Superconductivity and the high-field ordered phase in the heavy-fermion compound PrOs₄Sb₁₂

Superconductivity is observed in the filled skutterudite compound PrOs₄Sb₁₂ below a critical temperature T_c = 1.85 K and appears to develop out of a nonmagnetic heavy Fermi liquid with an effective mass m* ≈ 50 me_e, where m_e is the free electron mass. Features associated with a cubic crystalline electric field are present in magnetic susceptibility, specific heat, electrical resistivity, and inelastic neutron scattering measurements, yielding a Pr³⁺ energy level scheme consisting of a Γ₃ nonmagnetic doublet ground state, a low-lying Γ₅ triplet excited state at ∼ 10 K, and Γ₄ triplet and Γ₁ singlet excited states at much higher temperatures. Measurements also indicate that the superconducting state is unconventional and consists of two distinct superconducting phases. At high fields and low temperatures, an ordered phase of magnetic or quadrupolar origin is observed, suggesting that the superconductivity may occur in the vicinity of a magnetic or quadrupolar quantum critical point.