Broken time-reversal symmetry in superconducting Pr1-xLaxPt4Ge12

The superconducting state of the filled skutterudite alloy series Pr(1-x)La(x)Rt(4)Ge(12) has been systematically studied by specific heat and zero-field muon spin relaxation (mu SR), while the critical fields have been determined by magnetization measurements. An additional inhomogeneous local magnetic field, indicative of broken time-reversal symmetry, is observed in the superconducting states of the alloys. For x less than or similar to 0.5, the broken-TRS phase sets in below a temperature T-m, distinctly lower than the superconducting transition temperature T-c. For x greater than or similar to 0.5, T-m approximate to T-c. The local field strength decreases as x -> 1, where LaPt4Ge12 is characterized by conventional pairing. The lower critical field H-c1 (T) of PrPt4Ge12 shows the onset of a second quadratic temperature region below Tq similar to T-m. Upper critical field H-c2 (T ) measurements suggest multiband superconductivity, and point gap nodes are consistent with the specific heat data. In Pr(1-x)La(x)Rt(4)Ge(12), only a single specific-heat discontinuity is observed at T-c, in contrast to the second jump seen in PrOs4Sb12 below T-c. These results suggest that superconductivity in PrPt4Ge12 is characterized by a complex order parameter.