On the Dynamics of Crystal Electrons, high Momentum Regime
  We study the quantum dynamics generated by $H^{SW}=-{d^2\over dx^2}+V-x$ with $V$ a real periodic function of weak regularity. We prove that the continuous spectrum of $H^{SW}$ is never empty, and furthermore that for $V$ small enough there are no bound states. 
J. Asch, F. Bentosela, P. Duclos, G. Nenciu