My background is in ab initio calculations of atoms and molecules, and I consider myself to have a good understanding of the underlying quantum theory of electronic structure and electronic spectra when it comes to interpreting such calculations. I'm trying to understand some solid state physics now, and I have a hard time interpreting electronic band structures.
For example, let's take a plot from this paper:
I think I have an understanding on what the x axis entails when it comes to the paths in the reciprocal space (although I find it very hard to have an intuition about it), and the meaning of the energy is clear. I can also identify conducting and insulating materials by the absence/presence of the gap at 0 eV.
However, I am having difficulty in understanding the rest of the information content in the plot. Below I list out questions I have:
What is the meaning of the fact that there's a crossing around -8 eV at the L point?
What is that the gap between the two unoccupied levels varies heavily along the k point path?
Can I extract any interesting observation from the plot, apart from checking whether the material is conducting or not?