  Since the early days of Dirac flux quantization, magnetic monopoles have been
sought after as a potential corollary of quantized electric charge. As opposed
to magnetic monopoles embedded into the theory of electromagnetism, Weyl
crystals exhibit Berry flux monopoles in reciprocal parameter space. As a
function of crystal momentum, such monopoles locate at the degeneracy point of
the Weyl cone. Here, we report momentum-resolved spectroscopic signatures of
Berry flux monopoles in TaAs as a paradigmatic Weyl semimetal. We have probed
the orbital and spin angular momentum (OAM and SAM) of the Weyl-fermion states
by angle-resolved photoemission spectroscopy at bulk-sensitive soft X-ray
energies (SX-ARPES) combined with photoelectron spin detection and circular
dichroism. Supported by first-principles calculations, our measurements image
characteristics of a topologically non-trivial winding of the OAM at the Weyl
nodes and unveil a chirality-dependent SAM of the Weyl bands. Our results
experimentally visualize the non-trivial momentum-space topology in a Weyl
semimetal, promising to have profound implications for the study of
quantum-geometric effects in solids.
