Checkerboard order state in superconducting FeSe/SrTiO3(001) monolayer

TL;DR

This study uncovers a checkerboard charge order in superconducting FeSe/SrTiO3 monolayers using STM/STS, revealing a static electronic order that correlates with high-temperature superconductivity, and absent in non-superconducting layers.

Contribution

It reports the first observation of a checkerboard charge order in superconducting FeSe monolayers, linking it to the enhanced Tc and providing insights into electronic states in these materials.

Findings

Checkerboard charge order with four times the inter-Fe-atom period.

Charge order is present only in superconducting monolayers.

Charge order intensity peaks near the superconducting transition temperature.

Abstract

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 (Tc). Here, by using scanning tunneling microscopy/ spectroscopy (STM/STS), we reveal a checkerboard charge order in the epitaxial FeSe/STO monolayer, with a period of four times the inter-Fe-atom distance along two perpendicular directions of the Fe lattice. This ordered state is uniquely present in the superconducting FeSe/STO monolayer, even at liquid nitrogen temperature, but absent in the non-superconducting FeSe monolayer or bilayer. Quasiparticle interference (QPI) measurements further confirm it as a static order without an energy-dependent dispersion and 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-Tc superconductivity in the FeSe/STO monolayer. This study provides a new basis for exploring the ordered electronic states and their interplay with high-Tc superconductivity in the FeSe monolayer.