Development of a ferromagnetic component in the superconducting state of Fe-excess Fe_1.12 Te_1-x Se_x by electronic charge redistribution

The general picture established so far for the links between superconductivity and magnetic ordering in iron chalcogenide Fe_1+y (Te_1-x Se_x) is that the substitution of Se for Te directly drives the system from the antiferromagnetic end into the superconducting regime. Here, we report on the observation of a ferromagnetic component that developed together with the superconducting transition in Fe-excess Fe_1.12 Te_1-x Se_x crystals using neutron and x-ray diffractions, resistivity, magnetic susceptibility and magnetization measurements. The superconducting transition is accompanied by a negative thermal expansion of the crystalline unit cell and an electronic charge redistribution, where a small portion of the electronic charge flows from around the Fe sites toward the Te/Se sites. First-principles calculations show consistent results, revealing that the excess Fe ions play a more significant role in affecting the magnetic property in the superconducting state than in the normal state and the occurrence of an electronic charge redistribution through the superconducting transition.