title: Searching for the interlayer band and unravelling the bonding in beta-ThSi(2) and alpha-ThSi(2) with NMTO Wannier-like functions.
abstract: 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 ThSi(2) with six electrons per Si. We find that the Th s-like NMTO, not only in the graphite-structured beta-ThSi(2) but also in the twisted allotrope, alpha-ThSi(2), 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 ThSi(2) is similar to the one in intercalated graphites such as CaC(6) with 4(1)/(3) electrons per C. Here, C buckling and Ca in-plane phonons couple between interlayer and pi* bands. In ThSi(2), not only these bands but also the sigma* band cross the Fermi level, and intraband coupling via Si bond-stretching modes, like in MgB(2), is likely to contribute to the superconductivity. Finally, we investigate the bonding in ThSi(2) by seeking those NMTO basis sets with merely nine or six orbitals per ThSi(2), 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.
authors.affiliation: Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, 70569, Stuttgart, Germany. ezurek@buffalo.edu
authors.forename: Eva
authors.initials: E
authors.lastname: Zurek
authors.forename: Ove
authors.initials: O
authors.lastname: Jepsen
authors.forename: Ole Krogh
authors.initials: OK
authors.lastname: Andersen