873 research outputs found

    Critical behavior of plastic depinning of vortex lattices in two dimensions: Molecular dynamics simulations

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    Using molecular dynamics simulations, we report a study of the dynamics of two-dimensional vortex lattices driven over a disordered medium. In strong disorder, when topological order is lost, we show that the depinning transition is analogous to a second order critical transition: the velocity-force response at the onset of motion is continuous and characterized by critical exponents. Combining studies at zero and nonzero temperature and using a scaling analysis, two critical expo- nents are evaluated. We find v\sim (F-F_c)^\beta with \beta=1.3\pm0.1 at T=0 and F>F_c, and v\sim T^{1/\delta} with \delta^{-1}=0.75\pm0.1 at F=F_c, where F_c is the critical driving force at which the lattice goes from a pinned state to a sliding one. Both critical exponents and the scaling function are found to exhibit universality with regard to the pinning strength and different disorder realizations. Furthermore, the dynamics is shown to be chaotic in the whole critical region.Comment: 8 pages, 6 figure

    The elastic depinning transition of vortex lattices in two dimensions

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    Large scale numerical simulations are used to study the elastic dynamics of two-dimensional vortex lattices driven on a disordered medium in the case of weak disorder. We investigate the so-called elastic depinning transition by decreasing the driving force from the elastic dynamical regime to the state pinned by the quenched disorder. Similarly to the plastic depinning transition, we find results compatible with a second order phase transition, although both depinning transitions are very different from many viewpoints. We evaluate three critical exponents of the elastic depinning transition. β=0.29±0.03\beta = 0.29 \pm 0.03 is found for the velocity exponent at zero temperature, and from the velocity-temperature curves we extract the critical exponent δ1=0.28±0.05\delta^{-1} = 0.28 \pm 0.05. Furthermore, in contrast with charge-density waves, a finite-size scaling analysis suggests the existence of a unique diverging length at the depinning threshold with an exponent ν=1.04±0.04\nu= 1.04 \pm 0.04, which controls the critical force distribution, the finite-size crossover force distribution and the intrinsic correlation length. Finally, a scaling relation is found between velocity and temperature with the β\beta and δ\delta critical exponents both independent with regard to pinning strength and disorder realizations.Comment: 17 pages, 10 figure

    Hiding relativistic degrees of freedom in the early universe

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    We quantify the extent to which extra relativistic energy density can be concealed by a neutrino asymmetry without conflicting with the baryon asymmetry measured by the Wilkinson Microwave Anisotropy Probe (WMAP). In the presence of a large electron neutrino asymmetry, slightly more than seven effective neutrinos are allowed by Big Bang Nucleosynthesis (BBN) and WMAP at 2\sigma. The same electron neutrino degeneracy that reconciles the BBN prediction for the primordial helium abundance with the observationally inferred value also reconciles the LSND neutrino with BBN by suppressing its thermalization prior to BBN.Comment: 5 pages, 5 figure

    WMAPping out Neutrino Masses

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    Recent data from from the Wilkinson Microwave Anisotropy Probe (WMAP) place important bounds on the neutrino sector. The precise determination of the baryon number in the universe puts a strong constraint on the number of relativistic species during Big-Bang Nucleosynthesis. WMAP data, when combined with the 2dF Galaxy Redshift Survey (2dFGRS), also directly constrain the absolute mass scale of neutrinos. These results impinge upon a neutrino oscillation interpretation of the result from the Liquid Scintillator Neutrino Detector (LSND). We also note that the Heidelberg--Moscow evidence for neutrinoless double beta decay is only consistent with the WMAP+2dFGRS data for the largest values of the nuclear matrix element.Comment: 5 pages, 2 figures. v2:Streamlined discussion of thermalization; improved discussion of large lepton asymmetry. v3: Added References. v4: More conservative treatment of bia

    N=2 Current Algebras for Non-Semi-Simple Groups

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    We examine the problem of constructing N=2 superconformal algebras out of N=1 non-semi-simple affine Lie algebras. These N=2 superconformal theories share the property that the super Virasoro central charge depends only on the dimension of the Lie algebra. We find, in particular, a construction having a central charge c=9. This provides a possible internal space for string compactification and where mirror symmetry might be explored.Comment: 10 pages, BONN-HE-94-0

    Superstring Gravitational Wave Backgrounds with Spacetime Supersymmetry

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    We analyse the stringy gravitational wave background based on the current algebra E2cE^{c}_{2}. We determine its exact spectrum and construct the modular invariant vacuum energy. The corresponding N=1 extension is also constructed. The algebra is again mapped to free bosons and fermions and we show that this background has N=4 (N=2) unbroken spacetime supersymmetry in the type II (heterotic case).Comment: 14 pages, CERN-TH.7218/94, HUN-IEP-94/5, LPTENS-94/10 (references updated

    Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy

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    We consider the effect of inhomogeneous neutrino degeneracy on Big Bang nucleosynthesis for the case where the distribution of neutrino chemical potentials is given by a Gaussian. The chemical potential fluctuations are taken to be isocurvature, so that only inhomogeneities in the electron chemical potential are relevant. Then the final element abundances are a function only of the baryon-photon ratio η\eta, the effective number of additional neutrinos ΔNν\Delta N_\nu, the mean electron neutrino degeneracy parameter ξˉ\bar \xi, and the rms fluctuation of the degeneracy parameter, σξ\sigma_\xi. We find that for fixed η\eta, ΔNν\Delta N_\nu, and ξˉ\bar \xi, the abundances of helium-4, deuterium, and lithium-7 are, in general, increasing functions of σξ\sigma_\xi. Hence, the effect of adding a Gaussian distribution for the electron neutrino degeneracy parameter is to decrease the allowed range for η\eta. We show that this result can be generalized to a wide variety of distributions for ξ\xi.Comment: 9 pages, 3 figures, added discussion of neutrino oscillations, altered presentation of figure

    Exact four dimensional string solutions and Toda-like sigma models from `null-gauged' WZNW theories

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    We construct a new class of exact string solutions with a four dimensional target space metric of signature (,+,+,+-,+,+,+) by gauging the independent left and right nilpotent subgroups with `null' generators of WZNW models for rank 2 non-compact groups GG. The `null' property of the generators (Tr(NnNm)=0{\rm Tr }(N_n N_m)=0) implies the consistency of the gauging and the absence of \a'-corrections to the semiclassical backgrounds obtained from the gauged WZNW models. In the case of the maximally non-compact groups (G=SL(3),SO(2,2),SO(2,3),G2G= SL(3), SO(2,2), SO(2,3), G_2) the construction corresponds to gauging some of the subgroups generated by the nilpotent `step' operators in the Gauss decomposition. The rank 2 case is a particular example of a general construction leading to conformal backgrounds with one time-like direction. The conformal theories obtained by integrating out the gauge field can be considered as sigma model analogs of Toda models (their classical equations of motion are equivalent to Toda model equations). The procedure of `null gauging' applies also to other non-compact groups.Comment: 33 p., harvmac, Imperial/TP/93-94/17 (minor changes: few explanations added

    Supersymmetric Toda Field Theories

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    We present new supersymmetric extensions of Conformal Toda and AN(1)A^{(1)}_N Affine Toda field theories. These new theories are constructed using methods similar to those that have been developed to find supersymmetric extensions of two-dimensional bosonic sigma models with a scalar potential. In particular, we show that the Conformal Toda field theory admits a (1,1)-supersymmetric extension, and the AN(1)A^{(1)}_N Affine Toda field admits a (1,0)-supersymmetric extension.Comment: 13 page
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