The unconventional superconductivity of skutterudite PrOs ₄Sb ₁₂: Time-reversal symmetry breaking and adjacent field-induced quadrupole ordering

This review presents a summary and evaluation of the experimental properties of unconventional superconductivity in PrOs ₄Sb ₁₂. After a brief introduction of filled skutterudites, we argue that the normal-state properties of PrOs ₄Sb ₁₂ are quite different from ordinary heavy-fermion superconductors, in the sense that the 4f-electrons seem to be well localized even at low temperatures where the superconductivity appears. This is reflected in the existence of a field-induced antiferro-quadrupole ordering of Pr 4f-electrons. It is shown that the 4f collective excitations can be described as quadrupolar excitons. Their strong temperature dependence in close correlation with the superconductivity suggests a conjecture that the superconducting mechanism may involve quadrupolar-exciton- mediated pairing of some kind. Although the structure of the superconducting order parameter is far from conclusive, outstanding experimental observations reported to date reveal the uniqueness of the unconventional superconducting state. Special focus is given to time-reversal symmetry breaking in the superconducting state. The article concludes with a discussion of some key issues and desirable future work.