Enhancement of Transition Temperature in FexSe0.5Te0.5 Film via Iron   Vacancies
  The effects of iron deficiency in FexSe0.5Te0.5 thin films (0.8<x<1) on superconductivity and electronic properties have been studied. A significant enhancement of the superconducting transition temperature (TC) up to 21K was observed in the most Fe deficient film (x=0.8). Based on the observed and simulated structural variation results, there is a high possibility that Fe vacancies can be formed in the FexSe0.5Te0.5 films. The enhancement of TC shows a strong relationship with the lattice strain effect induced by Fe vacancies. Importantly, the presence of Fe vacancies alters the charge carrier population by introducing electron charge carriers, with the Fe deficient film showing more metallic behavior than the defect-free film. Our study provides a means to enhance the superconductivity and tune the charge carriers via Fe vacancy, with no reliance on chemical doping. 
J. C. Zhuang, W. K. Yeoh, X. Y. Cui, J. H. Kim, D. Q. Shi, Z. X. Shi,   S. P. Ringer, X. L. Wang, and S. X. Dou
cond-mat.supr-con