Two characteristic energies in the low-energy magnetic response of the electron-doped high-temperature superconductor Nd_2-xCe _xCuO_4+δ

Neutron scattering for Nd_2-xCe_xCuO _4+δ (x≈0.155, T_c =25 K) reveals two distinct magnetic energy scales in the superconducting state: ω₁ ≈6.4 meV and ω₂ ≈4.5 meV. These magnetic energies agree quantitatively with the B_1g/B_2g and A_1g features observed in electronic Raman scattering, where the former is believed to indicate the maximum superconducting gap and the origin of the latter has remained unexplained. The data are inconsistent with previous claims of the existence of a magnetic resonance mode near 10 meV, but consistent with a resonance at ω₂ and with the recently established universal ratio of resonance energy to superconducting gap in unconventional superconductors.