Checkerboard ordered state in a superconducting FeSe/SrTiO3(001) monolayer

Ordered electronic states have been extensively explored in cuprates and iron-based unconventional superconductors, but seldom observed in the epitaxial FeSe/SrTiO3(001) monolayer (FeSe/STO) with an enhanced superconducting transition temperature (T-c). Here, by using scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS), we reveal a checkerboard charge order in the epitaxial FeSe/STO monolayer, with a period of four times the inter-iron-atom distance along two perpendicular directions of the iron lattice. This ordered state is uniquely present in the superconducting FeSe/STO monolayer, even at liquid nitrogen temperature, but is absent in the nonsuperconducting FeSe monolayer or bilayer. Quasi-particle interference measurements further confirm it as a static order without an energy-dependent dispersion and is gapped out within the superconductivity gap. The intensity of the charge order shows an enhancement near the superconducting transition temperature, thus implying a correlation with the high-T-c superconductivity in the FeSe/STO monolayer. This study provides a new basis for exploring the ordered electronic states and their interplay with high-T-c superconductivity in the FeSe monolayer.

Automated summary

Using scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS), we discovered a checkerboard charge order in the epitaxial FeSe/STO monolayer, with a period of four times the inter-iron-atom distance. This ordered state is unique to the superconducting FeSe/STO monolayer and is absent in nonsuperconducting FeSe monolayers or bilayers. Quasi-particle interference measurements confirmed the static nature of this charge order, which is gapped out within the superconductivity gap. The intensity of the charge order is enhanced near the superconducting transition temperature, suggesting a correlation with the high-Tc superconductivity in the FeSe/STO monolayer.