  We fabricated single- and 3-core superconducting Fe-Se wires using a novel
process based on a chemical transformation from hexagonal FeSe1+d
(non-superconducting) to tetragonal FeSe (superconducting) via an optimal
supply of Fe from the Fe sheath by annealing. This process enhanced a packing
density of superconducting core inside the sheath, owing to an expansion of the
lattice volume via a chemical transformation from high-density hexagonal
FeSe1+d to low-density tetragonal FeSe. The obtained superconducting wire
showed superconductivity below ~10 K and the upper critical field was estimated
to be 19.3 T at 0 K. The obtained transport critical current densities at 0 T
were 588 A/cm2 for the 3-core wire and 218 A/cm2 for the single-core wire.
