Spin-to-charge-current conversion in altermagnetic candidate RuO$_2$ probed by terahertz emission spectroscopy

TL;DR

This study uses terahertz emission spectroscopy to analyze ultrafast spin-to-charge conversion in RuO₂ thin films, highlighting the dominant role of the anisotropic inverse spin Hall effect and disentangling it from other spin-dependent phenomena.

Contribution

First quantitative analysis of spin-to-charge conversion in RuO₂, identifying the inverse spin Hall effect as the main contributor and distinguishing it from other effects.

Findings

Inverse spin Hall effect dominates THz emission signals.

Measured spin-Hall angle of approximately 2.4×10⁻³ at room temperature.

Possible contribution from altermagnetic inverse spin-splitter effect is about 2-4×10⁻⁴.

Abstract

Using the THz emission spectroscopy, we investigate ultrafast spin-to-charge current conversion in epitaxial thin films of the altermagnetic candidate RuO$_2$. We perform a quantitative analysis of competing effects that can contribute to the measured anisotropic THz emission. These include the anisotropic inverse spin splitter and spin Hall effects in RuO$_2$, the anisotropic conductivity of RuO$_2$, and the birefringence of the TiO$_2$ substrate. We observe that the leading contribution to the measured signals comes from the anisotropic inverse spin Hall effect, with an average spin-Hall angle of $2.4\times 10^{-3}$ at room temperature. In comparison, a possible contribution from the altermagnetic inverse spin-splitter effect is found to be approximately $2-4\times 10^{-4}$. Our work stresses the importance of carefully disentangling spin-dependent phenomena that can be generated by the unconventional altermagnetic order, from the effects of the relativistic spin-orbit coupling.