Delocalization in harmonic chains with long-range correlated random masses

We study the nature of collective excitations in harmonic chains with masses exhibiting long-range correlated disorder with a power spectrum proportional to 1/k(alpha), where k is the wave vector of the modulations on the random masses landscape. Using a transfer-matrix method and exact diagonalization, we compute the localization length and participation ratio of eigenmodes within the band of allowed energies. We find extended vibrational modes in the low-energy region for alpha>1. In order to study the time evolution of an initially localized energy input, we calculate the second moment M(2)(t) of the energy spatial distribution. We show that M(2)(t), besides being dependent of the specific initial excitation and exhibiting an anomalous diffusion for weakly correlated disorder, assumes a ballistic spread in the regime alpha>1 due to the presence of extended vibrational modes.