4,287 research outputs found
Techniques for carrying out radiative transfer calculations for the Martian atmospheric dust
A description is given of the modification of a theory on the reflectance of particulate media so as to apply it to analysis of the infrared spectra obtained by the IRIS instrument on Mariner 9. With the aid of this theory and the optical constants of muscovite mica, quartz, andesite, anorthosite, diopside pyroxenite, and dunite, modeling calculations were made to refine previous estimates of the mineralogical composition of the Martian dust particles. These calculations suggest that a feldspar rich mixture is a very likely composition for the dust particles. The optical constants used for anorthosite and diopside pyroxenite were derived during this program from reflectance measurements. Those for the mica were derived from literature reflectance data. Finally, a computer program was written to invert the measured radiance data so as to obtain the absorption coefficient spectrum which should then be independent of the temperature profile and gaseous component effects
Dual Fronts Propagating into an Unstable State
The interface between an unstable state and a stable state usually develops a
single confined front travelling with constant velocity into the unstable
state. Recently, the splitting of such an interface into {\em two} fronts
propagating with {\em different} velocities was observed numerically in a
magnetic system. The intermediate state is unstable and grows linearly in time.
We first establish rigorously the existence of this phenomenon, called ``dual
front,'' for a class of structurally unstable one-component models. Then we use
this insight to explain dual fronts for a generic two-component
reaction-diffusion system, and for the magnetic system.Comment: 19 pages, Postscript, A
Erosion waves: transverse instabilities and fingering
Two laboratory scale experiments of dry and under-water avalanches of
non-cohesive granular materials are investigated. We trigger solitary waves and
study the conditions under which the front is transversally stable. We show the
existence of a linear instability followed by a coarsening dynamics and finally
the onset of a fingering pattern. Due to the different operating conditions,
both experiments strongly differ by the spatial and time scales involved.
Nevertheless, the quantitative agreement between the stability diagram, the
wavelengths selected and the avalanche morphology reveals a common scenario for
an erosion/deposition process.Comment: 4 pages, 6 figures, submitted to PR
Development of a theory of the spectral reflectance of minerals, part 2
Theory of diffuse reflectance of particulate media including garnet, glass, corundum powders, and mixture
Neutron, electron and X-ray scattering investigation of Cr1-xVx near Quantum Criticality
The weakness of electron-electron correlations in the itinerant
antiferromagnet Cr doped with V has long been considered the reason that
neither new collective electronic states or even non Fermi liquid behaviour are
observed when antiferromagnetism in CrV is suppressed to zero
temperature. We present the results of neutron and electron diffraction
measurements of several lightly doped single crystals of CrV in
which the archtypal spin density wave instability is progressively suppressed
as the V content increases, freeing the nesting-prone Fermi surface for a new
striped charge instability that occurs at x=0.037. This novel nesting
driven instability relieves the entropy accumulation associated with the
suppression of the spin density wave and avoids the formation of a quantum
critical point by stabilising a new type of charge order at temperatures in
excess of 400 K. Restructuring of the Fermi surface near quantum critical
points is a feature found in materials as diverse as heavy fermions, high
temperature copper oxide superconductors and now even elemental metals such as
Cr.Comment: 6 pages, 6 figures. Accepted to Physical Review
Thermal and electrical transport in the spin density wave antiferromagnet CaFeAs
We present here measurements of the thermopower, thermal conductivity, and
electrical resistivity of the newly reported compound CaFe4As3. Evidence is
presented from specific heat and electrical resistivity measurements that a
substantial fraction of the Fermi surface survives the onset of spin density
wave (SDW) order at the Neel temperature TN=88 K, and its subsequent
commensurate lockin transition at T2=26.4 K. The specific heat below T2
consists of a normal metallic component from the ungapped parts of the Fermi
surface, and a Bardeen-Cooper- Schrieffer (BCS) component that represents the
SDW gapping of the Fermi surface. A large Kadowaki-Woods ratio is found at low
temperatures, showing that the ground state of CaFe4As3 is a strongly
interacting Fermi liquid. The thermal conductivity of CaFe4As3 is an order of
magnitude smaller than those of conventional metals at all temperatures, due to
a strong phonon scattering. The thermoelectric power displays a sign change
from positive to negative indicating that a partial gap forms at the Fermi
level with the onset of commensurate spin density wave order at T2=26.4 K. The
small value of the thermopower and the enhancements of the resistivity due to
gap formation and strong quasiparticle interactions offset the low value of the
thermal conductivity, yielding only a modest value for the thermoelectric
figure of merit Z < 5x10^-6 1/K in CaFe4As3. The results of ab initio
electronic structure calculations are reported, confirming that the sign change
in the thermopower at T2 is reflected by a sign change in the slope of the
density of states at the Fermi level. Values for the quasiparticle
renormalization are derived from measurements of the specific heat and
thermopower, indicating that as T->0, CaFe4As3 is among the most strongly
correlated of the known Fe-based pnictide and chalcogenide systems.Comment: 8 pages with 5 figure
The influence of fractional diffusion in Fisher-KPP equations
We study the Fisher-KPP equation where the Laplacian is replaced by the
generator of a Feller semigroup with power decaying kernel, an important
example being the fractional Laplacian. In contrast with the case of the stan-
dard Laplacian where the stable state invades the unstable one at constant
speed, we prove that with fractional diffusion, generated for instance by a
stable L\'evy process, the front position is exponential in time. Our results
provide a mathe- matically rigorous justification of numerous heuristics about
this model
Avalanche of Bifurcations and Hysteresis in a Model of Cellular Differentiation
Cellular differentiation in a developping organism is studied via a discrete
bistable reaction-diffusion model. A system of undifferentiated cells is
allowed to receive an inductive signal emenating from its environment.
Depending on the form of the nonlinear reaction kinetics, this signal can
trigger a series of bifurcations in the system. Differentiation starts at the
surface where the signal is received, and cells change type up to a given
distance, or under other conditions, the differentiation process propagates
through the whole domain. When the signal diminishes hysteresis is observed
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