1Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190 2Collaborative Innovation Center of Quantum Matter, Beijing 100190 3School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049
Abstract：In the iron-based high-$T_{\rm c}$ bulk superconductors, $T_{\rm c}$ above 50 K was only observed in the electron-doped 1111-type compounds. Here we revisit the electron-doped SmFeAsO polycrystals to make a further investigation for the highest $T_{\rm c}$ in these materials. To introduce more electron carriers and less crystal lattice distortions, we study the Th and F codoping effects into the Sm-O layers with heavy electron doping. Dozens of Sm$_{1-x}$Th$_{x}$FeAsO$_{1-y}$F$_{y}$ samples are synthesized through the solid state reaction method, and these samples are carefully characterized by the structural, resistive, and magnetic measurements. We find that the codoping of Th and F clearly enhances the superconducting $T_{\rm c}$ more than the Th or F single-doped samples, with the highest record $T_{\rm c}$ up to 58.6 K when $x=0.2$ and $y=0.225$. Further element doping causes more impurities and lattice distortions in the samples with a weakened superconductivity.
Zhang S J, Wang X C, Liu Q Q, Lv Y X, Yu X H, Lin Z J, Zhao Y S, Wang L, Ding Y, Mao H K and Jin C Q 2009 Europhys. Lett.88 47008
[14]
Medvedev S, Mcqueen T M, Troyan I A, Palasyuk T, Eremets M I, Cava R J, Naghavi S, Casper F, Ksenofontov V, Wortmann G and Felser C 2009 Nat. Mater.8 630
Sun L L, Chen X J, Guo J, Gao P W, Huang Q Z, Wang H D, Fang M H, Chen X L, Chen G F, Wu Q, Zhang C, Gu D C, Dong X L, Wang L, Yang K, Li A G, Dai X, Mao H K and Zhao Z X 2012 Nature483 67
[17]
Ge J F, Liu Z L, Liu C H, Gao C L, Qian D, Xue Q K, Liu Y and Jia J F 2015 Nat. Mater.14 285
Katayama N, Kudo K, Onari S, Mizukami T, Sugawara K, Sugiyama Y, Kitahama Y, Iba K, Fujimura K, Nishimoto N, Nohara M and Sawa H 2013 J. Phys. Soc. Jpn.82 123702
Sefat A S, Huq A, McGuire M A, Jin R Y, Sales B C, Mandrus D, Cranswick L M D, Stephens P W and Stone K H 2008 Phys. Rev. B 78 104505
[31]
Zocco D A, Hamlin J J, Baumbach R E, Maple M B, Mcguire M A, Sefat A S, Sales B C, Jin R, Mandrus D, Jeffries J R, Weir S T and Vohra Y K 2008 Physica C 468 2229
[32]
Yi W, Sun L L, Ren Z A, Lu W, Dong X L, Zhang H J, Dai X, Fang Z, Li Z C, Che G C, Yang J, Shen X L, Zhou F and Zhao Z X 2008 Europhys. Lett.83 57002
2017, Vol. 34 
