Interface-induced two-dimensional altermagnetism in RuO₂/TiO₂superlattices

RuO₂ was initially proposed as an altermagnet, but this view is now contested and no longer widely accepted. In this work, we have designed and investigated a series of rutile (RuO₂)_m/(TiO₂)_nsuperlattices stacked in the (001) direction using density functional theory (DFT) calculations. Our calculations reveal that altermagnetism emerges in the two-dimensional (2D) RuO₂layers when isolated by sufficiently thick TiO₂spacers. Spin-real-space symmetry in even-numbered Ru layers drives this altermagnetism. The magnetic moments of Ru ions are relatively large at the interfacial layers (up to 0.8μ_B), primarily induced by interface effects, but decline to ∼0.1μ_Bin the central regions of the 2D RuO₂slab. This pronounced layer-dependent moment reduction and electronic structure variation are attributed to quantum confinement effects, demonstrating a significant difference compared to the bulk RuO₂. Furthermore, by accounting for electronic correlation effects (DFT +U), we observe not only an enhancement of the Ru magnetic moments to ∼1.4μ_B, but also thickness-driven phase transitions: insulating states dominate in thinner 2D RuO₂(m⩽ 6), while metallic behavior emerges in thicker cases (m> 8). Thus, the (RuO₂)_m/(TiO₂)_nsuperlattices are tunable platforms for engineering 2D altermagnetism, providing new insights into altermagnetic spintronics.