An electron of mass m moves in a square lattice of lattice spacing a.
The lattice potential is represented by Dirac-delta functions;
.
(a) For the nearly free-electron approximation to work, in what sense must
the potential be ``weak"? (What is the condition on 
?)
(b) How big is the energy gap between the second and third lowest energy bands?
Laszlo Mihaly
Thu Oct 31 13:23:11 EST 1996
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