Reactive molecular beam epitaxial growth of RuO₂superconducting thin films

RuO₂is remarkable not only for its strain-stabilized superconductivity, but also for being theoretically predicted to be an altermagnet, a third form of magnetism characterized by antiferromagnetism-like magnetic order with zero net magnetization, while exhibiting ferromagnetism-like lifted Kramers spin degeneracy in reciprocal space. Here, we have synthesized high-quality RuO₂films on TiO₂(110) substrates by reactive molecular beam epitaxy and thoroughly investigated their electronic structure. Through improving the quality of the interface, the transition temperature of superconductivity in the strained films increased significantly.In-situangle-resolved photoemission spectroscopy detected a nearly flat band close to the Fermi level, characterized by a high density of states that may play an important role in the emergence of superconductivity. This study highlights the importance of interface engineering in modulating the superconducting properties of RuO₂thin films, suggesting that strain engineering could be a powerful tool to unlock hidden properties in other materials.