Superconductivity mediated by spin fluctuations in the heavy-fermion compound UPd2Al3

It is well known that any weak attractive electron-electron interaction in metals can in principle cause the formation of Cooper pairs, which then con dense into a superconducting ground state(1). In conventional superconducto rs, this attractive interaction is mediated by lattice vibrations (phonons) . But for the heavy-fermion and high-temperature superconductors, alternati ve pairing interactions are considered to be possible(2). For example, the low-temperature properties of heavy-fermion systems are dominated by antife rromagnetic spin fluctuations, which have been considered theoretically(3) as a possible cause for Cooper-pair formation. This picture recently receiv ed some experimental support: the resistivity behaviour under pressure of t wo cerium-based heavy-fermion compounds was shown to be consistent with a m agnetically mediated pairing mechanism(4). Here we use tunnelling spectrosc opy to investigate the superconducting order parameter of a uranium-based h eavy-fermion superconductor-epitaxial thin films of UPd2Al3. Our observatio n of a strong-coupling feature in the tunnelling conductivity, combined wit h recent inelastic neutron scattering data(13-15) strongly suggest a pairin g interaction mediated by spin fluctuations.