Suppression of superconductivity in Fe chalcogenides by annealing: A reverse effect to pressure

Superconductivity in FeTe$_{1-x}$Se$_{x}$ can be controlled by annealing, in the absence of extrinsic influences. Using neutron diffraction, we show that T$_{C}$ sensitively depends on the atomic configurations of the Te and Se ions. Low temperature annealing not only homogenizes the Te and Se ion distribution as previously observed, it suppresses T$_{C}$ because of changes in the chalcogen ions' z-parameter. In particular, the height of Te from the Fe basal plane is much reduced while that for Se shows a modest increase. These trends are reverse of the effects induced by pressure.