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 sim2.0 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^2}-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 geq 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.