Coupling between Magnetic and Superconducting Order Parameters and Evidence for the Spin Excitation Gap in the Superconducting State of a Heavy Fermion Superconductor UPd_<b>2</b>Al_<b>3</b>

Abstract

Neutron scattering experiments have been carried out in order to study the interplay between magnetism and superconductivity in a heavy fermion superconductor, UPd 2 Al 3 . 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 Δ E g increases continuously from Δ E g = 0 to 0.4 meV with decreasing temperature from T c to 0.4 K. This gap energy corresponds to 2 k B T c , which is smaller than the superconducting gap expected from the BCS theory (3.5 k B T c ). These results are indicative of the strong interplay between magnetism and superconductivity.