Pair density wave state in a monolayer high-Tc iron-based superconductor*

The pair density wave (PDW) is an extraordinary superconducting state where Cooper pairs carry nonzero momentum. Theoretically, the PDW order is hypothesized to play a fundamental role in high-temperature (high-Tc) cuprate superconductors, wherein experimental evidence of the PDW state has been reported. However, the evidence of PDW order is scarce in iron-based high-Tc superconductors and low dimensional superconductors. Here, using scanning tunneling microscopy/spectroscopy, we report the discovery of the PDW state in monolayer iron-based high-Tc Fe(Te,Se) films grown on SrTiO3(001) substrates. The PDW state with a period of λ ~ 3.6aFe (aFe is the distance between neighboring Fe atoms) is observed at the domain walls by the spatial electronic modulations of the local density of states, superconducting coherence peak height and gap energy. Moreover, the π-phase shift boundaries of the PDW state are observed near the vortices of the induced secondary charge density wave state, further demonstrating that the PDW state at the domain wall is a primary state. The discovery of the primary PDW state in the monolayer Fe(Te,Se) film provides a low-dimensional platform to study the PDW state and its interplay with the topological electronic states and unconventional high-Tc superconductivity.