Broken Time-Reversal Symmetry in Superconducting Partially Filled Skutterudite Pr1−δPt4Ge12**Supported by the National Key Research and Development Program of China under Grant Nos 2017YFA0303104 and 2016YFA0300503, the National Natural Science Foundation of China under Grant No 11774061, and the Chinese Government Scholarship of China Scholarship Council.

Abstract

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 Pr1 − δPt4Ge12. The results from x-ray diffraction and magnetization measurements show that the [Pt4Ge12] cage-forming structure survives and bulk superconductivity is preserved below the superconducting transition temperature Tc = 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 PrPt4Ge12.