5,187 research outputs found
A Variational Principle for the Asymptotic Speed of Fronts of the Density Dependent Diffusion--Reaction Equation
We show that the minimal speed for the existence of monotonic fronts of the
equation with , and in
derives from a variational principle. The variational principle allows
to calculate, in principle, the exact speed for arbitrary . The case
when is included as an extension of the results.Comment: Latex, postcript figure availabl
Spontaneous flow transition in active polar gels
We study theoretically the effects of confinement on active polar gels such
as the actin network of eukaryotic cells. Using generalized hydrodynamics
equations derived for active gels, we predict, in the case of quasi
one-dimensional geometry, a spontaneous flow transition from a homogeneously
polarized immobile state for small thicknesses, to a perturbed flowing state
for larger thicknesses. The transition is not driven by an external field but
by the activity of the system. We suggest several possible experimental
realizations.Comment: 7 pages, 3 figures. To appear in Europhys. Let
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
A Nuclear Physics Program at the ATLAS Experiment at the CERN Large Hadron Collider
The ATLAS collaboration has significant interest in the physics of
ultra-relativistic heavy ion collisions. We submitted a Letter of Intent to the
United States Department of Energy in March 2002. The following document is a
slightly modified version of that LOI. More details are available at:
http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/SM/ionsComment: Letter of Intent submitted to the United States Department of Energy
Nuclear Physics Division in March 2002 (revised version
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
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Practice patterns and outcomes of equivocal bone scans for patients with castration-resistant prostate cancer: Results from SEARCH.
ObjectiveTo review follow-up imaging after equivocal bone scans in men with castration resistant prostate cancer (CRPC) and examine the characteristics of equivocal bone scans that are associated with positive follow-up imaging.MethodsWe identified 639 men from five Veterans Affairs Hospitals with a technetium-99m bone scan after CRPC diagnosis, of whom 99 (15%) had equivocal scans. Men with equivocal scans were segregated into "high-risk" and "low-risk" subcategories based upon wording in the bone scan report. All follow-up imaging (bone scans, computed tomography [CT], magnetic resonance imaging [MRI], and X-rays) in the 3 months after the equivocal scan were reviewed. Variables were compared between patients with a positive vs. negative follow-up imaging after an equivocal bone scan.ResultsOf 99 men with an equivocal bone scan, 43 (43%) received at least one follow-up imaging test, including 32/82 (39%) with low-risk scans and 11/17 (65%) with high-risk scans (p = 0.052). Of follow-up tests, 67% were negative, 14% were equivocal, and 19% were positive. Among those who underwent follow-up imaging, 3/32 (9%) low-risk men had metastases vs. 5/11 (45%) high-risk men (p = 0.015).ConclusionWhile 19% of all men who received follow-up imaging had positive follow-up imaging, only 9% of those with a low-risk equivocal bone scan had metastases versus 45% of those with high-risk. These preliminary findings, if confirmed in larger studies, suggest follow-up imaging tests for low-risk equivocal scans can be delayed while high-risk equivocal scans should receive follow-up imaging
The magnetic response at the metal–insulator transition in La1−xSrxTiO3 (abstract)
We report on recent inelastic neutron scattering measurements of the magnetic response close to the metal–insulator transition in La1−xSrxTiO3. Specific heat and susceptibility data from Tokura et al. give evidence for a divergent effective mass at the critical concentration xc=0.05 in agreement with recent mean‐field theories of the transition. The mass enhancement is believed to arise from the formation of a d‐electron resonance at the Fermi energy close to the transition. The aim of this investigation is to look for evidence of this resonance in the dynamic magnetic susceptibility. We studied samples with x=0, 0.05, and 0.2 using incident energies between 25 and 200 meV. After correction for the phonon scattering, we observe a broad response above a threshold of 20–30 meV extending to over 100 meV. In addition, the Mott insulating antiferromagnet (x=0) has a peak at 40 meV, consistent with the estimated activation energy derived from resistivity measurements. This feature becomes washed out with temperature and doping. Possible origins for this peak are discussed. © 1996 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70552/2/JAPIAU-79-8-6432-1.pd
On a Conjecture of Goriely for the Speed of Fronts of the Reaction--Diffusion Equation
In a recent paper Goriely considers the one--dimensional scalar
reaction--diffusion equation with a polynomial reaction
term and conjectures the existence of a relation between a global
resonance of the hamiltonian system and the asymptotic
speed of propagation of fronts of the reaction diffusion equation. Based on
this conjecture an explicit expression for the speed of the front is given. We
give a counterexample to this conjecture and conclude that additional
restrictions should be placed on the reaction terms for which it may hold.Comment: 9 pages Revtex plus 4 postcript figure
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
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