Re: CCL:Periodic systems: Fermi level at the band crossing

 On Tue, 4 Dec 2001, Konstantin N. Kudin wrote:
 >  I am looking for examples of metallic 2D and 3D periodic systems that
 > have an orbital crossing exactly at the Fermi level. In such a case if the
 > exact position of the crossing is missed, the system appears to be an
 > insulator (or semiconductor). Any examples, references or even cartesian
 > coordinates for unit cells are greatly appreciated.
 >
 >
 >  Example - 1D periodic (3,3) carbon nanotube
 >
 >   *     *
 >    *  *
 >     *   --- Fermi level
 >   *  *  --- seeming top of the occupied bands if the crossing is missed
 > *     *
 >
 A single sheet of graphite has pi and pi* bands touching at the Fermi
 level, at the edge of the BZ. That degeneracy arises from the crystal
 symmetry; isoelectronic BN has a band gap. Not sure what you mean by
 'missing the exact position' though, for the degeneracy and therefore
 metallic behaviour will remain while the symmetry does even if electrons
 are added or removed. Graphite is strictly a semimetal as its DOS is
 zero at the Fermi level (near zero for the 3D crystal).
 David
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