Abstract
Quantum materials such as antiferromagnets or superconductors are complex in that chemical,
electronic, and spin phenomena at atomic scales can manifest in their collective properties. Although
there are some clues for designing such materials, they remain mainly unpredictable. In this work, we
find that enhancement of transition temperatures in BaFe2As2-based crystals are caused by removing
local-lattice strain and electronic-structure disorder by thermal annealing. While annealing improves
Néel-ordering temperature in BaFe2As2 crystal (TN=132K to 136K) by improving in-plane electronic
defects and reducing overall a-lattice parameter, it increases superconducting-ordering temperature
in optimally cobalt-doped BaFe2As2 crystal (Tc=23 to 25K) by precipitating-out the cobalt dopants
and giving larger overall a-lattice parameter. While annealing improves local chemical and electronic
uniformity resulting in higher TN in the parent, it promotes nanoscale phase separation in the
superconductor resulting in lower disparity and strong superconducting band gaps in the dominant
crystal regions, which lead to both higher overall Tc and critical-current-density, Jc
