Emergent Weyl Fermions and the Metal to Weyl-Semimetal phase transition in WTe2,via broadband High Resolution NMR

Weyl Fermions (WFs) in the type-II Weyl Semimetal (WSM) WTe2 are difficult to resolve experimentally because the Weyl bands disperse in an extremely narrow region of the (E-k) space. Here, by using DFT-assisted high-resolution 125Te solid-state NMR (ssNMR) in the temperature range 50K - 700K, we succeeded in detecting low energy WF excitations and monitor their evolution with temperature. Remarkably, WFs appear to emerge at T∼120K; at lower temperatures WTe2 behaves as a metal. This intriguing metal-to-WSM phase transition is shown to be induced by the rapid raise of the Fermi level with temperature, crossing solely the electron and hole pockets in the low-T metallic phase, while crossing the Weyl bands near the nodal points - a prerequisite for the emergence of WFs - only for T>120K