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 NMR (ssNMR) in the temperature range $50$K - $700$K, we succeeded in detecting low energy WF excitations and monitor their evolution with temperature. Remarkably, WFs appear to emerge at T$\sim 120$K; at lower temperatures WTe${}_2$ 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$>120$K.

23 pages (5 main article, 18 supplementary material), 16 figures (4 main article, 12 supplementary material), 1 table (supplementary material)