title: Fe-vacancy order and superconductivity in tetragonal β-Fe1-xSe.
abstract: Several superconducting transition temperatures in the range of 30-46 K were reported in the recently discovered intercalated FeSe system (A1-xFe2-ySe2, A = K, Rb, Cs, Tl). Although the superconducting phases were not yet conclusively decided, more than one magnetic phase with particular orders of iron vacancy and/or potassium vacancy were identified, and some were argued to be the parent phase. Here we show the discovery of the presence and ordering of iron vacancy in nonintercalated FeSe (PbO-type tetragonal β-Fe1-xSe). Three types of iron-vacancy order were found through analytical electron microscopy, and one was identified to be nonsuperconducting and magnetic at low temperature. This discovery suggests that the rich-phases found in A1-xFe2-ySe2 are not exclusive in Fe-Se and related superconductors. In addition, the magnetic β-Fe1-xSe phases with particular iron-vacancy orders are more likely to be the parent phase of the FeSe superconducting system instead of the previously assigned β-Fe1+δTe.
authors.affiliation: Institute of Physics, Academia Sinica, Taipei 115, Taiwan.
authors.forename: Ta-Kun
authors.initials: TK
authors.lastname: Chen
authors.forename: Chung-Chieh
authors.initials: CC
authors.lastname: Chang
authors.forename: Hsian-Hong
authors.initials: HH
authors.lastname: Chang
authors.forename: Ai-Hua
authors.initials: AH
authors.lastname: Fang
authors.forename: Chih-Han
authors.initials: CH
authors.lastname: Wang
authors.forename: Wei-Hsiang
authors.initials: WH
authors.lastname: Chao
authors.forename: Chuan-Ming
authors.initials: CM
authors.lastname: Tseng
authors.forename: Yung-Chi
authors.initials: YC
authors.lastname: Lee
authors.forename: Yu-Ruei
authors.initials: YR
authors.lastname: Wu
authors.forename: Min-Hsueh
authors.initials: MH
authors.lastname: Wen
authors.forename: Hsin-Yu
authors.initials: HY
authors.lastname: Tang
authors.forename: Fu-Rong
authors.initials: FR
authors.lastname: Chen
authors.forename: Ming-Jye
authors.initials: MJ
authors.lastname: Wang
authors.forename: Maw-Kuen
authors.initials: MK
authors.lastname: Wu
authors.forename: Dirk
authors.initials: D
authors.lastname: Van Dyck
keywords: magnetism; phase diagram