1Institute of Physics and Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, Beijing 100190 2High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031 3Collaborative Innovation Center of Quantum Matter, Beijing 100190
Abstract：Single crystals of undoped CaFe$_{2}$As$_{2}$ are grown by an FeAs self-flux method, and the crystals are quenched in ice-water rapidly after high-temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature-dependent x-ray diffraction measurements. Superconductivity below 25 K is observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-II superconductor. For comparison, we systematically characterize the properties of the furnace-cooled, quenched, and post-annealed single crystals, and find strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe$_{2}$As$_{2}$ single crystals.
Kreyssig A, Green M A, Lee Y, Samolyuk G D, Zajdel P, Lynn J W, Bud'ko S L, Torikachvili M S, Ni N, Nandi S, Le?o J B, Poulton S J, Argyriou D N, Harmon B N, McQueeney R J, Canfield P C and Goldman A I 2008 Phys. Rev. B 78 184517
[12]
Goldman A I, Kreyssig A, Proke? K, Pratt D K, Argyriou D N, Lynn J W, Nandi S, Kimber S A J, Chen Y, Lee Y B, Samolyuk G, Le?o J B, Poulton S J, Bud'ko S L, Ni N, Canfield P C, Harmon B N and McQueeney R J 2009 Phys. Rev. B 79 024513
Goldman A I, Argyriou D N, Ouladdiaf B, Chatterji T, Kreyssig A, Nandi S, Ni N, Bud'ko S L, Canfield P C and McQueeney R J 2008 Phys. Rev. B 78 100506(R)
[22]
Pratt D K, Zhao Y, Kimber S A J, Hiess A, Argyriou D N, Broholm C, Kreyssig A, Nandi S, Bud'ko S L, Ni N, Canfield P C, McQueeney R J and Goldman A I 2009 Phys. Rev. B 79 060510(R)
Ortenzi L, Gretarsson H, Kasahara S, Matsuda Y, Shibauchi T, Finkelstein K D, Wu W, Julian S R, Kim Y J, Mazin I I and Boeri L 2015 Phys. Rev. Lett.114 047001
Ran S, Bud'ko S L, Pratt D K, Kreyssig A, Kim M G, Kramer M J, Ryan D H, Rowan-Weetaluktuk W N, Furukawa Y, Roy B, Goldman A I and Canfield P C 2011 Phys. Rev. B 83 144517
[27]
Saparov B, Cantoni C, Pan M, Hogan T C, Ratcliff W, 2nd, Wilson S D, Fritsch K, Tachibana M, Gaulin B D and Sefat A S 2014 Sci. Rep.4 4120
2016, Vol. 33 
