Phonon density of states, anharmonicity, electron-phonon coupling and   possible multigap superconductivity in the clathrate superconductors Ba8Si46   and Ba24Si100: Why is Tc different in these two compounds?
  We report a detailed study of specific heat, electrical resistivity and thermal expansion in combination with inelastic neutron and inelastic X-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 K and 1.5 K respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function and phonon anharmonicity from these measurements we discuss the important factors governing Tc and explain the difference between the two compounds. 
Rolf Lortz, Romain Viennois, Alexander Petrovic, Yuxing Wang, Pierre   Toulemonde, Christoph Meingast, Michael Marek Koza, H...