New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. The crystal structure and composition were investigated by x-ray diffraction and microprobe analysis. Magnetic susceptibility, specific heat, and electrical resistivity measurements evidence superconductivity in LaPt4Ge12 and PrPt4Ge12 below 8.3 K. The parameters of the normal and superconducting states were established. Strong coupling and a crystal electric field singlet ground state is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level. Similar behavior with lower Tc was observed for SrPt4Ge12 and BaPt4Ge12.

Zero-and longitudinal-field muon-spin-rotation ͑SR͒ experiments were performed on the superconductors PrPt 4 Ge 12 and LaPt 4 Ge 12 . In PrPt 4 Ge 12 below T c a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry ͑TRS͒ breaking in PrPt 4 Ge 12 below T c = 7.9 K. This remarkably high T c for an anomalous superconductor and the weak and gradual change in T c and of the related specific-heat anomaly upon La substitution in La 1−x Pr x Pt 4 Ge 12 suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs.

The filled skutterudite superconductor PrPt4Ge12 was studied in muon-spin rotation (µSR), specific heat, and electrical resistivity experiments. The continuous increase of the superfluid density with decreasing temperature and the dependence of the magnetic penetration depth λ on the magnetic field obtained by means of µSR, as well as the observation of a T 3 dependence of the electronic specific heat establish the presence of point-like nodes in the superconducting energy gap. The energy gap was found to be well described by ∆ = ∆0|kx ± iky| or ∆ = ∆0(1 −k 4 y ) functional forms, similar to that obtained for another skutterudite superconductor, PrOs4Sb12. The gap to Tc ratios were estimated to be ∆0/kBTc = 2.68(5) and 2.29(5), respectively. The filled skutterudite compounds RM 4 P n 12 (R = rare-earth or alkaline-earth, M = Fe, Ru, Os, and P n = P, As, Sb) have attracted much attention in the recent years. Depending on the composition they may undergo metal-insulator transitions [1], show conventional or unconventional superconductivity [2,3,4], become magnetic [5,6,7], semiconducting [8], or exhibit the Kondo effect [9] etc. One of the most interesting discoveries was superconducting Pr-filled skutterudites, since within the traditional mechanism of s−wave Cooper pairing the Pr magnetism would destroy superconductivity. The first compound demonstrating such unusual behavior was PrOs 4 Sb 12 [2]. The Cooper-pairing mechanism and the corresponding symmetry of the superconducting energy gap in PrOs 4 Sb 12 are still under debate. Studies of the thermal conductivity suggested the presence of two distinct superconducting phases [10]. Penetration depth experiments were pointing either to a possible nodal structure of the gap [11] or to a gap without nodes [3]. A number of experimental techniques such as scanning tunneling microscopy [12], thermal conductivity [13], and specific heat measurements [14] are in agreement with a fully developed isotropic s−wave superconducting gap.Recently, a new Pr-containing skutterudite superconductor PrPt 4 Ge 12 was discovered [15]. Specific heat experiments reveal strongly coupled superconductivity with a transition temperature T c ≃ 7.9 K, factor of 4 higher than that in PrOs 4 Sb 12 [2], thus making PrPt 4 Ge 12 more accessible for e.g. spectroscopic studies. Here, we report on a study of PrPt 4 Ge 12 by means of muon-spin rotation (µSR), specific heat, and electrical resistivity. The linear increase of the superfluid density (ρ s ) with decreasing temperature, its dependence on the magnetic field, as well as the observation of a T 3 dependence of the electronic specific heat document presence of the point-like nodes in the energy gap. The temperature dependence of ρ s was analyzed with various gap models suggested previously for PrOs 4 Sb 12 and found to be well described by two models of axial symmetry with point-like nodes:The sample preparation procedure of PrPt 4 Ge 12 is described in [15]. The transverse field (TF) µSR experiments were performed at the πM3 beam line at the ...