Broken Time-Reversal Symmetry in Superconducting Partially Filled Skutterudite Pr_1-\deltaPt_4Ge_12

Time reversal symmetry (TRS) is a key symmetry for classification of unconventional superconductors, and the violation of TRS often results in a wealth of novel properties. Here we report the synthesis and superconducting properties of the partially filled skutterudite Pr_1-\deltaPt_4Ge_12. The results from x-ray diffraction and magnetization measurements show that the Pt_4Ge_12 cage-forming structure survives and bulk superconductivity is preserved below the superconducting transition temperature T_\rm c=7.80 K. The temperature dependence of both the upper critical field and the electronic specific heat can be described in terms of a two-gap model, providing strong evidence of multi-band superconductivity. TRS breaking is observed using zero field muon-spin relaxation experiments, and the magnitude of the spontaneous field is nearly half of that in PrPt_4Ge_12.