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

TL;DR

This study demonstrates that doping and strain significantly enhance the upper critical fields in Fe-based superconductors, with measurements showing increased Hc2 and theoretical analysis predicting exotic superconducting states.

Contribution

It reveals that doping and strain can effectively increase Hc2 in Fe-based superconductors, surpassing traditional methods like increasing disorder.

Findings

Hc2 increased by nearly 25 T at 30 K in optimally-doped samples

High slopes of dHc2/dT indicate suppression of orbital pair-breaking

Prediction of Fulde-Ferrel-Larkin-Ovchinnikov state at low temperatures

Abstract

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.