◎代POST
◎急需
( A)(1)for the delta-function potential with P<<1,
find at k=0 the energy of the lowest energy band
(2) for the same problem find the band gap at k= π/a
( B)(3) show that for diamond stucture the Fourier component
Ug of the crystal potential seen by an electron is
equal to zero for G=2A
(4) show that in the usual first-order approximation to the
solutions of the wave equation in a periodic lattice the
energy gap vanishes at the zone boundary plane normal to
the end of the vector A
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