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arXiv 0910.4833cond-mat.dis-nn
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计算无序系统最大弛豫时间的特征值方法

Eigenvalue method to compute the largest relaxation time of disordered systems

Cecile Monthus, Thomas Garel

摘要:

我们考虑有限尺寸无序系统的动力学定义的主方程满足详细的平衡。主方程可以映射到位形空间中的薛定谔方程,其中量子哈密顿量$H$具有安德森局域紧束缚模型的一般形式。控制向玻尔兹曼平衡收敛的最大弛豫时间$t_{eq}$由$H$的最低非零本征值$E_1=1/t_{eq}$(最低本征值是$E_0=0$)确定。因此,弛豫时间$t_{eq}$可以通过能够计算第一激发能$E_1$的任何本征值方法来计算{ \it而不模拟动力学}。本文用共轭梯度法确定了每个无序样品的$E_1$,并给出了两种模型在给定尺寸的无序样品上的弛豫时间$t_{eq}$统计的数值结果:(i)对于二维$L \times L$上Hurst指数为$H$的自仿射势中的随机游动,我们发现$ψ=H$的激活标度$\ln t_{eq}(L) \sim L^ψ$与预期的一致;(ii)对于$N$自旋的Sherrington-Kirkpatrick自旋玻璃模型的动力学,我们发现用$ψ=1/3$的增长$\ln t_{eq}(N) \sim N^ψ$与大多数以前的Monte-Carlo测量一致。此外,我们发现,重新标度的分布$(\ln t_{eq})$衰减为$e^{- u^η}$的大$u$与顺序$η\simeq 1.36$的尾部指数。我们给出了一个罕见事件的解释这个值,指向一个样本到样本的波动指数为$ψ_{width} \simeq 0.26$的障碍。

英文摘要:

We consider the dynamics of finite-size disordered systems as defined by a master equation satisfying detailed balance. The master equation can be mapped onto a Schrödinger equation in configuration space, where the quantum Hamiltonian $H$ has the generic form of an Anderson localization tight-binding model. The largest relaxation time $t_{eq}$ governing the convergence towards Boltzmann equilibrium is determined by the lowest non-vanishing eigenvalue $E_1=1/t_{eq}$ of $H$ (the lowest eigenvalue being $E_0=0$). So the relaxation time $t_{eq}$ can be computed {\it without simulating the dynamics} by any eigenvalue method able to compute the first excited energy $E_1$. Here we use the 'conjugate gradient' method to determine $E_1$ in each disordered sample and present numerical results on the statistics of the relaxation time $t_{eq}$ over the disordered samples of a given size for two models : (i) for the random walk in a self-affine potential of Hurst exponent $H$ on a two-dimensional square of size $L \times L$, we find the activated scaling $\ln t_{eq}(L) \sim L^ψ$ with $ψ=H$ as expected; (ii) for the dynamics of the Sherrington-Kirkpatrick spin-glass model of $N$ spins, we find the growth $\ln t_{eq}(N) \sim N^ψ$ with $ψ=1/3$ in agreement with most previous Monte-Carlo measures. In addition, we find that the rescaled distribution of $(\ln t_{eq})$ decays as $e^{- u^η}$ for large $u$ with a tail exponent of order $η\simeq 1.36$. We give a rare-event interpretation of this value, that points towards a sample-to-sample fluctuation exponent of order $ψ_{width} \simeq 0.26$ for the barrier.

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