# Discovery of Weyl fermion semimetals and topological Fermi arc states

## TL;DR

This paper reviews the discovery and experimental investigation of Weyl semimetals, highlighting their topological Fermi arc surface states and Weyl nodes, with a focus on TaAs and NbAs materials.

## Contribution

It provides a comprehensive overview of the experimental discovery, characterization, and theoretical understanding of Weyl semimetals and their unique surface states.

## Key findings

- Observation of topological Fermi arcs in TaAs and NbAs
- Detection of Weyl nodes via photoemission spectroscopy
- Evidence of chiral anomaly in magnetotransport measurements

## Abstract

Weyl semimetals are conductors whose low-energy bulk excitations are Weyl fermions, whereas their surfaces possess metallic Fermi arc surface states. These Fermi arc surface states are protected by a topological invariant associated with the bulk electronic wavefunctions of the material. Recently, it has been shown that the TaAs and NbAs classes of materials harbor such a state of topological matter. We review the basic phenomena and experimental history of the discovery of the first Weyl semimetals, starting with the observation of topological Fermi arcs and Weyl nodes in TaAs and NbAs by angle and spin-resolved surface and bulk sensitive photoemission spectroscopy and continuing through magnetotransport measurements reporting the Adler-Bell-Jackiw chiral anomaly. We hope that this article provides a useful introduction to the theory of Weyl semimetals, a summary of recent experimental discoveries, and a guideline to future directions.