2,558 research outputs found
The SiC problem: astronomical and meteoritic evidence
Pre-solar grains of silicon carbide found in meteorites and interpreted as
having had an origin around carbon stars from their isotopic composition, have
all been found to be of the beta-SiC polytype. Yet to date fits to the 11.3
microns SiC emission band of carbon stars had been obtained only for alpha-SiC
grains. We present thin film infrared (IR) absorption spectra measured in a
diamond anvil cell for both the alpha- and beta- polymorphs of synthetic SiC
and compare the results with previously published spectra taken using the KBr
matrix method. We find that our thin film spectra have positions nearly
identical to those obtained previously from finely ground samples in KBr.
Hence, we show that this discrepancy has arisen from inappropriate `KBr
corrections' having been made to laboratory spectra of SiC particles dispersed
in KBr matrices. We re-fit a sample of carbon star mid-IR spectra, using
laboratory data with no KBr correction applied, and show that beta-SiC grains
fit the observations, while alpha-SiC grains do not. The discrepancy between
meteoritic and astronomical identifications of the SiC-type is therefore
removed. This work shows that the diamond anvil cell thin film method can be
used to produce mineral spectra applicable to cosmic environments without
further manipulation.Comment: to be published in Astrophysical Journal Letter 4 pages, 3 figure
On the hyperbolicity and causality of the relativistic Euler system under the kinetic equation of state
We show that a pair of conjectures raised in [11] concerning the construction
of normal solutions to the relativistic Boltzmann equation are valid. This
ensures that the results in [11] hold for any range of positive temperatures
and that the relativistic Euler system under the kinetic equation of state is
hyperbolic and the speed of sound cannot overcome .Comment: 6 pages. Abridged version; full version to appear in Commun. Pure
Appl. Ana
Measurement of Stochastic Entropy Production
Using fluorescence spectroscopy we directly measure entropy production of a
single two-level system realized experimentally as an optically driven defect
center in diamond. We exploit a recent suggestion to define entropy on the
level of a single stochastic trajectory (Seifert, Phys. Rev. Lett. {\bf 95},
040602 (2005)). Entropy production can then be split into one of the system
itself and one of the surrounding medium. We demonstrate that the total entropy
production obeys various exact relations for finite time trajectories.Comment: Phys. Rev. Lett., in pres
First-principles study of epitaxial strain in perovskites
Using an extension of a first-principles method developed by King-Smith and
Vanderbilt [Phys. Rev. B {\bf 49}, 5828 (1994)], we investigate the effects of
in-plane epitaxial strain on the ground-state structure and polarization of
eight perovskite oxides: BaTiO, SrTiO, CaTiO, KNbO, NaNbO,
PbTiO, PbZrO, and BaZrO. In addition, we investigate the effects of
a nonzero normal stress. The results are shown to be useful in predicting the
structure and polarization of perovskite oxide thin films and superlattices.Comment: 10 page
Effective free energy for pinned membranes
We consider membranes adhered through specific receptor-ligand bonds. Thermal
undulations of the membrane induce effective interactions between adhesion
sites. We derive an upper bound to the free energy that is independent of
interaction details. To lowest order in a systematic expansion we obtain
two-body interactions which allow to map the free energy onto a lattice gas
with constant density. The induced interactions alone are not strong enough to
lead to a condensation of individual adhesion sites. A measure of the thermal
roughness is shown to depend on the inverse square root of the density of
adhesion sites, which is in good agreement with previous computer simulations.Comment: to appear as a Rapid Communication in Phys. Rev.
The Curious Conundrum Regarding Sulfur Abundances In Planetary Nebulae
Sulfur abundances derived from optical emission line measurements and
ionization correction factors in planetary nebulae are systematically lower
than expected for the objects' metallicities. We have carefully considered a
large range of explanations for this "sulfur anomaly", including: (1)
correlations between the size of the sulfur deficit and numerous nebular and
central star properties; (2) ionization correction factors which under-correct
for unobserved ions; (3) effects of dielectronic recombination on the sulfur
ionization balance; (4) sequestering of S into dust and/or molecules; and (5)
excessive destruction of S or production of O by AGB stars. It appears that all
but the second scenario can be ruled out. However, we find evidence that the
sulfur deficit is generally reduced but not eliminated when S^+3 abundances
determined directly from IR measurements are used in place of the customary
sulfur ionization correction factor. We tentatively conclude that the sulfur
anomaly is caused by the inability of commonly used ICFs to properly correct
for populations of ionization stages higher than S^+2.Comment: 40 pages, 14 figures, 3 tables. Accepted for publication in the
Astrophysical Journa
The quasi-free-standing nature of graphene on H-saturated SiC(0001)
We report on an investigation of quasi-free-standing graphene on 6H-SiC(0001)
which was prepared by intercalation of hydrogen under the buffer layer. Using
infrared absorption spectroscopy we prove that the SiC(0001) surface is
saturated with hydrogen. Raman spectra demonstrate the conversion of the buffer
layer into graphene which exhibits a slight tensile strain and short range
defects. The layers are hole doped (p = 5.0-6.5 x 10^12 cm^(-2)) with a carrier
mobility of 3,100 cm^2/Vs at room temperature. Compared to graphene on the
buffer layer a strongly reduced temperature dependence of the mobility is
observed for graphene on H-terminated SiC(0001)which justifies the term
"quasi-free-standing".Comment: 3 pages, 3 figures, accepted for publication in Applied Physics
Letter
Linear response theory and transient fluctuation theorems for diffusion processes: a backward point of view
On the basis of perturbed Kolmogorov backward equations and path integral
representation, we unify the derivations of the linear response theory and
transient fluctuation theorems for continuous diffusion processes from a
backward point of view. We find that a variety of transient fluctuation
theorems could be interpreted as a consequence of a generalized
Chapman-Kolmogorov equation, which intrinsically arises from the Markovian
characteristic of diffusion processes
Irreversible effects of memory
The steady state of a Langevin equation with short ranged memory and coloured
noise is analyzed. When the fluctuation-dissipation theorem of second kind is
not satisfied, the dynamics is irreversible, i.e. detailed balance is violated.
We show that the entropy production rate for this system should include the
power injected by ``memory forces''. With this additional contribution, the
Fluctuation Relation is fairly verified in simulations. Both dynamics with
inertia and overdamped dynamics yield the same expression for this additional
power. The role of ``memory forces'' within the fluctuation-dissipation
relation of first kind is also discussed.Comment: 6 pages, 1 figure, publishe
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