It is burning hydrogen in a shell, and mass
transfer is driven by the advancement of the burning shell. Cyg X-2 therefore
is related to a previously little studied class of intermediate-mass X-ray
binaries (IMXBs). We suggest that perhaps a significant fraction of X-ray
binaries presently classified as low-mass X-ray binaries may be descendants of
IMXBs and discuss some of the implications
Globular Clusters in the dE,N galaxy NGC 3115 DW1: New Insights from Spectroscopy and HST Photometry
The properties of globular clusters in dwarf galaxies are key to
understanding the formation of globular cluster systems, and in particular in
verifying scenarios in which globular cluster systems of larger galaxies formed
(at least partly) from the accretion of dwarf galaxies. Here, we revisit the
globular cluster system of the dE,N galaxy NGC 3115 DW1 - a companion of the
nearby S0 galaxy NGC 3115 - adding Keck/LRIS spectroscopy and HST/WFPC2 imaging
to previous ground-based photometry. Spectra for seven globular clusters reveal
normal abundance ratios with respect to the Milky Way and M31 clusters, as well
as a relatively high mean metallicity ([Fe/H] = -1.0+/-0.1 dex). Crude
kinematics indicate a high velocity dispersion within 10 kpc which could either
be caused by dark matter dominated outer regions, or by the stripping of outer
globular clusters by the nearby giant galaxy NGC 3115. The total galaxy mass
out to 3 and 10 kpc lies between 10^10 and 10^11 solar masses and 2*10^10 and
4*10^11 solar masses, respectively, depending on the mass estimator used and
the assumptions on cluster orbits and systemic velocity. The HST imaging allows
measurement of sizes for two clusters, returning core radii around 2.0 pc,
similar to the sizes observed in other galaxies. Spectroscopy allows an
estimate of the degree of contamination by foreground stars or background
galaxies for the previous ground-based photometry, but does not require a
revision of previous results: NGC 3115 DW1 hosts around 60+/-20 clusters which
corresponds to a specific frequency of 4.9+/-1.9, on the high end for massive
dEs. Given its absolute magnitude (M_V=-17.7 mag) and the properties of its
cluster system, NGC 3115 DW1 appears to be a transition between a luminous dE
and low-luminosity E galaxy.Comment: 25 pages, 8 figures, accepted for publication in The Astronomical
Journal, August 2000 issu
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