1. Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
   
2. Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA
   
3. Neutron Scattering Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, USA

Correspondence to: V. Madhavan¹ Correspondence and requests for materials should be addressed to V.M. (Email: madhavan@bc.edu).

Despite recent advances in understanding high-transition-temperature (high-T _c) superconductors, there is no consensus on the origin of the superconducting 'glue': that is, the mediator that binds electrons into superconducting pairs. The main contenders are lattice vibrations^{1, 2} (phonons) and spin-excitations^{3, 4}, with the additional possibility of pairing without mediators⁵. In conventional superconductors, phonon-mediated pairing was unequivocally established by data from tunnelling experiments⁶. Proponents of phonons as the high-T _c glue were therefore encouraged by the recent scanning tunnelling microscopy experiments on hole-doped Bi₂Sr₂CaCu₂O_8- (BSCCO) that reveal an oxygen lattice vibrational mode whose energy is anticorrelated with the superconducting gap energy scale⁷. Here we report high-resolution scanning tunnelling microscopy measurements of the electron-doped high-T _c superconductor Pr_0.88LaCe_0.12CuO₄ (PLCCO) (T _c = 24 K) that reveal a bosonic excitation (mode) at energies of 10.5  2.5 meV. This energy is consistent with both spin-excitations in PLCCO measured by inelastic neutron scattering (resonance mode)⁸ and a low-energy acoustic phonon mode⁹, but differs substantially from the oxygen vibrational mode identified in BSCCO. Our analysis of the variation of the local mode energy and intensity with the local gap energy scale indicates an electronic origin of the mode consistent with spin-excitations rather than phonons.