Kramers nodal lines in 3R-TaS₂

It has recently been proposed that all non-centrosymmetric crystals host so-called Kramers nodal lines (KNLs), which are doubly degenerate band crossings connecting time-reversal invariant momenta (TRIMs) in the Brillouin zone. This proposal has sparked significant interest due to the inherent robustness of KNLs against spin-orbit coupling (SOC) and their potential to drive unconventional physical phenomena, such as unusual spin textures, magnetoelectric effects, and novel optical responses. Furthermore, if the enforced touching point along a KNL is between an electron Fermi pocket and a hole Fermi pocket, in the few layers limit the system is expected to exhibit a quantized optical conductivity. Unfortunately, realistic materials with purely this type of enforced crossing have not been identified so far.

Here we show evidence of a few-layers-thick 3R polytype of TaS2 (space-group 160) where the KNL would enforce a touching point between an electron and a hole pocket. Furthermore, since the Fermi Surfaces (FSs) enclose two TRIMs each, we expect the existence of the degeneracy at the Fermi level to be tunable by strain. Thus, whenever the nodal line enforces a touching point at the Fermi level, the resulting FS is homeomorphic to the wedge sum of 2-Toruses T².

These results motivate further exploration of these systems, both because of their potential tunability by strain and as a parent compound of KWSs, thereby significantly extending the range of candidate materials exhibiting Weyl physics.