  The pairing mechanism in iron-based superconductors is believed to be
unconventional, i.e. not phonon-mediated. The achieved transition temperatures
Tc in these superconductors are still significantly below those of some of the
cuprates, with the exception of single layer FeSe films on SrTiO3 showing a Tc
between 60 and 100 K, i.e. an order of magnitude larger than in bulk FeSe. This
enormous increase of Tc demonstrates the potential of interface engineering for
superconductivity, yet the underlying mechanism of Cooper pairing is not
understood. Both conventional and unconventional mechanisms have been
discussed. Here we report a direct measurement of the electron-boson coupling
function in FeSe on SrTiO3 using inelastic electron scattering which shows that
the excitation spectrum becomes fully gapped below Tc strongly supporting a
predominantly electronic pairing mechanism. We also find evidence for strong
electron-phonon coupling of low energy electrons, which is however limited to
regions near structural domain boundaries.
