Background: The tetrahedral coordination of Fe surrounded by 4 Se(Te) atoms is of crucial importance for the new high TC 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 can not be achieved by chemical substitution, which alters electronic properties, nor high pressure, which decreases both c and a.

Control of Tetrahedral Coordination in FeSe Superconductors MRSEC (Johns Hopkins University) DMR05-20491

S. X. Huang, C. L. Chien, V. Thampy, and C. Broholm, ”Control of tetrahedral coordination and superconductivity in FeSe0.5Te0.5 thin films” Phys. Rev. Lett. 104, 217001 (2010).

Fig. 1: Lattice constant c and a vs. substrate temperaturefor epitaxial FeSe0.5Te0.5 thin films.

Fig. 2: TC of epitaxial FeSe0.5Te0.5 vs. relative chalcogen height .

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 TS. We used high resolution x-ray and neutron diffraction to determine the structure details and correlate with TC. We show that the chalcogen height is the most essential quantity for TC.

Neutron diffraction