That limit suggests the choice of an atomic wave function as an approximate form for the Wannier function, the so-called tight binding approximation.
2.
The Rashba model in solids can be derived in the framework of the \ boldsymbol k \ cdot \ boldsymbol p theory or from the point of view of a tight binding approximation.
3.
With the band structure ( derived with the Kronig-Penny tight binding approximation ) model, you can attribute this negative differential mobility to a higher-energy, lower curvature band beginning to populate and thereby reduce the net differential velocity with higher field rather than increase it.
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