Two energy gaps in the tunneling-conductance spectra of the superconducting clathrate Ba8Si46

We have studied the quasiparticle excitation spectrum of the superconductor Ba8Si46 by local tunneling spectroscopy. Using high-energy resolution achieved in superconductor-insulator-superconductor junctions we observed tunneling conductance spectra of a nonconventional shape revealing two distinct energy gaps, Delta(L)=1.3 +/- 0.1 meV and Delta(S)=0.9 +/- 0.2 meV. The analysis of tunneling data identified Delta(L) as the leading superconducting gap in the bulk material. A smaller and more dispersive gap Delta(S) is interpreted as induced either in reciprocal space, via the quasiparticle interband scattering from the leading superconducting band, or in real space, by the proximity effect to a normal layer at the surface.