Coexistence of topological bosons and topological fermions in triple-point-metals*

Recent progress in the discovery of Dirac and Weyl fermions in crystal systems has inspired researchers to search for other novel fermionic quasi-particles that are topologically protected by the crystal symmetries, and are beyond the Dirac, Weyl and Majorana classification scheme. Recently, three-component fermions which show fermionic statistics with an integer (pseudo)spin degree of freedom have been discovered in the triple-point-metals (TPM) [1,2]. Akin to the topological fermionic systems, researchers have also theoretically envisioned and experimetnally detected topological bosonic quasiparticles in photonic and phononic crystals at kHz frequency [3,4]. Here we demonstrate the appearance a new three-component topological bosonic excitation at THz frequency near a triply-degenerate gapless point in the phonon spectra of special TPM. Interestingly, this three-component boson coexists with the three-component fermions, and both appear near the same point in the reciprocal space. Both type-I and type-II Weyl fermions have also been spotted the studied material.
[1] Phys. Rev. X 6, 031003 (2016); [2] Phys. Rev. B 95, 241116 (2017); [3] Nat. Mater. 12, 233 (2013); [4] Nat. Phys. 12, 621 (2016)