Significant enhancement of upper critical fields by doping and strain in   Fe-based superconductors
  We report measurements of Hc2(T) up to 85 Tesla on Ba1-xKxAs2Fe2 single crystals and FeSe1-xTex films tuned by doping and strain. We observed an Hc2 enhancement by nearly 25 T at 30 K for the optimally-doped Ba1-xKxAs2Fe2 as compared to the previous results and extraordinarily high slopes dHc2/dT = 250-500 T/K near Tc in FeSe1-xTex indicating an almost complete suppression of the orbital pair-breaking. Theoretical analysis of Hc2(T) in FeSe1-xTex and the optimally doped Ba1-xKxAs2Fe2 predicts an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state for H//ab and T < 3-10 K, and shows that Hc2 in multiband Fe based superconductor can be enhanced by doping and strain much more effectively than by the conventional way of increasing disorder. 
C. Tarantini, A. Gurevich, J. Jaroszynski, F. Balakirev, E.   Bellingeri, I. Pallecchi, C. Ferdeghini, B. Shen, H.H. Wen, D....
cond-mat.supr-con cond-mat.mtrl-sci