Evidences of subnanometre orbital diffusion length in heavy metals using terahertz emission spectroscopy

The orbital angular momentum of electrons offers a promising, yet underexplored, degree of freedom for ultrafast, energy-efficient information processing. As the foundation of orbitronics, understanding how orbital polarizations propagate and convert into charge currents is essential but remains elusive due to the challenge in disentangling orbital and spin dynamics in thin films. While some theoretical studies predict that orbital transport is constrained to sub-atomic-layer scales in materials, recent experiments have reported exceptionally long orbital diffusion lengths. To address this contradiction, we combine terahertz emission spectroscopy with a wedge-sample platform to systematically investigate spin and orbital transport in heavy metals with subnanometre resolution. Our measurements access the previously unexplored thin-film regimes (<3 nm), uncovering anomalous behaviours that challenge the prevailing interpretations of long-range orbital transport. We consistently find the orbital diffusion lengths (λ_L) to be substantially shorter than the spin diffusion lengths (λ_S) in heavy metals, with λ_L in W approaching 0.36 nm. Interface-sensitive control experiments further rule out interfacial orbital-to-charge conversion as the dominant mechanism, supporting the bulk inverse orbital Hall effect as the primary conversion process.