Pair density wave state in a monolayer high-T c iron-based superconductor.

The pair density wave (PDW) is an extraordinary superconducting state in which Cooper pairs carry non-zero momentum 1,2 . Evidence for the existence of intrinsic PDW order in high-temperature (high-T c ) cuprate superconductors 3,4 and kagome superconductors 5 has emerged recently. However, the PDW order in iron-based high-T c superconductors has not been observed experimentally. Here, using scanning tunnelling microscopy and spectroscopy, we report the discovery of the PDW state in monolayer iron-based high-T c Fe(Te,Se) films grown on SrTiO 3 (001) substrates. The PDW state with a period of λ ≈ 3.6a Fe (a Fe is the distance between neighbouring Fe atoms) is observed at the domain walls by the spatial electronic modulations of the local density of states, the superconducting gap and the π-phase shift boundaries of the PDW around the vortices of the intertwined charge density wave order. The discovery of the PDW state in the monolayer Fe(Te,Se) film provides a low-dimensional platform to study the interplay between the correlated electronic states and unconventional Cooper pairing in high-T c superconductors.