STARK LADDERS AND ZENER TUNNELING

In connection with the controversial question about the interband matrix elements of a stair-function potential in a crystal, the authors show analytically that they are non-vanishing, in general, for an arbitrary band model for both an infinite and a finite crystal. This is in accord with numerical studies of specialised models, but in contradiction to other analytical claims, which are analysed in some detail. The connection to the phenomena of Stark ladders and Zener tunnelling is discussed.