Phonon instability of insulating states in optical lattices
  The influence of collective phonon excitations, due to intersite atomic interactions, on the stability of optical lattices is analyzed. These phonon excitations are shown to essentially reduce the ability of atoms to be localized. The states that seem to be insulating in the absence of the phonon excitations can become delocalized when the latter are present. The delocalization effect exists for both long-range as well as local atomic interactions. 
V.I. Yukalov and K. Ziegler
cond-mat.quant-gas