Relationship between Structure and Superconductivity in FeSe_1-<i>x</i>Te_<i>x</i>

Abstract

We determined the crystal structural parameters of FeSe 1- x Te x ( x =0.5, 0.625, 0.75, 0.875) and FeSe by using the neutron powder diffraction technique. It was found that the superconducting transition temperature T c of FeSe 1- x Te x increased with the (Se,Te)–Fe–(Se,Te) bond angle α in the Te-doped samples. However, T c of FeSe did not exhibit a similar relation. In order to explain this contradiction, we studied the temperature dependence of the z position of chalcogens, which is strongly coupled to the spin fluctuation by theoretical calculations. The z position in FeSe showed a remarkable shift, indicating that strong spin fluctuation can affect the superconductivity in FeSe.