Abstract
The superconductivity in bulk FeSe depends sensitively on the stoichiometry, ditto for the monolayer FeSe films grown on $\mathrm{SrTi}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ 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 \ensuremath{\sim}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 \ensuremath{\sim}15 to 20 meV for the annealed films, the smaller superconducting gap indicates weakened FeSe-${\mathrm{TiO}}_{2}$ coupling due to interfacial Se atom substitution.
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