Disentangling Spin and Orbital Contributions in the Search for Altermagnetic Terahertz Conversion in RuO2

Abstract

Rutile RuO2 has emerged as a prominent candidate altermagnet, yet the microscopic origin of its reported charge-angular-momentum conversion remains unsettled. Recent theoretical and experimental advances have highlighted a potentially dominant role of orbital transport, raising a fundamental ambiguity between spin- and orbital-mediated conversion in RuO2. Here, we report the first experimental disentanglement of the spin- and orbital-to-charge conversion channels in RuO2 heterostructures using terahertz emission spectroscopy. We identify Fe as a predominantly spin-current injector, whereas Ni and permalloy predominantly inject orbital currents, revealing a substantial orbital contribution to ultrafast terahertz emission. Under controlled spin-current injection, we find no evidence for inverse altermagnetic spin-splitting effect. Instead, the observed anisotropy is consistent with an anisotropic inverse spin Hall response that is strongly conditioned by the near-interface environment. Under orbital-current injection, the anisotropy is markedly weaker, indicating that orbital-to-charge conversion is largely isotropic in RuO2. Our results establish orbital transport and interface-conditioned spin Hall conversion as key to terahertz emission from RuO2, underscoring the need to disentangle spin, orbital, and interfacial contributions before assigning anisotropy to altermagnetism.