Control of Tetrahedral Coordination in FeSe Superconductors

Background: The tetrahedral coordination of Fe surrounded by 4 Se(Te) atoms is of crucial importance for the new high T_C Fe pnictides superconductors with lattice parameters c and a. To reveal the essential aspects of the tetrahedron, one needs to vary the lattice parameter c and a in opposite manner, without altering the electronic properties.  This cannot be achieved by chemical substitution, which alters electronic properties, nor high pressure, which decreases both c and a.

This work: We used epitaxial thin films to achieve biaxial stress in material of the same composition but with lattice parameters c and a varying in opposite manner, as facilitated by the substrate temperature T_S.  We used high resolution x-ray and neutron diffraction to determine the structure details and correlate with T_C. We show that the chalcogen height is the most essential quantity for T_C.