  The iron-based superconductors are an ideal platform to reveal the enigma of
the unconventional superconductivity and potential topological
superconductivity. Among them, the monolayer Fe(Se,Te)/SrTiO3(001), which is
proposed to be topological nontrivial, shows interface-enhanced
high-temperature superconductivity in the two dimensional limit. However, the
experimental studies on the superconducting pairing mechanism of monolayer
Fe(Se,Te) films are still limited. Here, by measuring quasiparticle
interference in monolayer Fe(Se,Te)/SrTiO3(001), we report the observation of
the anisotropic structure of the large superconducting gap and the sign change
of the superconducting gap on different electron pockets. The results are well
consistent with the 'bonding-antibonding' s+- wave pairing symmetry driven by
spin fluctuations in conjunction with spin-orbit coupling. Our work is of basic
significance not only for a unified superconducting formalism in the iron-based
superconductors, but also for understanding of topological superconductivity in
high-temperature superconductors.
