LAUSR

The electron pairing mechanism has always been one of the most challenging problems in high-temperature superconductors. Fe(Te,Se), as the superconductor with intrinsic topological property, may host Majorana bound states and has attracted tremendous interest. While in bulk Fe(Te,Se), the pairing mechanism has been experimentally investigated, it remains little understood in its two-dimensional limit counterpart. Here, by in-situ scanning tunneling spectroscopy, we show clear evidences of the bosonic mode Ω beyond the superconducting gap Δ in monolayer FeTe0.5Se0.5/SrTiO3(001) high-temperature superconductor. Statistically, Ω shows an obvious anticorrelation with Δ and appears below 2Δ, consistent with the spin-excitation nature. Furthermore, the in-gap bound states induced by two types of magnetically different impurities support the sign-reversing pairing scenario. Our results not only suggest that the spin-excitation-like bosonic mode within a sign-reversing pairing plays an essential role in monolayer FeTe0.5Se0.5/SrTiO3(001), but also offer the crucial information for investigating the high-temperature superconductivity in interfacial iron selenides.