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Searching for the analogues of high-energy particles in solids has become a hot topic since the discovery of Dirac semimetals Na₃Bi and Cd₃As₂, and Weyl semimetals TaAs family. The massless Dirac fermions described by Dirac equation behave in the similar way as the low energy excitation around the four-fold degenerate nodal points formed by band crossing in solids. The chiral Weyl fermions are described by two-component Weyl equation with two nondegenerate bands touching at a point. In solids, the space group can protect energy nodes with other degeneracies, such as three, six and eight-fold one, which might lead to massless fermions that have no counterpart particles in high-energy physics[1]. In this talk, I will introduce our recent theoretical proposals of triply degenerate nodal point semimetals. They are crystals with symmorphic space group symmetry of WC type crystal structure, including TaN[2], ZrTe[3], HfC[4], MoP[5,6] and WC[7,8]. The unconventional three component fermions in them are formed by crossing of nondegenerate and double degenerate bands, protected by both rotational and mirror symmetries. As an intermediate fermion between Dirac and Weyl fermion, the host semimetal has different magnetoresistance from Dirac semimetal and Weyl semimetal.

[1] B. Bradlyn et al., 353, aaf 5037 (2016); H. Weng et al., Nat. Sci. Rev. doi:10.1093/nsr/nwx066
[2] H. Weng et al., Phys. Rev. B 93, 241202 (2016)
[3] H. Weng et al., Phys. Rev. B 94, 165201 (2016)
[4] R. Yu et al., Phys. Rev. Lett. 193, 036401 (2017)
[5] Z. Zhu et al. Phy. Rev. X 6, 031003 (2016); J. Sun et al. Chin. Phys. Lett. 34, 027102 (2017).
[6] B. Q. Lv et al., Nature 546, 627 (2017)
[7] J. B. He et al., Phys. Rev. B 95, 195165 (2016)
[8] J. Z. Ma et al., arXiv: 1706.02664

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