Searching for the Interlayer Band and Unravelling the Bonding in β-ThSi2 and α-ThSi2 with NMTO Wannier-like Functions

Using the newly developed NMTO (muffin-tin orbitals of order N) technique, which generates atom-centered, localized, Wannier-like functions, we investigate whether-such as in graphites-an interlayer band exists at the Fermi level in ThSi2 with six electrons per Si. We find that the Th s-like NMTO, not only in the graphite-structured beta-ThSi2 but also in the twisted allotrope, alpha-ThSi2, strikingly resembles the previously calculated interlayer NMTO in graphites and that the corresponding band cuts the Fermi level but is at many places gapped, particulaly in the twisted allotrope. This suggests that the mechanism of superconductivity in both allotropes of ThSi2 is similar to the one in intercalated graphites such as CaC6 with 4 1/3 electrons per C. Here, C buckling and Ca in-plane phonons couple between interlayer and pi* bands. In ThSi2, not only these bands but also the sigma* band cross the Fermi level, and intraband coupling via Si bond-stretching modes, like in MgB2, is likely to contribute to the superconductivity. Finally, we investigate the bonding in ThSi2 by seeking those NMTO basis sets with merely nine or six orbitals per ThSi2, which best span the occupied bands. Our results indicate that in both allotropes there are three weakened Si-Si sigma bonds, two Th-Si sp(z) bonds, and a nonbonding Si p(z)-orbital.