Superconductivity Dichotomy In K-Coated Single And Double Unit Cell Fese Films On Srtio3

We report the superconductivity evolution of one unit cell (1-UC) and 2-UC FeSe films on SrTiO3(001) substrates with potassium (K) adsorption. By in situ scanning tunneling spectroscopy measurement, we find that the superconductivity in 1-UC FeSe films is continuously suppressed with increasing K coverage, whereas nonsuperconducting 2-UC FeSe films become superconducting with a gap of similar to 17 meV or similar to 11 meV depending on whether the underlying 1-UC films are superconducting or not. This work explicitly reveals that the FeSe/STO interface plays a vital role in enhancing Cooper pairing in both 1-UC and 2-UC FeSe films.