title: Superconductivity at 44 K in K intercalated FeSe system with excess Fe.
abstract: We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 Å. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.
authors: Department of Physics, Renmin University of China, Beijing 100872, P. R. China.
authors: An-min
authors: AM
authors: Zhang
authors: Tian-long
authors: TL
authors: Xia
authors: Kai
authors: K
authors: Liu
authors: Wei
authors: W
authors: Tong
authors: Zhao-rong
authors: ZR
authors: Yang
authors: Qing-ming
authors: QM
authors: Zhang
references: Chem Commun (Camb). 2008 Nov 21;(43):5607-9
references: 18997967
references: Phys Rev B Condens Matter. 1993 Jan 1;47(1):558-561
references: 10004490
references: Sci Rep. 2012;2:426
references: 22645642
references: Phys Rev B Condens Matter. 1987 Nov 1;36(13):7241-7243
references: 9942475
references: Phys Rev Lett. 2009 Jun 19;102(24):247001
references: 19659037
references: Phys Rev B Condens Matter. 1996 Oct 15;54(16):11169-11186
references: 9984901
references: Phys Rev B Condens Matter. 1994 Dec 15;50(24):17953-17979
references: 9976227
references: Nature. 2012 Feb 22;483(7387):67-9
references: 22367543
references: Proc Natl Acad Sci U S A. 2008 Sep 23;105(38):14262-4
references: 18776050
references: Phys Rev Lett. 1996 Oct 28;77(18):3865-3868
references: 10062328