Strain-enhanced superconductivity in RuO 2 thin films: Joint experimental and theoretical studies

We investigate the superconducting properties of RuO 2 films grown on TiO 2 substrates by combining film fabrication and transport measurement with ab initio calculation. Experimentally, we grow several RuO 2 (110) thin films on TiO 2 (110) substrates and observe superconductivity with critical temperature (T c) of about 2.0 K in 20-nm and 30-nm RuO 2 (110) thin films. Theoretically, we study the superconductivity in both RuO 2 (110) and RuO 2 (100) epitaxial films within McMillan formalism of isotropic Migdal-Eliashberg theory. Among the salient results, the calculated phonon dispersion relations of all the considered RuO 2 films show that the epitaxial films are dynamically stable. Importantly, our ab initio calculations reveal that independent of the film orientation, the [001] compressive strain ε[001] pushes the flat Ru d z squared-dominant bands towards the Fermi level (E F), and significantly soften the out-of-plane acoustic phonon mode near the R point in the Brillouin zone, which substantially enhances the electron-phonon coupling (EPC) strength λ and hence the calculated T c in the RuO 2 films. Remarkably, the amount of the phonon softening and the enhanced T c increase almost linearly with the increasing ε[001]. The calculated T c for the films with thickness of at least 20 nm agree well with the experimental T c. These findings demonstrate that among other things, lattice mismatch strain provides an effective route for tuning phonon-mediated superconductivity in RuO 2 and other thin films.