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arXiv 1508.05097cond-mat.dis-nn
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Anderson和多体局域化跃迁的前向近似作为平均场近似

The forward approximation as a mean field approximation for the Anderson and Many Body Localization transitions

Francesca Pietracaprina, Valentina Ros, Antonello Scardicchio

摘要:

在本文中,我们分析了在一些模型,表现出安德森(单)或多体局域相的前向近似的预测。这种近似,其中包括在求和的振幅只有最短的路径在定位器的扩展,是已知的高估的临界值的障碍,确定的发病的本地化阶段。然而,近似提供的结果变得越来越准确的局部协调(维数)的图形,定义的跳跃矩阵,是更大的。在这个意义上,前向近似可以看作是无限维中Anderson跃迁的平均场论。的总和可以有效地计算使用传输矩阵技术,并与最精确的精确对角化的结果进行比较。对于安德森问题,我们发现一个临界值的障碍,这是$0.9\%$关闭最精确的可用数值已经在5个空间维度,而对于多体局域相的海森堡模型与随机场的临界障碍$h_c=4.0\pm 0.3$是惊人的接近最近的结果通过精确对角化。在这两种情况下,我们得到一个临界指数$ν=1$。在安德森的情况下,后者不显示依赖于维数,因为它是常见的平均场近似。我们讨论了单体和多体问题的最短路径之间的相关性,并评论我们的结果与随机介质中定向聚合物问题的连接。

英文摘要:

In this paper we analyze the predictions of the forward approximation in some models which exhibit an Anderson (single-) or many-body localized phase. This approximation, which consists in summing over the amplitudes of only the shortest paths in the locator expansion, is known to over-estimate the critical value of the disorder which determines the onset of the localized phase. Nevertheless, the results provided by the approximation become more and more accurate as the local coordination (dimensionality) of the graph, defined by the hopping matrix, is made larger. In this sense, the forward approximation can be regarded as a mean field theory for the Anderson transition in infinite dimensions. The sum can be efficiently computed using transfer matrix techniques, and the results are compared with the most precise exact diagonalization results available. For the Anderson problem, we find a critical value of the disorder which is $0.9\%$ off the most precise available numerical value already in 5 spatial dimensions, while for the many-body localized phase of the Heisenberg model with random fields the critical disorder $h_c=4.0\pm 0.3$ is strikingly close to the most recent results obtained by exact diagonalization. In both cases we obtain a critical exponent $ν=1$. In the Anderson case, the latter does not show dependence on the dimensionality, as it is common within mean field approximations. We discuss the relevance of the correlations between the shortest paths for both the single- and many-body problems, and comment on the connections of our results with the problem of directed polymers in random medium.

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