Phonon-induced topological transition to a type-II Weyl semimetal

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied here, as exemplified in NbNiTe₂, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For the NbNiTe₂ structure in space-group 53 (Pmna)− having an inversion center arising from two glide planes and one mirror plane with a two-fold rotation and screw axis−a full gap opening exists between two band manifolds near the Fermi energy. Upon atomic displacements by optical phonons, the symmetry lowers to space-group 28 (Pma2), eliminating one glide plane along c, the associated rotation and screw axis, and the inversion center. As a result, 20 Weyl points emerge, including four type-IIWeyl points in the Γ-X direction at the boundary between a pair of adjacent electron and hole bands. Thus, optical phonons may offer control of the transition to a Weyl fermion state.