Spatiotemporally Resolved Orbital Hall Effect in a Topological Semimetal.

Orbital angular momentum (OAM), once considered a marginal property in solids, has recently gained importance in various physical phenomena. Although optical probes couple to OAM directly, comparable low-energy electrodynamic methods are still in the early stages of development. To exploit the potential of OAM for information science and technology, angular momentum-resolved transport methods must be established. Here, a principle is presented for an angular momentum-resolved method to observe orbital Hall effect (OHE) in real space. The orbital conduction in the topological semimetal Td-WTe2 is measured using contact-free polarimetric terahertz probe, showing that two distinct regions of ±Lz with a diffusion length of ≈130 µm due to the extremely low scattering rate of Weyl fermions form in real space. The observed orbital conduction can be explained by OHE, according to theoretical calculations. By altering the polarization and intensity of the terahertz electric field input, it is also demonstrated that this orbital conduction can be engineered. Using a spatiotemporal low-energy electrodynamic probe, the work reveals direct evidence of coherent OHE in an inversion-broken topological semimetal with strong spin-orbit coupling-an experimentally elusive regime until now.