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# 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 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 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 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.