We report on muon-spin relaxation measurements of the $4f^2$-based heavy-fermion superconductor filled-skutterudite ${\mathrm{Pr}\mathrm{}\mathrm{O}\mathrm{s}}_4{\mathrm{S}\mathrm{b}}_{12}$. The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature, providing unambiguous evidence for the breaking of time-reversal symmetry in the superconducting state. A discussion is made on which of the spin or orbital component of Cooper pairs carries a nonzero momentum.

We have studied Fermi surface properties of LaRhIn 5 and CeTIn 5 (T: Co, Rh and Ir) via the de Haas–van Alphen (dHvA) experiment. The Fermi surface of a non-4 f reference compound LaRhIn 5 is quasi-two dimensional, reflecting the unique tetragonal crystal structure, which is consistent with the result of FLAWP energy band calculations. The Fermi surface of an antiferromagnet CeRhIn 5 is similar to that of LaRhIn 5 , although the cyclotron mass in CeRhIn 5 is roughly by one order larger than that in LaRhIn 5 . Both Fermi surfaces are, however, different from those of CeCoIn 5 and CeIrIn 5 which are well explained by the 4 f -itinerant band model. The 4 f electrons in CeRhIn 5 thus do not contribute to the volume of the Fermi surface. We have also studied magnetic and superconducting properties of CeCoIn 5 and CeIrIn 5 .

Specific heat and transport measurements have revealed an unconventional heavy-fermion (HF) state in SmOs 4 Sb 12 single crystals. The electronic specific-heat coefficient (γ=0.82 J/K 2 mol) and the coefficient ( A ) of the quadratic temperature dependence of electrical resistivity are largely enhanced, although the ratio A γ -2 is reduced from the Kadowaki–Woods ratio of HF materials. Both γ and A do not show any significant decrease in applied field in contrast with Ce-based HF compounds, suggesting an unconventional origin of the heavy quasiparticles. A weak ferromagnetic ordering sets in below ∼3 K, probably originating in the itinerant quasiparticles.

We have investigated the resistivity $\left(\rho \right),$ thermoelectric power (TEP), and Hall coefficient ${(R}_H)$ on high-quality single crystals of $R{\mathrm{Fe}}_4{\mathrm{P}}_{12}.$ TEP in ${\mathrm{CeFe}}_4{\mathrm{P}}_{12}$ is extremely large $(\sim 0.5$ mV/K at 290 K) with a peak of $\sim 0.75$ mV/K at around 65 K. The Hall mobility also shows a peak at $\sim 65$ K, suggesting carriers with heavy masses developed at lower temperatures related to the f-hybridized band. Both Pr and Nd systems exhibit an apparent increase of $\rho$ with decreasing temperature far above their magnetic transition temperatures. In the same temperature ranges, TEP exhibits unusually large absolute values of $-50\hspace{0.5em}\mu \mathrm{V}/\mathrm{K}$ for ${\mathrm{PrFe}}_4{\mathrm{P}}_{12}$ and $-15\hspace{0.5em}\mu \mathrm{V}/\mathrm{K}$ for ${\mathrm{NdFe}}_4{\mathrm{P}}_{12},$ respectively. For ${\mathrm{PrFe}}_4{\mathrm{P}}_{12},$ such anomalous transport properties suggest an unusual ground state, possibly related to the quadrupolar Kondo effect.

Neutron diffraction experiments on the heavy-fermion superconductor PrOs 4 Sb 12 revealed that a small antiferromagnetic moment, parallel to [010] ( y -direction), is induced in the field-induced phase ( H ∥ z ). The analysis, based on the Γ 1 singlet ground state crystal field model, shows that antiferro-order of O y z -type quadrupole moments of Pr ions is formed in the field-induced phase. This strongly suggests that the crystal field ground-state of Pr ions in PrOs 4 Sb 12 is the Γ 1 singlet and the quadrupolar ordering is induced due to level crossing with an excited state under a magnetic field.