Identification of Excess Fe in Superconducting Single-layer FeSe/SrTiO₃ Films

The discovery of high temperature superconductivity with a superconducting transition temperature (T_C) above 65 K in single-layer FeSe films grown on the SrTiO₃ substrate (FeSe/STO films) has attracted much interest. It is usually believed that the composition of the epitaxially-grown FeSe films is stoichiometric, i. e., the ratio of iron and selenium is 1:1. We will report identification of a large amount of excess iron in the superconducting single-layer FeSe/STO film by in situ scanning tunneling microscopy (STM). The discovery of such a large amount of excess iron in superconducting single-layer FeSe/STO film should be taken into consideration in understanding the high T_C superconductivity in the single-layer FeSe/STO film.