The absolute density of states of isostructural ${\mathrm{Ba}}_{24}{\mathrm{Si}}_{100}$ and ${\mathrm{Ba}}_{24}{\mathrm{Ge}}_{100}$ near the Fermi level, which is very relevant for discussion on phonon-mediated superconductivity, is quantitatively evaluated to be $0.28\mathrm{\text{states}}{\mathrm{eV}}^{-1}/\mathrm{Ge}\mathrm{\text{atom}}$ and $0.18\mathrm{\text{states}}{\mathrm{eV}}^{-1}/\mathrm{Si}\mathrm{\text{atom}}$ by combining soft x-ray photoelectron spectroscopy and magnetic susceptibility measurements. The energetics of the anharmonic motions of the endohedral Ba atoms are also discussed on the basis of the $4d$-core-level spectra. The discussion based on these important physical parameters concludes that the unique electron-phonon interactions do not occur due to the local Jahn-Teller distortion of the fragile open cage structure in the phonon-mediated BCS formalism. Compounds with a rigid cage would be essential to give rise to anticipated unique interactions as a key ingredient favored for superconducting paring.

• Received 30 July 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.176402