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A century on from its discovery, a complete fundamental understanding ofsuperconductivity is still missing. Considerable research efforts are currentlydevoted to elucidating mechanisms by which pairs of electrons can bind togetherthrough the mediation of a boson field different than the one associated to thevibrations of a crystal lattice. PuCoGa_5, a 5f-electron heavy-fermionsuperconductor with a record critical temperature T_c=18.5 K, is one of themany compounds for which the short-range, isotropic attraction provided bysimple electron-phonon coupling does not appear as an adequate glue forelectron pairing. Here, we report the results of point-contact spectroscopymeasurements in single crystals of PuCoGa_5. Andreev reflection structures areclearly observed in the low-temperature spectra, and unambiguously prove thatthe paired superconducting electrons have wavefunction with the d-wave symmetryof a four-leaf clover. A straightforward analysis of the spectra provide theamplitude of the gap and its temperature dependence, \Delta(T). We obtain\Delta(T -> 0) = 5.1 \pm 0.3 meV and a gap ratio, 2\Delta/k_B T_c = 6.5 \pm0.3, indicating that the compound is in the regime of strong electron-bosoncoupling. The gap value and its temperature dependence can be well reproducedwithin the Eliashberg theory for superconductivity if the spectral function ofthe mediating bosons has a spin-fluctuations-like shape, with a peak energy of6.5 meV. Electronic structure calculations, combining the local densityapproximation with an exact diagonalization of the Anderson impurity model,provide a hint about the possible origin of the fluctuations.

Strong-coupling d-wave superconductivity in PuCoGa_5 probed by point contact spectroscopy