  We report an in situ scanning tunneling spectroscopy study of one-unit-cell
(1-UC) FeSe film on SrTiO3(001) (STO) substrate. In quasiparticle density of
states, bosonic excitation mode characterized by the "dip-hump" structure is
detected outside the larger superconducting gap with energy comparable with
phonon and spin resonance modes in heavily electron-doped iron selenides.
Statistically, the excitation mode, which is intimately correlated with
superconductivity, shows an anticorrelation with pairing strength and yields an
energy scale upper-bounded by twice the superconducting gap coinciding with the
characteristics of magnetic resonance in cuprates and iron-based
superconductors. The local response of tunneling spectra to magnetically
different Se defects all exhibits the induced in-gap quasiparticle bound
states, indicating an unconventional sign-reversing pairing. These results
support the magnetic nature of the excitation mode and possibly reveal a
signature of electron-magnetic-excitation coupling in high-temperature
superconductivity of 1-UC FeSe/STO.
