Metastable superconducting state in quenched KxFe2-ySe2
  By direct quenching or post-annealing followed by quenching, we have successfully obtained a series of KxFe2-ySe2 samples with different properties. It is found that the samples directly quenched in the cooling process of growth show superconductivity and the one cooled with furnace is insulating even though their stoichiometries are similar. The sample cooled with furnace can be tuned from insulating to superconducting by post-annealing and then quenching. Based on the two points mentioned above, we conclude that the superconducting state in KxFe2-ySe2 is metastable, and quenching is the key point to achieve the superconducting state. The similar stoichiometries of all the non-superconducting and superconducting samples indicate that the iron valence doesn't play a decisive role in determining whether a KxFe2-ySe2 sample is superconducting. Combining with the result got in the KxFe2-ySe2 thin films prepared by molecular beam epitaxy (MBE), we argue that our superconducting sample partly corresponds to the phase without iron vacancies as evidenced by scanning tunnelling microscopy (STM) and the insulating sample mainly corresponds to the phase with the $\sqrt{5}\times\sqrt{5}$ vacancy order. Quenching may play a role of freezing the phase without iron vacancies. 
Fei Han, Huan Yang, Bing Shen, Zheng-Yu Wang, Chun-Hong Li and Hai-Hu   Wen
cond-mat.supr-con