67,757 research outputs found

    Driven interfaces in random media at finite temperature : is there an anomalous zero-velocity phase at small external force ?

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    The motion of driven interfaces in random media at finite temperature TT and small external force FF is usually described by a linear displacement hG(t)V(F,T)th_G(t) \sim V(F,T) t at large times, where the velocity vanishes according to the creep formula as V(F,T)eK(T)/FμV(F,T) \sim e^{-K(T)/F^{\mu}} for F0F \to 0. In this paper, we question this picture on the specific example of the directed polymer in a two dimensional random medium. We have recently shown (C. Monthus and T. Garel, arxiv:0802.2502) that its dynamics for F=0 can be analyzed in terms of a strong disorder renormalization procedure, where the distribution of renormalized barriers flows towards some "infinite disorder fixed point". In the present paper, we obtain that for small FF, this "infinite disorder fixed point" becomes a "strong disorder fixed point" with an exponential distribution of renormalized barriers. The corresponding distribution of trapping times then only decays as a power-law P(τ)1/τ1+αP(\tau) \sim 1/\tau^{1+\alpha}, where the exponent α(F,T)\alpha(F,T) vanishes as α(F,T)Fμ\alpha(F,T) \propto F^{\mu} as F0F \to 0. Our conclusion is that in the small force region α(F,T)<1\alpha(F,T)<1, the divergence of the averaged trapping time τˉ=+\bar{\tau}=+\infty induces strong non-self-averaging effects that invalidate the usual creep formula obtained by replacing all trapping times by the typical value. We find instead that the motion is only sub-linearly in time hG(t)tα(F,T)h_G(t) \sim t^{\alpha(F,T)}, i.e. the asymptotic velocity vanishes V=0. This analysis is confirmed by numerical simulations of a directed polymer with a metric constraint driven in a traps landscape. We moreover obtain that the roughness exponent, which is governed by the equilibrium value ζeq=2/3\zeta_{eq}=2/3 up to some large scale, becomes equal to ζ=1\zeta=1 at the largest scales.Comment: v3=final versio

    Coherence Resonance in Chaotic Systems

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    We show that it is possible for chaotic systems to display the main features of coherence resonance. In particular, we show that a Chua model, operating in a chaotic regime and in the presence of noise, can exhibit oscillations whose regularity is optimal for some intermediate value of the noise intensity. We find that the power spectrum of the signal develops a peak at finite frequency at intermediate values of the noise. These are all signatures of coherence resonance. We also experimentally study a Chua circuit and corroborate the above simulation results. Finally, we analyze a simple model composed of two separate limit cycles which still exhibits coherence resonance, and show that its behavior is qualitatively similar to that of the chaotic Chua systemComment: 4 pages (including 4 figures) LaTeX fil

    Notes on Decoherence at Absolute Zero

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    The problem of electron decoherence at low temperature is analyzed from the perspective of recent experiments on decoherence rate measurement and on related localization phenomena in low-dimensional systems. Importance of decoherence at zero temperature, perhaps induced by quantum fluctuations, is put in a broader context.Comment: 7 pages in PRB format, 1 figur

    Dynamical correlation functions of one-dimensional superconductors and Peierls and Mott insulators

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    I construct the spectral function of the Luther-Emery model which describes one-dimensional fermions with one gapless and one gapped degree of freedom, i.e. superconductors and Peierls and Mott insulators, by using symmetries, relations to other models, and known limits. Depending on the relative magnitudes of the charge and spin velocities, and on whether a charge or a spin gap is present, I find spectral functions differing in the number of singularities and presence or absence of anomalous dimensions of fermion operators. I find, for a Peierls system, one singularity with anomalous dimension and one finite maximum; for a superconductor two singularities with anomalous dimensions; and for a Mott insulator one or two singularities without anomalous dimension. In addition, there are strong shadow bands. I generalize the construction to arbitrary dynamical multi-particle correlation functions. The main aspects of this work are in agreement with numerical and Bethe Ansatz calculations by others. I also discuss the application to photoemission experiments on 1D Mott insulators and on the normal state of 1D Peierls systems, and propose the Luther-Emery model as the generic description of 1D charge density wave systems with important electronic correlations.Comment: Revtex, 27 pages, 5 figures, to be published in European Physical Journal

    Coherence scale of coupled Anderson impurities

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    For two coupled Anderson impurities, two energy scales are present to characterize the evolution from local moment state of the impurities to either of the inter-impurity singlet or the Kondo singlet ground states. The high energy scale is found to deviate from the single-ion Kondo temperature and rather scales as Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction when it becomes dominant. We find that the scaling behavior and the associated physical properties of this scale are consistent with those of a coherence scale defined in heavy fermion systems.Comment: 10 pages, 7 figures, extended versio
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