COUPLING BETWEEN MAGNETIC AND SUPERCONDUCTING ORDER PARAMETERS AND EVIDENCE FOR THE SPIN EXCITATION GAP IN THE SUPERCONDUCTING STATE OF A HEAVY-FERMION SUPERCONDUCTOR UPD2AL3

Neutron scattering experiments have been carried out in order to study the interplay between magnetism and superconductivity in a heavy ferm ion superconductor, UPd2Al3. We have observed 1% suppression of the (0 0 0.5) magnetic peak intensity below the superconducting transition t emperature T-c. This is direct evidence for the coupling of the magnet ic order parameter with the superconducting one, Furthermore, we have observed a spin excitation gap associated with superconductivity. The gap energy Delta E-g increases continuously from Delta E-g = 0 to 0.4 meV with decreasing temperature from T-c to 0.4 K. This gap energy cor responds to 2k(B)T(c), which is smaller than the superconducting gap e xpected front the BCS theory (3.5k(B)T(c)). These results are indicati ve of the strong interplay between magnetism and superconductivity.