科研之友
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摘要
Elastic systems, such as magnetic domain walls, density waves, contact lines, and cracks, are pinned by substrate disorder. When driven, they move via avalanches, with power law distributions of size, duration and velocity. Their exponents, and the shape of an avalanche, defined as its mean velocity as a function of time, were studied. They are known approximatively from experiments and simulations, and were predicted from mean- field models, such as the Brownian force model (BFM), where each point of the elastic interface sees a force field which itself is a random walk. As we showed in EPL, 97 (2012) 46004, the BFM is the starting point for an epsilon = d(c) - d expansion around the upper critical dimension, with d(c) = 4 for short- ranged elasticity, and d(c) = 2 for long-ranged elasticity. Here we calculate analytically the O(epsilon), i.e. 1-loop, correction to the avalanche shape at fixed duration T, for both types of elasticity. The exact expression, though different from the phenomenological form presented by Laurson et al. in Nat. Commun., 4 (2013) 2927, is well approximated by <(u)over dot (t = xT)>(T) similar or equal to [Tx(1 - x)](gamma-1) exp(A[1/2 - x]), 0 < x < 1. The asymmetry A approximate to -0.336(1- d/d(c)) is negative for d close to d(c), skewing the avalanche towards its end, as observed in numerical simulations in d = 2 and 3. The exponent gamma = (d + zeta)/z is given by the two independent exponents at depinning, the roughness zeta and the dynamical exponent z. We propose a general procedure to predict other avalanche exponents in terms of zeta and z. We finally introduce and calculate the shape at fixed avalanche size, not yet measured in experiments or simulations.
- 出版日期2014-12
