Multiple superconducting phases in skutterudite PrOs4Sb12

The skutterudite PrOs4Sb12 is a Heavy Fermion (HF) superconductor (sc) below Tc=1.8 K [Phys. Rev. B 65 (2002) 100506(R)]. Specific heat measurements indicate two separate sc transitions. Recent angle dependent magnetothermal conductivity experiments [Phys. Rev. Lett. 90 (2003) 117001] have shown that PrOs4Sb12 is another of the very few examples of multiphase-superconductivity. Two phases A and B with different node structures have been identified. We propose a model for their anisotropic gap function Δ(k). While the high-field A-phase has fourfold planar symmetry (tetragonal s + g-wave) with four point nodes along two cubic axis, the low-field B-phase has lower symmetry with only two point nodes. We discuss the field-angle dependent magnetothermal conductivity and propose a microscopic pairing mechanism