Exploring the Fragility of Altermagnetism in RuO₂ using Quantum Monte Carlo

Altermagnetism corresponds to the absence of Kramer’s degeneracy in an antiferromagnet, leading to a spin-split band structure that have important technological implications, specifically in achieving antiferromagnetic high-speed low-power spintronic devices.  Since the discovery of collinear antiferromagnetism in RuO₂, it has been widely discussed as a candidate altermagnet. However, recent μSR and neutron diffraction experiments found no evidence of local magnetic moments in both bulk and thin-film samples, casting down on its intrinsic magnetism.  Ab initio Density functional theory (DFT) based calculations show a magnetic ground-state when correlations are included, which appears to be critical in explaining observed lattice-dynamics.  We use many-body diffusion quantum monte-carlo approach to assess the true magnetic state of stoichiometric RuO₂. We find that bulk RuO₂ shows a non-magnetic metallic ground-state, with the nodal structure from PBE0 giving the lowest energy, consistent with experiments. We further demonstrate that compressive strain stabilizes a sizable local magnetic moment on Ru-d to realize a stable AFM metallic ground-state, which indicates that RuO₂ is intrinsically non-magnetic but lies close to a magnetic instability.  These findings reconcile previous discrepancies between theory and experiment, provide a viable pathway for creating an altermagnetic RuO₂ and demonstrate the importance of beyond-DFT methods for such correlated solids.