Thank you for your responses. After some effort, I still do not know enough to fully understand what you mean. I think an implication of your statement could be that under certain conditions, for an occupied valence band wave packet with a k-space spread determined by a vector Δk at an initial...
Perturbation theory predicts rates of transitions between eigenstates of the unperturbed Hamiltonian, which in the independent electron model for a crystal are nonlocal Bloch wave functions or linear combinations of them that extend throughout the crystal. However, photon absorption is...