Heavy Fermion Superconductivity in the Filled Skutterudite Compound PrOs4Sb12

The filled skutterudite compound PrOs4Sb12 exhibits superconductivity, with a supercon.ducting critical temperature T-c approximate to 1.85 K. that appears to involve heavy fermion quasiparticles with an effective mass m* approximate to 10(2)m(e), where m(e) is the free electron mass. Evidence for the heavy fermion state in PrOs4Sb12, 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 similar to 20 kbar, the normal state resistivity decreases with decreasing temperature below similar to 1 - 2 K as a power law T-n with n similar to 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 arc two distinct superconducting phases. Inelastic neutron scattering measurements are consistent with a Pr3+ energy level scheme in the cubic crystalline electric field with a Gamma(3) nonmagnetic doublet ground state that carries an electric quadrupole moment, in excellent overall agreement with the Pr3+ energy level scheme inferred from the magnetic susceptibility and specific heat measurements.