Title: 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 NbNiTe2, the interplay of
crystal symmetry, atomic displacements (lattice vibration), band degeneracy,
and band topology. For NbNiTe2 structure in space group 53 (Pmna) - having an
inversion center arising from two glide planes and one mirror plane with a
2-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, twenty Weyl points emerge, including four type-II Weyl points in the
G-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.