Lyapunov exponent for the laser speckle potential: a weak disorder expansion

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Anderson localization of matter waves was recently observed with cold atoms in a weak 1D disorder realized with laser speckle potential [J. Billy et al., Nature 453, 891 (2008)]. The latter is special in that it does not have spatial frequency components above certain cutoff qc. As a result, the Lyapunov exponent (LE), or inverse localization length, vanishes in Born approximation for particle wavevector k>1/2qc, and higher orders become essential. These terms, up to the order four, are calculated analytically and compared with numerical simulations. For very weak disorder, LE exhibits a sharp drop at k =1/2qc. For moderate disorder (a) the drop is less dramatic than expected from the fourth order approximation and (b) LE becomes very sensitive to the sign of the disorder skewness (which can be controlled in cold atom experiments). Both observations are related to the strongly non-Gaussian character of the speckle intensity.