Spin-momentum locking describes a fixed directional relationship between the spin and the crystal momentum of an electronic band over its entire Fermi surface. The most well-known form of spin-momentum locking is of the orthogonal Rashba-type, which has been discovered over 30 years ago and has inspired numerous fundamental insights and applications, ranging from topological superconductivity to the recently discovered Josephson diodes that could revolutionize superconducting electronics. Whilst other forms of spin-momentum locking have also been known for a long time, purely parallel spin-momentum locking - which can be considered the natural counterpart of the Rashba-type - has remained elusive in experiments. Finding empirical evidence for such monopole-like spin-textures would be of fundamental as well as practical interest since they have been predicted to realize various new phenomena, such as novel spin-orbit torques for high-density memory storage or exotic forms of superconductivity.
Our group and simultaneously other groups from the USA, China, and Japan have recently discovered topological multifold fermions in a family of chiral topological semimetals by employing angle-resolved photoelectron spectroscopy. In my talk, I will review these discoveries and present the first experimental evidence that these multifold fermions exhibit parallel spin-momentum locking.