Preprint reports near-quantized photocurrent in nickel-doped RhSi

Researchers report in a new arXiv preprint that they have measured a near-quantized, light-driven photocurrent in nickel-doped RhSi, a gapless topological semimetal — a result that, if it holds up, would be evidence that topology can lock in a universal response even without the energy gap usually associated with such effects. The work, posted in July 2026 and not yet peer reviewed, is titled “Quantized Photocurrents in Gapless Topological Matter.”

The paper, by Byunghoon Kim and colleagues from Princeton University, Los Alamos National Laboratory, the Max Planck Institute for Chemical Physics of Solids and Nanyang Technological University, focuses on the circular photogalvanic effect, or CPGE. In plain terms, that means circularly polarized light shining on a crystal generates a photocurrent whose direction flips with the light’s handedness. In the paper’s abstract, the authors write, “Here we observe a quantized circular photogalvanic effect in the chiral topological semimetal Rh0.95Ni0.05Si.” The broader claim is that this would amount to an all-optical analogue of the quantum Hall effect, where topology fixes a material response — but here in a gapless material rather than a gapped one.

The experiment did not measure electrical conductance directly. Instead, the team sent near- to mid-infrared circularly polarized laser pulses into Rh0.95Ni0.05Si and detected the resulting helicity-dependent terahertz emission from the crystal. The authors say three features point to quantization: a sharp onset of the signal, a plateau that stays flat as photon energy changes, and an abrupt long-wavelength cutoff consistent with Pauli blocking, a quantum effect that prevents certain optical transitions once electronic states are filled. The plateau spans roughly 2,000 to 5,300 nanometers, or about 0.62 to 0.234 electron volts, with a cutoff near 6,000 nanometers. After calibration, the average plateau value is reported as about 2.39 in units of the predicted quantum response. A first-principles calculation gives about 2.17, while the ideal value expected from the relevant topological charge is 2.

That near-quantized behavior depends on a materials-engineering step at the center of the paper. The researchers replaced 5% of the rhodium in RhSi with nickel, producing Rh0.95Ni0.05Si. According to density functional theory calculations and angle-resolved photoemission spectroscopy, or ARPES, that substitution shifts the electronic bands downward by about 220 millielectron volts. The authors argue that this isolates optical transitions at the Γ-point multifold node — a topological crossing in the band structure — while suppressing competing transitions from an opposite-chirality node at the R point over a finite photon-energy window. In effect, the doping is meant to create the “single-node” regime that theory says is required for a quantized CPGE.

That idea has been around for several years. In 2017, Fernando de Juan and colleagues predicted that Weyl and other chiral semimetals could show a quantized CPGE if optical transitions could be restricted to a single chiral node. Reaching that limit experimentally has proved difficult because real materials tend to have overlapping bands, extra transitions and Fermi levels that are not aligned where they need to be. Previous experiments in related chiral semimetals, including RhSi, CoSi, PdGa and PtGa, reported strong CPGE signals, but this paper presents its result as the first clear near-quantized plateau in a gapless semimetal.

The main caveat is that the absolute “quantized” scale is not measured in a fully independent way. What the terahertz experiment directly detects is a signal proportional to the CPGE multiplied by the carrier relaxation time, usually written as tau, or τ. The paper says the existence of the onset, plateau and cutoff is directly measured. But to place the plateau on the claimed quantum scale, the authors set τ by matching theory and experiment. That means the strongest part of the evidence is the shape of the response across photon energies, while the absolute value — and how close it sits to the ideal quantized number — depends in part on that calibration assumption. For that reason, “near-quantized” is the more accurate description than exact quantization.

If confirmed, the result would mark an important step in showing that topological quantization can survive in gapless matter, extending a concept best known from the quantum Hall effect into an all-optical setting. For now, though, the work is available only as an arXiv preprint, and no journal publication or institutional press release for the paper was publicly available as of July 21, 2026.