Heavy Fermion Superconductivity in the Filled Skutterudite Compound PrOs₄Sb₁₂

The filled skutterudite compound PrOs 4 Sb 12 exhibits superconductivity, with a superconducting critical temperature T c ≈1.85 K, that appears to involve heavy fermion quasiparticles with an effective mass m * ≈10 2 m e , where m e is the free electron mass. Evidence for the heavy fermion state in PrOs 4 Sb 12 based on electrical resistivity, specific heat, and magnetic susceptibility measurements in the normal and superconducting states, is reviewed. New measurements are presented that reveal that T c decreases linearly with pressure to ∼20 kbar, the normal state resistivity decreases with decreasing temperature below ∼1-2 K as a power law T n with n ∼3-2.6 in magnetic fields from 30 to 80 kOe, and there is structure in the specific heat jump at T c , suggesting that there are two distinct superconducting phases. Inelastic neutron scattering measurements are consistent with a Pr 3+ energy level scheme in the cubic crystalline electric field with a Γ 3 nonmagnetic doublet ground state that carries an electric quadrupole moment, in excellent overall agreement with the Pr 3+ energy level scheme inferred from the magnetic susceptibility and specific heat measurements.