Superconductivity in Uniquely Strained RuO2 Films

We report on strain engineering of superconductivity in RuO2 single-crystal films, which are epitaxially grown on rutile TiO2 and MgF2 substrates with various crystal orientations. Systematic mappings between the superconducting transition temperature and the lattice parameters reveal that shortening of specific ruthenium-oxygen bonds is a common feature among the superconducting RuO2 films. Ab initio calculations of electronic and phononic structures for the strained RuO2 films suggest the importance of soft phonon modes for emergence of the superconductivity. The findings indicate that simple transition metal oxides such as those with a rutile structure may be suitable for further exploring superconductivity by controlling phonon modes through the epitaxial strain.