“ a two-dimensional material ”
Yeah, I’m calling BS.”
Mathematically, you’re 100% correct.
This means a material where the atoms are bonded horizontally, with no vertical bonds, like graphene. The bonds are much stronger in this format.
It does have a non-zero height dimension, but it is so small, you can treat it as zero.
Graphene is a zero-gap semiconductor, because its conduction and valence bands meet at the Dirac points. The Dirac points are six locations in momentum space, on the edge of the Brillouin zone, divided into two non-equivalent sets of three points. The two sets are labeled K and K'. The sets give graphene a valley degeneracy of gv = 2. By contrast, for traditional semiconductors the primary point of interest is generally Γ, where momentum is zero. Four electronic properties separate it from other condensed matter systems.
In case that helps.