Stoichiometry and defect superstructures in epitaxial FeSe films on SrTiO3

TL;DR

This study uses cryogenic scanning tunneling microscopy to explore how annealing affects stoichiometry, defects, and superconductivity in epitaxial FeSe films on SrTiO3, revealing critical insights into their phase transitions.

Contribution

It provides new understanding of how annealing conditions influence defect structures and superconducting phases in FeSe films, clarifying previous controversies.

Findings

Low-temperature annealing induces Fe vacancies and superstructures.

High-temperature annealing restores superconductivity by eliminating vacancies.

Multilayer FeSe formation links to Fe vacancy and adatom migration.

Abstract

Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on SrTiO3(001) substrates under various post-growth annealing conditions. Low-temperature annealing with an excess supply of Se leads to formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films.