Controllable orbital angular momentum monopoles in chiral topological semimetals

The emerging field of orbitronics aims to generate and control orbital angular momentum for information processing. Chiral crystals are promising orbitronic materials because they have been predicted to host monopole-like orbital textures, where the orbital angular momentum aligns isotropically with the electron's crystal momentum. However, such monopoles have not yet been directly observed in chiral crystals. Here, we use circular dichroism in angle-resolved photoelectron spectroscopy to image orbital angular momentum monopoles in the chiral topological semimetals PtGa and PdGa. The spectra show a robust polar texture that rotates around the monopole as a function of photon energy. This is a direct consequence of the underlying magnetic orbital texture and can be understood from the interference of local atomic contributions. Moreover, we also demonstrate that the polarity of the monopoles can be controlled through the structural handedness of the host crystal by imaging orbital angular moment monopoles and antimonopoles in the two enantiomers of PdGa, respectively. Our results highlight the potential of chiral crystals for orbitronic device applications, and our methodology could enable the discovery of even more complicated nodal orbital angular momentum textures that could be exploited for orbitronics. The motion of electrons in solids can be described by wave packets, which may have a self-rotation around their centre of mass. This self-rotation gives rise to an orbital magnetic moment that is proportional to the orbital angular momentum (OAM) of the Bloch states2. OAM currents have been proposed as a viable alternative to spin currents for manipulating and controlling magnetism in nanoscale memory devices by giving rise to large spin and orbital torques3–8. However, most recent investigations of orbital currents and the related orbital Hall effect have mainly focused on non-magnetic centrosymmetric materials where the OAM is suppressed in the equilibrium ground state and can be generated only with the application of an electrical field or at surfaces9–11. Magnetic materials, which have an intrinsic orbital moment, have also been shown to exhibit the orbital Hall effect12,13. The common key feature for a sizeable orbital Hall effect is a non-trivial momentum dependence of magnetic orbitals (known as orbital texture)14, which can enable orbital currents even in centrosymmetric crystals15,16. In non-magnetic helical molecules and non-magnetic structurally chiral crystals, the Bloch bands have such an OAM texture in the ground state, which leads to orbital polarization when an electric field is applied. This can be exploited for launching and injecting large orbital currents and creating orbital polarization in orbitronic devices. In helical molecules, this orbital polarization has recently been attributed to the observation of the chiral-induced spin selectivity effect17 due to the conversion of OAM to spin in contacts with strong

Yun Yen, Jonas A. Krieger, Mengyu Yao, Iñigo Robredo, Kaustuv Manna, Qun Yang, Emily C. McFarlane, Chandra Shekhar, Horst Borrmann, Samuel Stolz, Roland Widmer, Oliver Gröning, Vladimir N. Strocov, Stuart S. P. Parkin, Claudia Felser, Maia G. Vergniory, Michael Schüler, Niels B. M. Schröter