Topological Nontrivial Surface Plasmon on a Type-II Weyl Semimetal

Type-II Weyl semimetals (WSMs), with the Lorentz invariance broken and hosting tilted Weyl cones giving rise to non-point-like Fermi surfaces (FSs), have attracted plentiful attention recently. The orthorhombic phase of (W, Mo)Te₂ is predicted to be a prototypical system of Type-II WSM. However, the topological signatures are not clear enough from the single-electron measurements due to its complicated band structure. Meanwhile, measurements of electronic collective excitations (i. e. plasmons) in topological bands may provide more evidence of the topological nature, since they possess some exotic properties distinct from those in standard electron gases. Here, using momentum resolved inelastic electron scattering, we report an observation of a topological nontrivial surface plasmon mode originated from the topological bands of MoTe₂, which displays unique super-linear temperature-dependence matching the predictions for that in a WSM. Moreover, the damping of this nontrivial mode matches well with the boundary of electron-hole particle continuum from Weyl band. These findings provide consolidated evidences of the existence of topological nontrivial bands in MoTe₂.