Absence of Transport Altermagnetic Spin-Splitting Effect in RuO 2

Altermagnets have attracted significant interest recently. Through the altermagnetic spin-splitting effect (ASSE), a longitudinal spin-polarized or a transverse pure spin current can be generated upon charge current injection. The ASSE is a key experimental feature for altermagnets but is often mixed with the spin Hall effect (SHE). Here, we present a comprehensive study of spin-to-charge conversion in epitaxial ruthenium dioxide (RuO2) thin films using the ferromagnetic insulator yttrium iron garnet (YIG) as the spin current source. We conclusively show the absence of the ASSE in RuO2 films grown with three different crystal orientations. Instead, we attribute the spin-to-charge conversion signals solely to the SHE. Moreover, we reveal a negative spin Hall angle in RuO2 when it is adjacent to YIG, which reverses the sign when interfaced with Py. Our study provides crucial insights into the recent arguments on RuO2 and advances the understanding of spin-to-charge conversion in low-symmetry materials.