Transport properties of single- and three-core FeSe wires fabricated by a novel chemical-transformation PIT process

We fabricated single- and three-core superconducting FeSe wires by a novel process based on a chemical transformation from hexagonal FeSe 1 + d (non-superconducting) to tetragonal FeSe (superconducting) via an optimal supply of Fe from the Fe sheath by annealing. This process enhanced the packing density of the core inside the sheath, owing to an expansion of lattice volume via a chemical transformation from high-density hexagonal FeSe 1 + d to low-density tetragonal FeSe. The obtained wire showed superconductivity below ∼ 10 K and the obtained transport critical current densities at 0 T and 4.2 K were 588 A cm − 2 for the three-core wire and 218 A cm − 2 for the single-core wire.