Superconductivity driven by symmetry breaking in RuO₂

The electronic state in tetragonal RuO₂ is metallic and remains as such even under compressive as well as tensile strain. We have synthesized the rutile phase of RuO₂ by molecular beam epitaxy. Epitaxial RuO₂ thin films are grown onto (001), (111), (101), and (100) TiO₂ substrates. Two Wyckoff positions define the crystal structure of the rutile phase where ruthenium sits at 2a (0.000, 0.000, 0.000) and oxygen at 4f (x, x, 0.000). For the four different growth orientations, the tetragonal symmetry persists, as confirmed by high-resolution reciprocal space mapping (HRRSM). We performed further epitaxy experiments of RuO₂ films on (110) TiO₂ substrates. HRRSM measurements clearly show that d_{110}≠ d_{-110} for RuO₂ films grown on (110) TiO₂ substrates. Most importantly: If d_{110}≠ d_{-110}, we discovered that RuO₂ films are superconducting, in contrast to films where d_{110}= d_{-110}. It is therefore that the induction of superconductivity in RuO₂ is driven by symmetry breaking rather than strain.