Enhanced upper critical field in Co-doped Ba122 superconductors by lattice defect tuning

Nanoscale defects in superconductors play a dominant role in enhancing superconducting properties through electron scattering, modulation of coherence length, and correlation with quantized magnetic flux. For iron-based superconductors (IBSCs) that are expected to be employed in high-field magnetic applications, a fundamental question is whether such defects develop an upper critical field (H-c2) similar to that of conventional BCS-type superconductors. Herein, we report the first demonstration of a significantly improved H-c2 in a 122-phase IBSC by introducing defects through high-energy milling. Co-doped Ba122 polycrystalline bulk samples [Ba(Fe, Co)(2)As-2] were prepared by sintering powder which was partially mechanically alloyed through high-energy milling. A remarkable increase in the full-width at half maximum of X-ray powder diffraction peaks, anomalous shrinkage in the a-axis, and elongation in the c-axis were observed. When lattice defects are introduced into the grains, the semiconductor behavior of the electric resistivity at a low temperature (T < 100 K), a slight decrease in transition temperature (T-c), an upturn of H-c2(T) near T-c, and a large increase in the H-c2(T) slope were observed. The slope of H-c2(T) increased approximately by 50%, i.e., from 4 to 6 T/K, and exceeded that of single crystals and thin films. Defect engineering through high-energy milling is expected to facilitate new methods for the designing and tuning of H-c2 in 122-phase IBSCs. (C) 2019 Author(s).