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 fermion superconductor, UPd2Al3. We have observed 1% suppression of the (0 0 0.5) magnetic peak intensity below the superconducting transition temperature T-c. This is direct evidence for the coupling of the magnetic 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 corresponds to 2k(B)T(c), which is smaller than the superconducting gap expected front the BCS theory (3.5k(B)T(c)). These results are indicative of the strong interplay between magnetism and superconductivity.