Optimally Tensile Strained La_{3}Ni_{2}O_{7} Films as Candidate High-Temperature Superconductors on Designer...

High-temperature superconductivity in La_{3}Ni_{2}O_{7}-derived films with critical temperatures (T_{c}) of 40-50 K has so far been realized only under substrate-induced compressive strain. Here we use first-principles calculations to predict that such La_{3}Ni_{2}O_{7} films can be stably grown on designer substrates Ba_{1-x}Sr_{x}O (x=1-0) and SrO-terminated SrTiO_{3} (SrO-SrTiO_{3}) with improved film quality and continuously tunable epitaxial strain, offering physically realistic materials platforms to achieve enhanced superconductivity. In particular, under the optimal tensile strain of ∼2% imposed by Ba_{0.75}S_{0.25}O or SrO-SrTiO_{3}, the La_{3}Ni_{2}O_{7} films are energetically stable within a desirable thickness range, and more resilient against oxygen vacancy formation. Concomitantly, the lattice constant normal to the films is effectively reduced, and the Ni d_{z^{2}} orbital is peaked near the Fermi level and hybridizes with the Ni d_{x^{2}-y^{2}} orbital, features that closely resemble their bulk counterparts at high pressure and strongly point to superconductivity with higher T_{c}. These findings establish innovative routes toward realizing enhanced superconductivity in tensile-strained La_{3}Ni_{2}O_{7} films, and allow to critically assess the role of the Ni d_{z^{2}} orbital in superconducting pairing.