Tom S. Seifert and colleagues published their latest work on ultrafast orbital currents in the journal “Nature Nanotechnology”. Using terahertz-emission spectroscopy, the researchers found strong evidence for optically triggered orbital angular momentum currents in Ni|W thin-film stacks. Surprisingly, those currents seem to travel for 10s of nm without undergoing any scattering.

The researchers observed terahertz emission signals from optically excited Ni|W|SiO_x stacks that are consistently assigned to the ultrafast injection of currents of orbital angular momentum (OAM) from Ni into W and long-distance ballistic transport through W. Remarkably, they found strong indications for a dominant OAM-to-charge-current conversion at the W/SiO_x interface (the so-called inverse orbital Rashba Edelstein effect) in both experiment and theory. This result can be considered as time-domain signature of the long-range nature of OAM currents and the IOREE in W.

As illustrated by the figure above, upon ultrafast laser excitation of the Ni layer, an excess of the Ni magnetization arises, leading to an accumulation μ_L of OAM and the injection of an OAM current j_L into the W layer. At the back surface, the IOREE generates an ultrafast in-plane charge current j_C that emits a terahertz electromagnetic pulse with electric-field amplitude E.

This study highlights the power of broadband terahertz emission spectroscopy in disentangling the transport of OAM and spin angular momentum (SAM) as well as Hall-like and Rashba–Edelstein-like conversion processes based on their different dynamics. Seifert and coworkers find that Py versus Ni are, respectively, attractive SAM and OAM sources, whereas Pt versus W are, respectively, good SAM-to-charge and OAM-to-charge converters with distinctly different efficiency and dynamics for SAM versus OAM.

These results are a significant step toward the identification of ideal sources and detectors of either SAM or OAM currents, which will strongly benefit from accurate theoretical predictions.

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Original publication:

T. S. Seifert, D. Go, H. Hayashi,  R. Rouzegar, F. Freimuth, K. Ando, Y. Mokrousov, and T. Kampfrath: Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten - Nat. Nanotechnol. (2023) - DOI: 10.1038/s41565-023-01470-8