6,906 research outputs found

    Development of a theory of the spectral reflectance of minerals, part 4

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    A theory of the spectral reflectance or emittance of particulate minerals was developed. The theory is expected to prove invaluable in the interpretation of the remote infrared spectra of planetary surfaces

    A Variational Principle for the Asymptotic Speed of Fronts of the Density Dependent Diffusion--Reaction Equation

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    We show that the minimal speed for the existence of monotonic fronts of the equation ut=(um)xx+f(u)u_t = (u^m)_{xx} + f(u) with f(0)=f(1)=0f(0) = f(1) = 0, m>1m >1 and f>0f>0 in (0,1)(0,1) derives from a variational principle. The variational principle allows to calculate, in principle, the exact speed for arbitrary ff. The case m=1m=1 when f′(0)=0f'(0)=0 is included as an extension of the results.Comment: Latex, postcript figure availabl

    Contest based on a directed polymer in a random medium

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    We introduce a simple one-parameter game derived from a model describing the properties of a directed polymer in a random medium. At his turn, each of the two players picks a move among two alternatives in order to maximize his final score, and minimize opponent's return. For a game of length nn, we find that the probability distribution of the final score SnS_n develops a traveling wave form, Prob(Sn=m)=f(m−vn){\rm Prob}(S_n=m)=f(m-v n), with the wave profile f(z)f(z) unusually decaying as a double exponential for large positive and negative zz. In addition, as the only parameter in the game is varied, we find a transition where one player is able to get his maximum theoretical score. By extending this model, we suggest that the front velocity vv is selected by the nonlinear marginal stability mechanism arising in some traveling wave problems for which the profile decays exponentially, and for which standard traveling wave theory applies

    Magnetotransport in the low carrier density ferromagnet EuB_6

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    We present a magnetotransport study of the low--carrier density ferromagnet EuB_6. This semimetallic compound, which undergoes two ferromagnetic transitions at T_l = 15.3 K and T_c = 12.5 K, exhibits close to T_l a colossal magnetoresistivity (CMR). We quantitatively compare our data to recent theoretical work, which however fails to explain our observations. We attribute this disagreement with theory to the unique type of magnetic polaron formation in EuB_6.Comment: Conference contribution MMM'99, San Jos

    Spontaneous flow transition in active polar gels

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    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

    Neutron diffraction in a model itinerant metal near a quantum critical point

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    Neutron diffraction measurements on single crystals of Cr1-xVx (x=0, 0.02, 0.037) show that the ordering moment and the Neel temperature are continuously suppressed as x approaches 0.037, a proposed Quantum Critical Point (QCP). The wave vector Q of the spin density wave (SDW) becomes more incommensurate as x increases in accordance with the two band model. At xc=0.037 we have found temperature dependent, resolution limited elastic scattering at 4 incommensurate wave vectors Q=(1+/-delta_1,2, 0, 0)*2pi/a, which correspond to 2 SDWs with Neel temperatures of 19 K and 300 K. Our neutron diffraction measurements indicate that the electronic structure of Cr is robust, and that tuning Cr to its QCP results not in the suppression of antiferromagnetism, but instead enables new spin ordering due to novel nesting of the Fermi surface of Cr.Comment: Submitted as a part of proceedings of LT25 (Amsterdam 2008

    Traveling Wave Solutions of Advection-Diffusion Equations with Nonlinear Diffusion

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    We study the existence of particular traveling wave solutions of a nonlinear parabolic degenerate diffusion equation with a shear flow. Under some assumptions we prove that such solutions exist at least for propagation speeds c {\in}]c*, +{\infty}, where c* > 0 is explicitly computed but may not be optimal. We also prove that a free boundary hy- persurface separates a region where u = 0 and a region where u > 0, and that this free boundary can be globally parametrized as a Lipschitz continuous graph under some additional non-degeneracy hypothesis; we investigate solutions which are, in the region u > 0, planar and linear at infinity in the propagation direction, with slope equal to the propagation speed.Comment: 40 pages, 1 figur
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