Topological Semimetals in the SnTe Material Class: Nodal Lines and Weyl Points

We theoretically show that IV-VI semiconducting compounds with lowerature rhombohedral crystal structure represent a new potential platform for topological semimetals. By means of minimal k·p models, we find that the two-step structural symmetry reduction of the higherature rocksalt crystal structure, comprising a rhombohedral distortion along the [111] direction followed by a relative shift of the cation and anion sublattices, gives rise to topologically protected Weyl semimetal and nodal line semimetal phases. We derive general expressions for the nodal features and apply our results to SnTe, showing explicitly how Weyl points and nodal lines emerge in this system. Experimentally, the topological semimetals could potentially be realized in the lowerature ferroelectric phase of SnTe, GeTe, and related alloys.