Bosonic Mode and Impurity-Scattering in Monolayer Fe(Te,Se) High-Temperature Superconductors

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 evidence of the bosonic mode Omega beyond the superconducting gap Delta in monolayer FeTe0.5Se0.5/SrTiO3 (001) high-temperature superconductor. Statistically, Omega shows an obvious anticorrelation with Delta and appears below 2 Delta, 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.