  The filled skutterudite compound \PrOsSb{} exhibits superconductivity below a
critical temperature $T_\mathrm{c} = 1.85$ K that develops out of a nonmagnetic
heavy Fermi liquid with an effective mass $m^{*} \approx 50 m_\mathrm{e}$,
where $m_\mathrm{e}$ 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$^{3+}$ energy level scheme that consists of a $\Gamma_{3}$
nonmagnetic doublet ground state that carries an electric quadrupole moment, a
low lying $\Gamma_{5}$ triplet excited state at $\sim 10$ K, and $\Gamma_{4}$
triplet and $\Gamma_{1}$ 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.
