The electronic structure of heavy-fermion compounds arises from the interaction of nearly localized 4f- or 5f-shell electrons (with atomic magnetic moments) with the free-electron-like itinerant conduction-band electrons. In actinide or rare-earth heavy-fermion materials, this interaction yields itinerant electrons having an effective mass about 100 times (or more) the bare electron mass. Moreover, the itinerant electrons in UPd2Al3 are found to be superconducting well below the magnetic ordering temperature of this compound, whereas magnetism generally suppresses superconductivity in conventional metals. Here we report the detection of a dispersive excitation of the ordered f-electron moments, which shows a strong interaction with the heavy superconducting electrons. This 'magnetic exciton' is a localized excitation which moves through the lattice as a result of exchange forces between the magnetic moments. By combining this observation with previous tunnelling measurements on this material, we argue that these magnetic excitons 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.

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The heavy fermion superconductor UPd 2 Al 3 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. [S0031-9007 (98)07531-0] PACS numbers: 74.70.Tx, 78.70.NxThe discovery of the heavy fermion superconductor UPd 2 Al 3 [1] which exhibits both an antiferromagnetic phase transition, T N 14.3 K, and a superconducting phase transition below T c 2 K has aroused great interest. The compound crystallizes in the hexagonal PrNi 2 Al 3 structure (space group P6͞mmm) with lattice constants a 5.350 Å and c 4.185 Å at room temperature. Neutron and x-ray scattering measurements have revealed unusually large low temperature ordered moments of 0.85m B which are coupled in ferromagnetic sheets in the basal plane; these ferromagnetic planes are then stacked along the c axis with a wave vector Q 0 ͑0 0 0.5͒ [2-4]. The measured discontinuity in the heat capacity at T c is large, DC 1.2gT c ͑g 140 mJ͞mole K 2 ͒ [1,5] and suggests that the superconducting ground state evolves out of interactions between quasiparticles located in strongly renormalized states in a low energy shell around the Fermi surface.In the hope of finding a connection between antiferromagnetism and superconductivity there have been many investigations in this compound since its discovery. Changes in antiferromagnetic Bragg peak intensities on passing through T c were reported [6]; however, these results are questioned by independent measurements using both neutron diffraction and resonant magnetic x-ray scattering [3,7]. Pioneering inelastic neutron scattering experiments under relatively coarse energy resolution [8] revealed no changes in the spectrum around the antiferromagnetic zone center on cooling through T c . An additional low energy component to the spectral response was first reported by Sato et al. [9], and Metoki et al. [10] have reported a gap opening below T c and the disappearance of the low energy response when a magnetic field is applied.In this Letter we report on new, high resolution, inelastic neutron scattering experiments carried out on the IN14 spectrometer at the Institut Laue-Langevin, Grenoble. With full neutron polarization analysis we have been able to identify the transverse nature of the dominant fluctuations at all temperatures below T N and the opening of a gap in the energy spectrum of magnetic fluctuations at the lowest temperatures (150 mK) in the superconducting state. The spectra consist of a damped spin wave coupled to a low energy mode. In the normal state we develo...

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