Phase Separation and Magnetic Order in K-doped Iron Selenide Superconductor

TL;DR

This study uses advanced microscopy techniques to clarify the phase separation, vacancy order, and magnetic interactions in potassium-doped iron selenide superconductors, revealing how these factors influence superconductivity.

Contribution

It provides definitive evidence of phase separation and vacancy effects in K-doped iron selenide, clarifying previous debates and linking magnetic order to superconductivity.

Findings

Superconducting phase KFe2Se2 has no iron vacancies.

Vacancies are detrimental to superconductivity.

Magnetic bipartite order is present in the superconducting phase.

Abstract

Alkali-doped iron selenide is the latest member of high Tc superconductor family, and its peculiar characters have immediately attracted extensive attention. We prepared high-quality potassium-doped iron selenide (KxFe2-ySe2) thin films by molecular beam epitaxy and unambiguously demonstrated the existence of phase separation, which is currently under debate, in this material using scanning tunneling microscopy and spectroscopy. The stoichiometric superconducting phase KFe2Se2 contains no iron vacancies, while the insulating phase has a \surd5\times\surd5 vacancy order. The iron vacancies are shown always destructive to superconductivity in KFe2Se2. Our study on the subgap bound states induced by the iron vacancies further reveals a magnetically-related bipartite order in the superconducting phase. These findings not only solve the existing controversies in the atomic and electronic structures in KxFe2-ySe2, but also provide valuable information on understanding the superconductivity and its interplay with magnetism in iron-based superconductors.