Strong coupling between local moments and superconducting 'heavy' electrons in UPd2Al3

The electronic structure of heavy-fermion compounds arises from the interac tion of nearly localized 4f- or 5f-shell electrons (with atomic magnetic mo ments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields iti nerant electrons having an effective mass about 100 times (or more) the bar e electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature(1,2) of thi s compound, whereas magnetism generally suppresses superconductivity in con ventional metals. Here we report the detection of a dispersive excitation o f the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This 'magnetic exciton' is a localized ex citation which moves through the lattice as a result of exchange forces bet ween the magnetic moments. By combining this observation with previous tunn elling measurements on this material(3), we argue that these magnetic excit ons may produce effective interactions between the itinerant electrons, and so be responsible for superconductivity in a manner analogous to the role played by phonons in conventional superconductors.