Interface-induced two-dimensional altermagnetism in RuO2/TiO2 superlattices

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)n superlattices stacked in the (001) direction using density functional theory (DFT) calculations. Our calculations reveal that altermagnetism emerges in the two-dimensional (2D) RuO2 layers when isolated by sufficiently thick TiO2 spacers. 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 0.8 μB), primarily induced by interface effects, but decline to ~0.1 μB in the central regions of the 2D RuO2 slab. 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 ~1.4 μB, 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)n superlattices are tunable platforms for engineering 2D altermagnetism, providing new insights into altermagnetic spintronics.