Abstract
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.
| Original language | English |
|---|---|
| Article number | 186801 |
| Journal | Physical Review Letters |
| Volume | 122 |
| Issue number | 18 |
| DOIs | |
| Publication status | Published - 7 May 2019 |
Funding
We have shown that systems in the SnTe material class are a new potential platform for Weyl and nodal line semimetals. The key ingredients are a rhombohedral distortion, induced by an elastic strain along the [111] direction, followed by inversion-symmetry breaking. By deriving and analyzing a minimal k · p model, we have demonstrated how this two-step symmetry-reduction process leads to the appearance of topologically stable Weyl nodes. Complementary, we find that the mirror symmetry in group-IV tellurides also gives rise to a semimetallic phase featuring topologically protected nodal lines, a class of systems for which only a few candidate materials have been put forward. There are various, feasible ways to realize our proposal experimentally. First, the conditions for the symmetry reduction are naturally provided in the low-temperature ferroelectric phase of SnTe and GeTe, which could therefore represent novel Weyl ferroelectric semimetals [60,61] . Moreover, additional external strain, pressure, or alloying [48,49,62–65] could be employed to tune the parameters of the systems. In fact, a recent experimental report supports the existence of semimetallic phases in Pb-alloyed SnTe under pressure [66] . Another route is the use of substrates with different lattice structures. This could induce an inhomogeneous strain close to the substrate interface mimicking a ferroelectric distortion. Finally, our proposal could also be applied to group-V semimetals such as Bi and Sb [67] . These materials are in the rhombohedral space group R 3 ¯ m with an inversion center and have a similar band structure as group-IV tellurides [68] . To realize Weyl nodes or nodal lines, one could therefore use thin films where inversion symmetry can be broken by either using substrates, as explained above, or externally applying a perpendicular electric field. We thank D. Varjas and T. 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