Evidence for time-reversal symmetry breaking in superconducting PrPt 4 Ge12

Abstract

Zero- and longitudinal-field muon-spin-rotation (μSR) experiments were performed on the superconductors PrPt4 Ge12 and LaPt 4 Ge12. In PrPt4 Ge12 below Tc a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in PrPt4 Ge12 below Tc =7.9 K. This remarkably high Tc for an anomalous superconductor and the weak and gradual change in Tc and of the related specific-heat anomaly upon La substitution in La1-x Prx Pt4 Ge12 suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs. © 2010 The American Physical Society.