AUTHOR=Yeh Keng-Yu , Chen Yan-Ruei , Lo Tung-Sheng , Wu Phillip M. , Wang Ming-Jye , Chang-Liao Kuei-Shu , Wu Maw-Kuen 

TITLE=Fe-Vacancy-Ordered Fe4Se5: The Insulating Parent Phase of FeSe Superconductor

JOURNAL=Frontiers in Physics

VOLUME=8

YEAR=2020

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2020.567054

DOI=10.3389/fphy.2020.567054

ISSN=2296-424X

ABSTRACT=<p>We carried out a detailed study to investigate the existence of an insulating parent phase for FeSe superconductors. The insulating Fe<sub>4</sub>Se<sub>5</sub> with √5 × √5 Fe-vacancy order shows a 3D-Mott variable-range-hopping behavior with a Verwey-like electronic correlation at around 45 K. The application of the RTA process at 450°C results in the destruction of Fe-vacancy order and induces more electron carriers by increasing the Fe<sup>3+</sup> valence state. Superconductivity emerges with <italic>T</italic><sub>c</sub> ∼ 8 K without changing the chemical stoichiometry of the sample after the RTA process, resulting in the addition of extra carriers in favor of superconductivity.</p>