title: Emergence of Multiple Superconducting Phases in (NH3)yMxFeSe (M: Na and Li).
abstract: We previously discovered multiple superconducting phases in the ammoniated Na-doped FeSe material, (NH3)yNaxFeSe. To clarify the origin of the multiple superconducting phases, the variation of Tc was fully investigated as a function of x in (NH3)yNaxFeSe. The 32 K superconducting phase is mainly produced in the low-x region below 0.4, while only a single phase is observed at x  =  1.1, with Tc =  45 K, showing that the Tc depends significantly on x, but it changes discontinuously with x. The crystal structure of (NH3)yNaxFeSe does not change as x increases up to 1.1, i.e., the space group of I4/mmm. The lattice constants, a and c, of the low-Tc phase (Tc = 32.5 K) are 3.9120(9) and 14.145(8) Å, respectively, while a = 3.8266(7) Å and c = 17.565(9) Å for the high-Tc phase (~46 K). The c increases in the high Tc phase, implying that the Tc is directly related to c. In (NH3)yLixFeSe material, the Tc varies continuously within the range of 39 to 44 K with changing x. Thus, the behavior of Tc is different from that of (NH3)yNaxFeSe. The difference may be due to the difference in the sites that the Na and Li occupy.
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Lu
authors.initials: L
authors.lastname: Zheng
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Xiao
authors.initials: X
authors.lastname: Miao
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Sakai
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Masanari
authors.initials: M
authors.lastname: Izumi
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Hidenori
authors.initials: H
authors.lastname: Goto
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Saki
authors.initials: S
authors.lastname: Nishiyama
authors.affiliation: Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan.
authors.forename: Eri
authors.initials: E
authors.lastname: Uesugi
authors.affiliation: Department of Applied physics, The University of Tokyo, Tokyo 113-8564, Japan.
authors.forename: Yuichi
authors.initials: Y
authors.lastname: Kasahara
authors.affiliation: Department of Applied physics, The University of Tokyo, Tokyo 113-8564, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Iwasa
authors.affiliation: 1] Research Laboratory for Surface Science, Okayama University, Okayama 700-8530, Japan [2] Research Centre of New Functional Materials for Energy Production, Storage and Transport, Okayama University, Okayama 700-8530, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Kubozono