Doping-driven structural phase transition and loss of superconductivity in MxFe1-xSeδ (M=Mn, Cu)

In this paper, we report the results of detailed studies on Mn and Cu substitution to Fe site of beta-FeSe, namely, MnxFe(1-x)Se(1-delta) and CuxFe1-xSe1-delta (delta equals to 0.03-0.05 based on our neutron-diffraction refinements). The results show that with only 10 at. % Cu doping the compound becomes a Mott insulator. Detailed temperature-dependent structural analyses of these Mn- and Cu-substituted compounds show that the structural transition, which is associated with the changes in the building block FeSe4 tetrahedron, is essential to the occurrence of superconductivity in beta-FeSe.