Superconductivity in brookite RuO₂

We synthesized epitaxial RuO₂ thin films using molecular beam epitaxy onto rutile TiO₂ substrates. Subject to the strain direction, the crystallographic space group of RuO₂ films changes. Earlier [a], we reported that for superconducting RuO₂ thin films, the lattice spacing along d₁₁₀ ≠ d_-110. Our results from first-principle calculations of elastic properties, formation enthalpy, and electronic structure indicate that orthorhombic brookite RuO₂ is stable for distinguished strain directions. In contrast, if isotropic pressure is applied, marcasite RuO₂ becomes stable. For RuO₂ films grown onto (110) TiO₂ substrates, brookite RuO₂, where d₁₁₀ ≠ d_-110, stabilizes and a superconducting state evolves at 2 K. If RuO₂ films are grown onto (100), (101), (111) or (001) TiO₂ substrates, the tetragonal (rutile) symmetry persists, as confirmed by high-resolution reciprocal space mapping. Yet, these RuO₂ films remain metallic down to 50 mK.

[a] Y. Krockenberger et al., APS March Meeting 2024, B16.00007