S/Te co-doping in tetragonal FeSe with unchanged lattice parameters: Effects on superconductivity and electronic structure

Previous study suggested that Se-site doping with Te or S in tetragonal FeSe adjusts the superconductivity only by expanding or compressing the lattice parameters . However, three chalcogens X ( X  = S, Se, Te) show different electronegativity, which could directly change the interaction between Fe & X , and their electronic structures. In order to understand this underlying mechanism, the samples with different anion compositions but the same lattice parameters are required. In this work, two superconductors : FeSe 0.95 Te 0.03 S 0.02 and FeSe 0.9 Te 0.057 S 0.043 have been synthesized by solid state reaction. The thus-obtained samples have the same lattice parameters in tetragonal single phase as that of undoped FeSe. It is found that despite the unchanged lattice, S/Te co-doping on Se-site still increased the density of the electronic states (DOS) near the Fermi level, thereby enhanced the superconductivity. Our results suggest that the influence of Se-site doping with the same valence chalcogen in superconducting FeSe is not simply in the view of the lattice size, effects on electronic structures originated from different electronegativity during Se-site doping should also be considered.