A New Heavy Fermion Superconductor: the Filled Skutterudite Compound Pros 4 Sb 12 *

The filled skutterudite compound PrOs4Sb12 exhibits superconductivity below a critical temperature Tc = 1.85 K that develops out of a nonmagnetic heavy Fermi liquid with an effective mass m∗ ≈ 50 me, where me is the free electron mass. Analysis of magnetic susceptibility, specific heat, electrical resistivity and inelastic neutron scattering measurements within the context of a cubic crystalline electric field yields a Pr energy level scheme that consists of a Γ3 nonmagnetic doublet ground state that carries an electric quadrupole moment, a low lying Γ5 triplet excited state at ∼ 10 K, and Γ4 triplet and Γ1 singlet excited states at much higher temperatures. The superconducting state appears to be unconventional and to consist of two distinct superconducting phases. An ordered phase of magnetic or quadrupolar origin occurs at high fields and low temperatures, suggesting that the superconductivity may occur in the vicinity of a magnetic or electric quadrupolar quantum critical point.