  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 (phonons) and spin-excitations with
the additional possibility of pairing without mediators. In conventional
superconductors, phonon-mediated pairing was unequivocally established by data
from tunnelling experiments. Proponents of phonons as the high-T c glue were
therefore encouraged by the recent scanning tunnelling microscopy experiments
on hole-doped Bi2Sr2CaCu2O8-delta (BSCCO) that reveal an oxygen lattice
vibrational mode whose energy is anticorrelated with the superconducting gap
energy scale. Here we report high-resolution scanning tunnelling microscopy
measurements of the electron-doped high-T c superconductor Pr0.88LaCe0.12CuO4
(PLCCO) (T c = 24 K) that reveal a bosonic excitation (mode) at energies of
10.5 plus/minus 2.5 meV. This energy is consistent with both spin-excitations
in PLCCO measured by inelastic neutron scattering (resonance mode) and a
low-energy acoustic phonon mode, 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.
