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.