Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten

Abstract: The emerging field of orbitronics exploits the electron orbital momentum $\textit{L}$. Compared to spin-polarized electrons, $\textit{L}$ may allow magnetic-information transfera with significantly higher density over longer distances in more materials. However, direct experimental observation of $\textit{L}$ currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO$_2$ thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with W thickness. We consistently ascribe these observations to a ballistic $\textit{L}$ current from Ni through W with giant decay length (~80 nm) and low velocity (~0.1 nm/fs). At the W/SiO$_2$ interface, the $\textit{L}$ flow is efficiently converted into a charge current by the inverse orbital Rashba-Edelstein effect, consistent with ab-initio calculations. Our findings establish orbitronic materials with long-distance ballistic $\textit{L}$ transport as possible candidates for future ultrafast devices and an approach to discriminate Hall- and Rashba-Edelstein-like conversion processes.