Observation of mirror-odd and mirror-even spin texture in ultra-thin epitaxially-strained RuO₂ films

Recently, rutile RuO₂ has attracted renewed interest due to expectations of prominent altermagnetic spin-splitting. However, accumulating experimental evidence suggests that in its bulk and thick-film forms, RuO₂ does not display any form of magnetic ordering. Despite this, the spin structure of RuO₂ remains largely unexplored in the ultra-thin limit, where substrate-imposed epitaxial strain can be substantial. Here, we employ spin-resolved angle-resolved photoemission spectroscopy, supported by ab-initio calculations, to reveal the electronic structure of 2.7 nm-thick epitaxial RuO₂ heterostructures. We observe an unconventional spin texture characterized by the coexistence of mirror-even and mirror-odd momentum-dependent components. A comprehensive symmetry analysis rules out nonmagnetic origins of this spin texture. These findings suggest an emergent non-relativistic spin structure enabled by epitaxial strain in the ultra-thin limit, marking a distinct departure from the behavior of relaxed or bulk RuO₂. Our work opens new perspectives for exploring symmetry-breaking mechanisms and spin textures in oxide heterostructures.