Mott localization in a pure stripe antiferromagnet Rb$_{1-delta}$Fe$_{1.5-sigma}$S$_2$

A combination of neutron diffraction and angle-resolved photoemission spectroscopy measurements on a pure antiferromagnetic stripe Rb$_{1-delta}$Fe$_{1.5-sigma}$S$_2$ is reported. A neutron diffraction experiment on a powder sample shows that a 98$%$ volume fraction of the sample is in the antiferromagnetic stripe phase with rhombic iron vacancy order and a refined composition of Rb$_{0.66}$Fe$_{1.36}$S$_{2}$, and that only 2$%$ of the sample is in the block antiferromagnetic phase with $sqrt{5}times sqrt{5}$ iron vacancy order. Furthermore, a neutron diffraction experiment on a single crystal shows that there is only a single phase with the stripe antiferromagnetic order with the refined composition of Rb$_{0.78}$Fe$_{1.35}$S$_2$, while the phase with block antiferromagnetic order is absent. Angle-resolved photoemission spectroscopy measurements on the same crystal with the pure stripe phase reveal that the electronic structure is gapped at the Fermi level with a gap larger than 0.325 eV. The data collectively demonstrates that the extra 10$%$ iron vacancies in addition to the rhombic iron vacancy order effectively impede the formation of the block antiferromagnetic phase; the data also suggest that the stripe antiferromagnetic phase with rhombic iron vacancy order is a Mott insulator.