Detection of Weyl fermions and the metal to Weyl-semimetal phase transition in WTe_{2} via broadband high resolution NMR

Abstract

Weyl fermions (WFs) in the type-II Weyl semimetal (WSM) WTe_{2} 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 ^{125}Te 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 WTe_{2} 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.