Growth of charging-free monolayer RuO₂ and its metallic electronic structure

Rutile RuO₂ thin films are known to exhibit strain-induced superconductivity, yet their relatively low critical temperature (~ 1-2 K) limits comprehensive study. Since superconductors in the 2D limit often behave distinctly from the bulk state, it might be important to characterize monolayer RuO₂ for understanding dimensionality effects on the strain-induced superconductivity. In this report, we investigated the electronic band structure of the monolayer RuO₂ by fabricating a charging-free RuO₂-TiO₂ heterostructure via pulsed laser deposition (PLD) and performing in-situ angle-resolved photoemission spectroscopy (ARPES). We successfully resolved the electronic band structure of atomically thin RuO₂ films and observed a Fermi edge for the monolayer, providing direct spectroscopic evidence that monolayer RuO₂ retains its metallicity under coherent epitaxial strain. Our findings provide an experimental basis for investigating superconductivity in the two-dimensional limit and offer a platform to examine strain effects in monolayer oxide systems.