Vibrational modes in silicon clathrate compounds: A key to understanding superconductivity

Abstract

Several doped type-I silicon clathrates $({\mathrm{Na}}_{8}@{\mathrm{Si}}_{46},$ ${\mathrm{K}}_{8}@{\mathrm{Si}}_{46},$ ${\mathrm{Ba}}_{8}@{\mathrm{Si}}_{46}$ and ${\mathrm{I}}_{8}@{\mathrm{Si}}_{46})$ have been investigated both experimentally by inelastic neutron scattering and x-ray-absorption spectroscopy and theoretically by ab initio calculations. We find that Ba atoms have a stronger coupling with the host lattice than Na or K ones. We show that in the superconductor ${\mathrm{Ba}}_{8}@{\mathrm{Si}}_{46}$ the coupling is effectively done by host acoustic phonons exciting guest acoustic modes. These features suggest that the host-guest vibrational coupling is a crucial parameter to explain the appearance of superconductivity in addition to the density of states near the Fermi level.