We report neutron scattering and transport measurements on semiconducting Rb$_{ 0.8}$Fe$_{ 1.5}$S$_2$, a compound isostructural and isoelectronic to the well-studied $A_{ 0.8}$Fe$_{ y}$Se$_2 (A=$ K, Rb, Cs, Tl/K) superconducting systems. Both resistivity and DC susceptibility measurements reveal a magnetic phase transition at $T=275$ K. Neutron diffraction studies show that the 275 K transition originates from a phase with rhombic iron vacancy order which exhibits an in-plane stripe antiferromagnetic ordering below 275 K. In addition, interdigitated mesoscopically with the rhombic phase is an ubiquitous phase with $\sqrt{ 5}\times\sqrt{ 5}$ iron vacancy order. This phase has a magnetic transition at $T_N=425$ K and an iron vacancy order-disorder transition at $T_{ S}=600$ K. These two different structural phases are closely similar to those observed in the isomorphous Se materials. Based on the close similarities of the in-plane antiferromagnetic structures, moments sizes, and ordering temperatures in semiconducting Rb$_{ 0.8}$Fe$_{ 1.5}$S$_2$ and K$_{ 0.81}$Fe$_{ 1.58}$Se$_2$, we argue that the in-plane antiferromagnetic order arises from strong coupling between local moments. Superconductivity, previously observed in the $A_{ 0.8}$Fe$_{ y}$Se$_{ 2-z}$S$_z$ system, is absent in Rb$_{ 0.8}$Fe$_{ 1.5}$S$_2$, which has a semiconducting ground state. The implied relationship between stripe/block antiferromagnetism and superconductivity in these materials as well as a strategy for further investigation is discussed in this paper.
Categories: cond-mat.supr-con