High-temperature superconductivity emerges on doping holes or electrons into antiferromagnetic copper oxides. The large energy scale of magnetic excitations, for example, compared with phonon energies, is thought to drive superconductivity with high transition temperatures (T c). Comparing high-energy magnetic excitations of hole- and electron-doped superconductors provides an opportunity to test this hypothesis. Here, we use resonant inelastic X-ray scattering at the Cu L 3 -edge to reveal collective excitations in the electron-doped cuprate Nd 2-x Ce x CuO 4. Surprisingly, magnetic excitations harden significantly across the antiferromagnetic high-temperature superconductivity phase boundary despite short-ranged antiferromagnetic correlations, in contrast to the hole-doped cuprates. Furthermore, we find an unexpected branch of collective modes in superconducting compounds, absent in hole-doped cuprates. These modes emanate from the zone centre and possess a higher temperature scale than T c, signalling a distinct quantum phase. Despite their differences, the persistence of magnetic excitations and the existence of a distinct quantum phase are apparently universal in both hole- and electron-doped cuprates.
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The authors appreciate Y. Lu’s support in characterizing the doping levels of the measured samples. This work was supported by the US Department of Energy, Office of Basic Energy Science, Division of Materials Science and Engineering under contract no. DE-AC02-76SF00515. The work at University of Minnesota was supported by the NSF and the NSF MRSEC program. S.G. acknowledges support from the Swiss NSF (Contract No. P2EZP2_148737). The authors appreciate the experimental support from the ADRESS beamline of the Swiss Light Source (SLS) at the Paul Scherrer Institut, Switzerland and the beamline BL05A1 at the National Synchrotron Radiation Research Center (NSRRC), Taiwan.