Control of Tetrahedral Coordination and Superconductivity inFeSe0.5Te0.5Thin Films

We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the scenario that (π, π) spin fluctuations promote superconductivity in Fe superconductors.