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The single-layer FeSe films grown on SrTiO3 (STO) substrates have attractedmuch attention because of its record high superconducting critical temperature(Tc). It is usually believed that the composition of the epitaxially grownsingle-layer FeSe/STO films is stoichiometric, i.e., the ratio of Fe and Se is1:1. Here we report the identification of a large amount of excess Fe in thesuperconducting single-layer FeSe/STO films. By depositing Se onto thesuperconducting single-layer FeSe/STO films, we find by in situ scanningtunneling microscopy (STM) the formation of the second-layer FeSe islands onthe top of the first layer during the annealing process at a surprisingly lowtemperature ($\sim$150{\deg}C) which is much lower than the usual growthtemperature ($\sim$490{\deg}C). This observation is used to detect excess Feand estimate its quantity in the single-layer FeSe/STO films. The amount ofexcess Fe detected is at least 20% that is surprisingly high for thesuperconducting single-layer FeSe/STO films. The discovery of such a largeamount of excess Fe should be taken into account in understanding the high-Tcsuperconductivity and points to a likely route to further enhance Tc in thesuperconducting single-layer FeSe/STO films.

Identification of a Large Amount of Excess Fe in Superconducting Single-Layer FeSe/SrTiO3 Films