  High-temperature superconductivity (HTSC) mysteriously emerges upon doping
holes or electrons into insulating copper oxides with antiferromagnetic (AFM)
order. It has been thought that the large energy scale of magnetic excitations,
compared to phonon energies for example, lies at the heart of an
electronically-driven superconducting phase at high temperatures. However,
despite extensive studies, little information is available for comparison of
high-energy magnetic excitations of hole- and electron-doped superconductors to
assess a possible correlation with the respective superconducting transition
temperatures. Here, we use resonant inelastic x-ray scattering (RIXS) at the Cu
L3-edge to reveal high-energy collective excitations in the archetype
electron-doped cuprate Nd2-xCexCuO4 (NCCO). Surprisingly, despite the fact that
the spin stiffness is zero and the AFM correlations are short-ranged, magnetic
excitations harden significantly across the AFM-HTSC phase boundary, in stark
contrast with the hole-doped cuprates. Furthermore, we find an unexpected and
highly dispersive mode in superconducting NCCO that is undetected in the
hole-doped compounds, which emanates from the zone center with a characteristic
energy comparable to the pseudogap, and may signal a quantum phase distinct
from superconductivity. The uncovered asymmetry in the high-energy collective
excitations with respect to hole and electron doping provides additional
constraints for modeling the HTSC cuprates.
