Observation of Topological Surface States in the Candidate Altermagnet RuO₂ via Spin-Resolved ARPES

In recent years, altermagnets have emerged as a promising third class of magnetic materials, following ferromagnets and antiferromagnets. Among them, RuO₂ has attracted particular attention as a prototypical candidate, showing intriguing transport phenomena such as spin conduction and signatures of the anomalous Hall effect, indicative of spin-split band structures. However, μSR and neutron scattering experiments have repeatedly reported the absence of long-range antiferromagnetic order, raising questions about the true nature of its ground state. To address this, we performed micro-ARPES measurements on bulk single crystals of RuO₂ with three different surface orientations to directly probe the electronic structure related to the proposed altermagnetism. The band dispersions of the (100) surface were compared with first-principles calculations for both paramagnetic and antiferromagnetic ground states, revealing good agreement with a nonmagnetic (paramagnetic) ground state. In addition, spin-resolved ARPES experiments were conducted to examine possible spin polarization in the bands. In this talk, we present our experimental findings, comparisons with theory, and discuss their implications for the magnetic properties and topological electronic states of RuO₂, providing insights into the ongoing controversy regarding its magnetism.