Quantized circular photogalvanic effect in chiral multifold semimetals
In plain words
In chiral crystals such as RhSi and CoSi, circularly polarized light should drive a current whose growth rate equals a whole number (a topological charge) times fundamental constants, and this quantization has not been confirmed.
Precise statement
For chiral multifold semimetals (RhSi, CoSi, PdGa), the trace of the circular photogalvanic injection tensor is predicted to be quantized, $\operatorname{tr}\beta = i \pi e^3 C / h^2$ with $C$ the Chern number of the Fermi surface around the multifold node, within the frequency window where only transitions near that node are allowed. Determine whether such a plateau exists experimentally and to what accuracy. An answer is the measured $\operatorname{tr}\beta$ in units of $\pi e^3/h^2$ with error over the predicted window.
What would settle it
Broadband, absolutely calibrated terahertz-emission or photocurrent measurements showing a frequency-independent plateau at the quantized value.
Status in the literature
Unverified note
RhSi data (Rees et al. 2020) agree with the predicted magnitude in a window below 0.65 eV, but a plateau at the quantized value with stated accuracy is not established, and interaction corrections are predicted, as of 2026.