Superconducting Energy Gap Observed in the Magnetic Excitation Spectra of a Heavy Fermion SuperconductorUPd2Al3

High resolution neutron scattering experiments have been carried out in order to study the interplay between magnetism and superconductivity in the heavy fermion superconductor ${\mathrm{UPd}}_2{\mathrm{Al}}_3$. We found direct evidence for a magnetic excitation gap associated with superconductivity. We observed $1\%$ suppression of the antiferromagnetic Bragg intensities below the superconducting transition temperature $T_c=1.9\mathrm{}\mathrm{K}$. We also observed the increases of the spin wave excitation energy and its linewidth in the superconducting state. These results indicate a strong coupling between magnetism and superconductivity in this compound.