Detection of 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 Te125 solid-state nuclear magnetic resonance (ssNMR) in the temperature range 50-700 K, we succeeded in detecting low energy WF excitations and monitoring their evolution with temperature. Remarkably, WFs are observed to emerge at T∼120 K, while at lower temperatures WTe2 behaves as a trivial metal. This intriguing phenomenon is induced by the rapid raise of the Fermi level upon heating, which crosses the Weyl bands only for T>120 K. The abrupt change of the NMR parameters at this temperature is signature of a topological Lifshitz transition instead of a cursive energy-bands crossing.