Two-electron theory of surface and impurity states

Abstract

The effect of electron-electron interactions is taken into account, by employing LÖwdin's alternant molecular orbital method, within the framework of the tight-binding approximation. The energy spectrum of the α- and β-spin electrons in a contaminated finite linear crystal is examined, and found to consist of two bands separated by a forbidden energy gap, whose width is proportional to the amount of electron-electron interaction present. Depending on certain conditions, the surface energy levels can lie outside the bands (outer states) and/or in the gap between them (inner states), while at the impurity the electrons occupy only outer or only inner states. In the case of surface states, both Tamm and Shockley types are discussed.