Bulk superconductivity in PuCoGa[sub 5] at 18.5[sup +]

Elemental plutonium is a fascinating metal yet poorly understood. A significant aspect of its complexity is the ambivalent nature of Pu's 5f electrons, which frequently appear to be neither completely localized nor hlly itinerant. This issue is emphasized in PuCoGas, the first Pu-based superconductor and with a T, exceeding 18 K. Though conventional phonon-mediated pairing may be responsible for its superconductivity, this view must be reconciled with three observations: (1) above T,, the static susceptibility of PuCoGa5 is Curie-Weiss-like with an effective moment of 0.68 p {approx}c,lo se to that expected for Pu'; (2) its Sommerfeld specific heat coefficient of 77 mJ/moleK2 indicates that electronic correlations cannot be ignored; and, (3) isostructural UCoGa5 is a Pauli paramagnet with much smaller Sommerfeld coefficient and is not superconducting above 0.3 K. Rather than a conventional superconductor, PuCoGa5 appears to be more nearly analogous to the isostructural heavy-fmion superconductor CeCoIn5 in which antiferromagnetic spin fluctuations mediate Cooper pairing. Also as in CeCoIn5, the T=O orbital upper critical field of 74 T in PuCoGas exceeds the Pauli limit by nearly a factor of two. In a scenario of magnetically mediated superconductivity, the almost order of magnitude higher T, in PuCoGas would be attributed to stronger 5f-ligand hybridization. Perhaps PuCoGas bridges two classes of unconventional superconductors-the heavy-fermion systems and high-Tc cuprates