Fig. 1

The crystal lattice of graphene and its elementary cell. Basic distance between atoms is denoted by a. The vectors b ₁, b ₂ determines the crystallographic directions. The unit cell contains two atoms which are marked by numerals “0” (basic atom) and “1” (an extra atom). Numerals “0 ^/”, “0 ^//” denote the atoms of neighboring unit cells equivalent to atom “0”. Numerals “1 ^/”, “1 ^//” denote the atoms of neighboring unit cells equivalent to atom “1”. Two circles with centers in atoms “0” and “1” shows the first two coordinative “spheres,” which limit the consideration by the approximation of the nearest neighbors