Strain-dependent superconductivity in RuO₂

Recent discovery of superconductivity in strained (110)-oriented RuO₂ films grown on TiO₂ (110) single-crystal substrates, with a superconducting transition temperature T_c of 2 K, renewed interest in this material. First-principles calculations revealed an enhancement of the density of states and softening of the phonon modes upon (110)-strain. Does RuO₂ become superconducting under different strain conditions? We study strained (100)-oriented RuO₂ thin films grown on TiO₂ (100) single-crystal substrates. While our density functional theory predicts the T_c of strained RuO₂ (100) films to be higher than the RuO₂ (110) films, the transport measurements find the T_c to be about 1 K in strained RuO₂ (100) films grown on TiO₂ (100) substrates. Nonetheless, the thickness dependence of the T_c follows a similar trend in both cases. Informed by the experimental results including angle-resolved photoemission spectroscopy, we further explore the strain-dependence of the electronic and vibrational structure of transition-metal oxides from first principles.