J. Asch, F. Bentosela, P. Duclos, G. Nenciu
On the Dynamics of Crystal Electrons, 
 high Momentum Regime
(414K, .pdf)

ABSTRACT.  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.