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Excitations in modulated (quasi-periodic) systems: collision-induced effects

It is argued that collision damping of excitations in a modulated (quasi-periodic) system has a pronounced effect on results for the frequency- and wave-vector-dependent response function observed in scattering experiments. Even a relatively small damping can remove all vestige of some of the striking features predicted for the undamped model. Results are obtained for a model that can be solved without approximation in the absence of damping. The latter is introduced by extending the frequency to complex values, and the magnitude of the imaginary component is viewed as the damping control parameter.