Natural sulfur-containing minerals as topological insulators with a wide band gap

The electronic structure of quaternary natural minerals PbBi$_2$Te$_2$S$_2$ (aleksite), Pb$_2$Bi$_2$Te$_2$S$_3$ (saddlebackite), and PbBi$_4$Te$_4$S$_3$ ($C$ phase) has been theoretically studied. These compounds have a layered structure: aleksite and saddlebackite are formed by seven- and nine-layer blocks, respectively, and $C$ phase consists of five- and seven-layer blocks separated by van der Waals gaps. It has been shown that all studied materials are three-dimensional topological insulators with a wide band gap in the electronic spectrum and have a high spin polarization. For these reasons, they are promising for applications.