title: Superconducting phases in potassium-intercalated iron selenides.
abstract: The ubiquitous coexistence of majority insulating 245 phases and minority superconducting (SC) phases in A(x)Fe(2-y)Se(2) (A = K, Cs, Rb, Tl/Rb, Tl/K) formed by high-temperature routes makes pure SC phases highly desirable for studying the intrinsic properties of this SC family. Here we report that there are at least two pure SC phases, K(x)Fe(2)Se(2)(NH(3))(y) (x ≈ 0.3 and 0.6), determined mainly by potassium concentration in the K-intercalated iron selenides formed via the liquid ammonia route. K(0.3)Fe(2)Se(2)(NH(3))(0.47) corresponds to the 44 K phase with lattice constant c = 15.56(1) Å and K(0.6)Fe(2)Se(2)(NH(3))(0.37) to the 30 K phase with c = 14.84(1) Å. With higher potassium doping, the 44 K phase can be converted into the 30 K phase. NH(3) has little, if any, effect on superconductivity. Thus, the conclusions should apply to both K(0.3)Fe(2)Se(2) and K(0.6)Fe(2)Se(2) SC phases. K(0.3)Fe(2)Se(2)(NH(3))(0.47) and K(0.6)Fe(2)Se(2)(NH(3))(0.37) stand out among known superconductors as their structures are stable only at particular potassium doping levels, and hence the variation of T(c) with doping is not dome-like.
authors: Research & Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors: Tianping
authors: T
authors: Ying
authors: Xiaolong
authors: X
authors: Chen
authors: Gang
authors: G
authors: Wang
authors: Shifeng
authors: S
authors: Jin
authors: Xiaofang
authors: X
authors: Lai
authors: Tingting
authors: T
authors: Zhou
authors: Han
authors: H
authors: Zhang
authors: Shijie
authors: S
authors: Shen
authors: Wanyan
authors: W
authors: Wang