Example 13.4: The insulator versus superconductor Why is an insulator not a perfect superconductor?

[Hint: Recall Eq. (13.23). If the conduction band is full, the velocity of the electrons at the upper edge is zero. No current is induced by a magnetic field in a completely filled conduction band. The kinetic energy has a point of inflection and hence a horizontal tangent at the upper end].