Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten
Tom S. Seifert, Dongwook Go, Hiroki Hayashi, Reza Rouzegar, Frank Freimuth, Kazuya Ando, Yuriy Mokrousov, Tobias Kampfrath
Abstract
Abstract The emerging field of orbitronics exploits the electron orbital momentum L . Compared to spin-polarized electrons, L may allow the transfer of magnetic information with considerably higher density over longer distances in more materials. However, direct experimental observation of 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 the W thickness. We consistently ascribe these observations to a ballistic L current from Ni through W with a giant decay length (~80 nm) and low velocity (~0.1 nm fs −1 ). At the W/SiO 2 interface, the 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 L transport as possible candidates for future ultrafast devices and an approach to discriminate Hall-like and Rashba–Edelstein-like conversion processes.