  Weyl semimetals are crystals in which electron bands cross at isolated points
in momentum space. Associated with each crossing point (or Weyl node) is an
integer topological invariant known as the Berry monopole charge. The discovery
of new classes of Weyl materials is driving the search for novel properties
that derive directly from the Berry charge. The circular photogalvanic effect
(CPGE), whereby circular polarized light generates a current whose direction
depends on the helicity of the absorbed photons, is a striking example of a
macroscopic property that emerges from Weyl topology. Recently, it was
predicted that the rate of current generation associated with optical
transitions near a Weyl node is proportional to its monopole charge and
independent of material-specific parameters. In Weyl semimetals that retain
mirror symmetry this universal photogalvanic current is strongly suppressed by
opposing contributions from energy equivalent nodes of opposite charge.
However, when all mirror symmetries are broken, as in chiral Weyl systems,
nodes with opposite topological charge are no longer degenerate, opening a
window of photon energies where the topological CPGE can emerge. In this work
we test this theory through measurement of the photon-energy dependence of the
CPGE in the chiral Weyl semimetal RhSi. The spectrum is fully consistent with a
topological CPGE, as it reveals a response in a low-energy window that closes
at 0.65 eV, in quantitative agreement with the theoretically-derived
bandstucture.
