Chiral topological semimetals are a new class of topological matter that host chiral multifold fermions in a chiral crystal structure. These new fermionic quasiparticles can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Their large topological charge has been predicted to give rise to unusual phenomena, such as giant quantized photocurrents, long fermi-arc surface states, unusual magnetotransport signatures, new spin-orbit torques, or unconventional and topological superconductivity. Whilst there have been many theoretical predictions related to multifold fermions in previous years, they have so far remained elusive in experiments.
Here I will report the experimental observation of multifold fermions in a chiral topological semimetal. Using angle-resolved photoelectron spectroscopy, we directly visualize their long fermi-arc surface states and resolve a band splitting that indicates that they carry the largest topological charge that can be realized for quasiparticles in any material. We are also able to show experimentally that there is a direct relationship between the handedness of the crystal structure and the electronic chirality (i.e. the Chern number sign) of the multifold fermions, which indicates that structural chirality can be used as a control parameter to manipulate phenomena that are sensitive to the electronic chirality, such as the direction of topological photocurrents. I will then also present our latest experimental results about new directions in the field of chiral topological semimetals.