Circular Dichroism and Orbital Angular Momentum Monopoles in Chiral Topological Semimetals PdGa/PtGa

Orbital angular momentum (OAM) monopoles have been predicted to exist in chiral crystals but have not been directly observed. Here we use circular dichroism angle resolved photoemission spectroscopy (CD-ARPES) to map such OAM monopoles in chiral topological semimetals PdGa/PtGa. The CD spectra shows a robust polar texture that rotates around the monopole as a function of photon energy, which can be well reproduced with our CD intensity simulation. We find that the correspondence between local OAM and dipole matrix elements depends on orbital characters and experimental geometry. The rotation of CD can be understood from the interference of local atomic contributions. On the other hand, the robust polar texture is a direct consequence of the magnetic orbital texture and OAM monopoles. Our results could enable the identification for complex nodal orbital textures and further highlight the potential of chiral crystals for orbitronic applications.