Unconventional pairing in single FeSe layers

Here we report measurements of the electron-boson coupling in single-layer FeSe films grown on SrTiO3 by means of inelastic electron scattering using two complementary spectroscopic techniques. Scanning tunneling spectroscopy shows that the excitation spectrum becomes fully gapped below T-c whereas significant electron-phonon coupling is limited to spatially confined regions. Further, high-resolution electron energy loss spectroscopy shows neither strong nor k-dependent electron-phonon coupling. Our results strongly support a predominantly electronic pairing mechanism.