The centrosymmetric Li2NaN material: an ideal topological semimetal with critical-type triply degenerate nodal points
Triply degenerate band crossing can generate novel triply degenerate nodal point (TDNP) fermions. For the TDNP character to the best manifest in physical properties, it desires that the TDNPs are near the Fermi level, far away with each other in the reciprocal space, and not coexisting with other quasiparticles or extraneous bands. Here we predict that the centrosymmetic material Li2NaN is a realistic TDNP semimetal that meets all these requirements. Li2NaN features only a single pair of TDNPs at the Fermi level, under the protection of the C6v symmetry. Interestingly, the TDNPs identified here show the critical-type band crossing, which is different from those in previously reported TDNP semimetals. The Fermi arc surface states for the TDNPs have been identified. Under lattice strain, we show that the position of the TDNPs can be tuned, and the TDNPs can even be annihilated. The centrosymmetry in Li2NaN makes the TDNP transform into a critical type Dirac point under the spin-orbit coupling, which is drastically distinct from those in noncentrosymmetric systems. Such centrosymmetic TDNP semimetal has been rarely identified in realistic materials.