Physics is amazing!

And again I realized how wonderful physics is. In condensed matter theory we talked about graphene and the different band structures which develop, how we can describe a one-dimensional layer and a two-dimensional one. Well, the 2d-model is the homework and you get a nice four-band-structure plot :-)

Upper and lower electron bands (in momentum space) in graphene, touching at an infinitesimally small point.

Yes, what am I talking about... You can imagine a band for an electron as the orbit around an atom where the electron is approximately located. Electrons have got very special properties and two electrons with exactly the same properties should not be at the same place (Pauli's principle), for example, so you have to be careful with the calculation. Anyway, there's a point where the band structure of the upper layer of graphene touches the lower one, and that's what people call the Dirac fermion. Thus, with some tricks you can get an electron hopping to the other layer :-) That's cool and you can calculate it!