Enhancement of magnetic fluctuations on passing below Tc in the heavy fermion superconductor UPd2Al3

The heavy fermion superconductor UPd2Al3 exhibits the unusual combination of an antiferromagnetic phase transition, at T-N = 14.3 K, followed by a superconducting phase transition below 2 K without destruction of the ordered magnetic moment. Polarized inelastic neutron scattering reveals the presence of two coupled modes, both transverse to the sublattice magnetization. On Passing into the superconducting phase an abrupt change is observed in the magnetic inelastic response. We show that it is reasonable to consider the superconducting state as arising out of interactions between quasiparticles which are strongly renormalized by the low-frequency exchange field.