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

Abstract

We report measurements up to 85 Tesla of the upper critical fields ${H}_{c2}$($T$) on Ba${}_{1\ensuremath{-}x}$K${}_{x}$As${}_{2}$Fe${}_{2}$ single crystals and FeSe${}_{1\ensuremath{-}x}$Te${}_{x}$ films tuned by doping and strain. We observed an ${H}_{c2}$ enhancement by more than 25 T at low temperatures for the optimally doped Ba${}_{1\ensuremath{-}x}$K${}_{x}$As${}_{2}$Fe${}_{2}$ as compared to the previous measurements and extraordinarily high slopes of $d{H}_{c2}$/$\mathit{dT}$ $=$ 250--500 T/K near ${T}_{c}$ in FeSe${}_{1\ensuremath{-}x}$Te${}_{x}$, indicating almost-complete suppression of orbital pair breaking. Theoretical analysis of ${H}_{c2}$($T$) suggests an inhomogeneous Fulde-Ferrel-Larkin-Ovchinnikov state below 10K for $H//ab$ in the optimally doped Ba${}_{1\ensuremath{-}x}$K${}_{x}$As${}_{2}$Fe${}_{2}$ and below 3K for $H//c$ and 9K for $H//ab$ in FeSe${}_{1\ensuremath{-}x}$Te${}_{x}$. The analysis also shows that ${H}_{c2}$ 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 ${H}_{c2}$ by nonmagnetic impurities.