Broken time-reversal symmetry in superconducting Pr1-xCexPt4Ge12

We report results of zero-field muon spin relaxation experiments on the filled-skutterudite superconductors Pr1-xCexPt4Ge12, x = 0, 0.07, 0.1, and 0.2, to investigate the effect of Ce doping on broken time-reversal symmetry (TRS) in the superconducting state. In these alloys broken TRS is signaled by the onset of a spontaneous static local magnetic field Bs below the superconducting transition temperature. We find that Bs decreases linearly with x and -> 0 at x approximate to 0.4, close to the concentration above which superconductivity is no longer observed. The (Pr,Ce)Pt4Ge12 and isostructural (Pr,La)Os4Sb12 alloy series both exhibit superconductivity with broken TRS, and in both the decrease of Bs is proportional to the decrease of Pr concentration. This suggests that Pr-Pr intersite interactions are responsible for the broken TRS. The two alloy series differ in that the La-doped (a)lloys are superconducting for all La concentrations, suggesting that in (Pr,Ce)Pt4Ge12 pair-breaking by Ce doping suppresses superconductivity. For all x the dynamic muon spin relaxation rate decreases somewhat in the superconducting state. This may be due to Korringa relaxation by conduction electrons, which is reduced by the opening of the superconducting energy gap.