Post-growth Fe deposition on the superconductivity of monolayer FeSe films on SrTiO3-δ

The superconductivity in bulk FeSe depends sensitively on the stoichiometry, ditto for the monolayer FeSe films grown on SrTiO3-delta and other substrates. To tune the stoichiometry, we deposit Fe onto monolayer FeSe films after epitaxial growth. Using in situ low-temperature scanning tunneling microscopy/spectroscopy, we find that the postdeposited Fe atoms incorporate into monolayer films and combine with excess Se to form FeSe, with an upper coverage limit of similar to 0.4 unit-cell. Consequently, the superconducting gap concentrates in 10 to 12 meV. The work demonstrates a simple method for stoichiometry tuning and improving the spatial uniformity of monolayer FeSe films. Compared with the gap of similar to 15 to 20 meV for the annealed films, the smaller superconducting gap indicates weakened FeSe-TiO2 coupling due to interfacial Se atom substitution.