Detection of Weyl Fermions and the Metal to Weyl-Semimetal phase transition in WTe$_2$ via broadband High Resolution NMR

TL;DR

This study uses broadband high-resolution NMR to detect Weyl fermions in WTe$_2$ and observes a temperature-induced transition from metal to Weyl semimetal around 120K, revealing the evolution of electronic states.

Contribution

It demonstrates a novel NMR approach to directly detect Weyl fermions and characterizes the temperature-driven phase transition in WTe$_2$.

Findings

Weyl fermions emerge at approximately 120K.

WTe$_2$ behaves as a metal below 120K.

The phase transition is driven by Fermi level shifts.

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.