Centrosymmetric Li₂NaN: a superior topological electronic material with critical-type triply degenerate nodal points

Abstract

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 Li 2 NaN is a realistic TDNP semimetal that meets all these requirements. Li 2 NaN features only a single pair of TDNPs at the Fermi level, under the protection of the C 6v 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 Li 2 NaN 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.Materials with nontrivial band topology have attracted great interests in recent years. Beside their potential technological applications, topological materials provide the possibility to study the fundamental physics theories in the relatively convenient condensed matter scale. For example, Weyl (Dirac) semimetals 1-14 host Weyl (Dirac) points with doubly (fourfold-) degenerate linear band crossings near the Fermi level, and their low-energy excitations can serve as the analogues of Weyl (Dirac) electrons in high-energy theories. More interestingly, three types of quasiparticle excitations, namely triply, six, and eightfold-degenerate nodal points, which even do not have high-energy counterparts, have also been proposed in realistic topological materials 15,16 . In particular, the triply degenerate nodal points (TDNPs) 17-26 , which are formed by the crossing between a non-degenerate band and a doubly-degenerate band, are proposed to host intriguing physics, such as large negative magnetoresistance 27 , helical anomaly 20 , exotic Fermi surface transitions 19,20 , and unconventional quantum Hall effects 28 .Initially, TDNP semimetals are predicted in some inversion-asymmetric materials, including the strained-HgTe 17 , WC-type materials 19-22 , Heusler compounds 24,25 , NaCu 3 Te 2 family 26 , PtBi 2 compound 29 , etc. Recently, Zhang et al. 23 first reported the existence of TDNPs in the centrosymmetric TiB 2 materials by theoretical prediction. They found the TDNPs in these centrosymmetric materials can transform into a novel type of Dirac points, which are drastically different from those in inversion-asymmetric systems. Beside theoretical predictions, several angle-resolved photoemission spectroscopy (ARPES) and transport experiments have also been carried out to probe the electronic structures around TDNPs in WC-type materials 27,30-33 and the a) Electronic t...

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