Significant enhancement of upper critical fields by doping and strain in iron-based superconductors

We report measurements up to 85 Tesla of the upper critical fields Hc2(T) on Ba_1-xKₓAs₂Fe₂ single crystals and FeSe_1-xTeₓ films tuned by doping and strain. We observed an Hc2 enhancement by more than 25 T at low temperatures for the optimally doped Ba_1-xKₓAs₂Fe₂ as compared to the previous measurements and extraordinarily high slopes of dHc2/dT $=$ 250--500 T/K near Tc in FeSe_1-xTeₓ, indicating almost-complete suppression of orbital pair breaking. Theoretical analysis of Hc2(T) suggests an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state below 10K for $H//ab$ in the optimally doped Ba_1-xKₓAs₂Fe₂ and below 3K for $H//c$ and 9K for $H//ab$ in FeSe_1-xTeₓ. The analysis also shows that Hc2 in a multiband Fe-based superconductor can be significantly enhanced by doping and strain by shrinking and expanding different pockets of the Fermi surface, which can be more effective than the conventional way of increasing Hc2 by nonmagnetic impurities.