@@ -197,11 +197,7 @@ A joint absorbtion of a photon and a phonon collision may excite an electron acr
## Summary
* In periodic potential all electron states are **Bloch waves**
* Electron dispersion is organized into **energy bands** that may overlap, or may be separated by **band gaps**
* If the lattice potential is weak, the dispersion can be obtained by copying $p^2/2m$ into different Brillouin zones, and opening gaps at every level crossing. Each gap is equal to the Fourier component of the lattice potential.
* If the number of electrons per unit cell is odd, the material must be conducting.
* Each band hosts $2N$ electrons, therefore a material with odd number of electrons is a metal; that with an even number of electrons may be an insulator.
* Each band can host two electrons per unit cell, therefore a material with an odd number of electrons is a metal; that with an even number of electrons may be an insulator.
* Light absorption is a tool to measure the band gap, and it distinguishes **direct** from **indirect** band gaps.
