Abstract

Superconductivity has been observed in ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ at ${T}_{C}=1.85 \mathrm{K}$ and appears to involve heavy fermion quasiparticles with an effective mass ${m}^{*}\ensuremath{\sim}50 {m}_{e}$ as inferred from the jump in the specific heat at ${T}_{C},$ the upper critical field near ${T}_{C},$ and the normal state electronic specific heat. Thermodynamic and transport measurements suggest that the heavy fermion state has a quadrupolar origin, although a magnetic origin cannot be completely ruled out.