Crystal chemistry, superconductivity and magnetism in iron chalcogenides

The structural simplicity of FeCh (Ch = S, Se, Te), in which the conducting layers are not separated by any third-atom layers, offers the best tool for investigating the nature of superconductivity and magnetism in Fe-based compounds. Either the pressure or the chemical composition distort the FeCh4 tetrahedron and tune the electronic properties. When partially substituting Se for Te in the antiferromagnetic Fe1+xTe, the excess of Fe is reduced and superconductivity appears over a wide range of compositions. Both the Fe excess and the Se substitution affect the structure and must be kept under control for tuning the structure deformation and the electronic properties. The excess of Fe stretches the FeCh4 tetrahedron, thus inducing spin and charge localization. Below a critical Fe-Ch distance, the antiferromagnetism is weakened and superconductivity occurs, mediated by spin fluctuations as in the similar families of Fe-based oxy-pnictides. Uniaxial rather than hydrostatic pressure has a strong effect on the magnetic and superconducting properties of FeCh.