Spin excitations in the optimally electron-doped superconductor Nd1.85Ce0.15CuO4-δ

The magnetic excitation spectrum of an optimally electron-doped superconductor Nd1.85Ce0.15CuO4-δ at Tc=25K has been measured using inelastic neutron scattering. On cooling from the normal to the superconducting state, a spin gap below 4meV appears in the system and a resonance occurs at about 9.5meV centered at Q=(1/2,1/2,0). The intensity of the resonance evolves like a superconducting order parameter, similar to those for hole-doped superconductors and electron-doped Pr0.88LaCe0.12CuO4. The resonance is therefore a universal feature of cuprate superconductors, and seems to be fundamental to the mechanism of high-Tc superconductivity.