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

Abstract

Given the importance of crystal symmetry for the emergence of topological quantum states, we have studied, as exemplified in $\mathrm{NbNiT}{\mathrm{e}}_{2}$, the interplay of crystal symmetry, atomic displacements (lattice vibration), band degeneracy, and band topology. For the $\mathrm{NbNiT}{\mathrm{e}}_{2}$ structure in space-group 53 (Pmna)---having an inversion center arising from two glide planes and one mirror plane with a twofold 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-II Weyl points in the \ensuremath{\Gamma}-$X$ 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.