A route for simultaneous increase of T_c and J_c in iron-based superconductors by low-energy proton irradiation

We have grown iron-chalcogenide FeSe_0.5Te_0.5 (FST) superconducting films on various single crystal and flexible metal substrates [Applied Physics Letters 95, 052504 (2009); Rep. Prog. Phys. 74, 124510 (2011)]. The FST films on CeO₂ buffer layer exhibit enhanced transition temperature T_c, which is about 30% higher than that found in the bulk materials, and carry high critical current density J_c [Nat. Commun. 4, 1347 (2013)]. In this talk, I will present a new route for simultaneous increase of T_c and J_c in FST films by low-energy proton irradiation [Nat. Commun. 7, 13036 (2016).]. We provide direct atomic-scale imaging of cascade defects and mapping of the surrounding nanoscale strain field produced by low-energy proton irradiation. T_c is enhanced due to the nanoscale compressive strain and proximity effect, while J_c is doubled at zero field and 4.2 K through strong vortex-pinning by the cascade defects and surrounding nanoscale strain. We observed an increase of J_c at 12 K by one order of magnitude after the irradiation at magnetic fields over 15 T for H//ab and over 6 T for H//c. This study opens up the possibility to achieve significant enhancement of J_c without T_c reduction through the design of vortex pinning landscape by ion irradiation in iron-based superconductors