Reactive Molecular Beam Epitaxial Growth of RuO2 Superconductivity thin Films

Abstract RuO2 is 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 RuO2 films on TiO2 (110) substrates by reactive molecular beam epitaxy and thoroughly investigated their electronic structures. Through improving the quality of the interface, the transition temperature of superconductivity in the strained films increased significantly. In-situ angle-resolved photoemission spectroscopy detected a nearly flat band close to the Fermi level, the high density of states may play an important role in the emergence of superconductivity. This study highlights the importance of interface engineering in modulating the superconducting properties of RuO2 thin films, and strain engineering could be a powerful tool to unlock hidden properties in other materials.