A route for a strong increase of critical current in nanostrained iron-based superconductors

The critical temperature T_c and the critical current density J_c determine the limits to large-scale superconductor applications. Superconductivity emerges at T_c. The practical current-carrying capability, measured by J_c, is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce T_c. Simultaneous increase of T_c and J_c in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both T_c and J_c together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe_0.5Te_0.5 films. T_c is enhanced due to the nanoscale compressive strain and proximity effect, whereas J_c is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of J_c enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.