  The emerging field of orbitronics aims at generating and controlling currents
of electronic orbital angular momentum (OAM) for information processing.
Structurally chiral topological crystals could be particularly suitable
orbitronic materials because they have been predicted to host topological band
degeneracies in reciprocal space that are monopoles of OAM. Around such a
monopole, the OAM is locked isotopically parallel or antiparallel to the
direction of the electron's momentum, which could be used to generate large and
controllable OAM currents. However, OAM monopoles have not yet been directly
observed in chiral crystals, and no handle to control their polarity has been
discovered. Here, we use circular dichroism in angle-resolved photoelectron
spectroscopy (CD-ARPES) to image OAM monopoles in the chiral topological
semimetals PtGa and PdGa. Moreover, we also demonstrate that the polarity of
the monopole can be controlled via the structural handedness of the host
crystal by imaging OAM monopoles and anti-monopoles in the two enantiomers of
PdGa, respectively. For most photon energies used in our study, we observe a
sign change in the CD-ARPES spectrum when comparing positive and negative
momenta along the light direction near the topological degeneracy. This is
consistent with the conventional view that CD-ARPES measures the projection of
the OAM monopole along the photon momentum. For some photon energies, however,
this sign change disappears, which can be understood from our numerical
simulations as the interference of polar atomic OAM contributions, consistent
with the presence of OAM monopoles. Our results highlight the potential of
chiral crystals for orbitronic device applications, and our methodology could
enable the discovery of even more complicated nodal OAM textures that could be
exploited for orbitronics.
