Strain-tuning for superconductivity in La$_3$Ni$_2$O$_7$ thin films

Abstract: The recent discovery of high-transition temperature ($T_\mathrm{c}$) superconductivity in pressurized La${3}$Ni${2}$O${7}$ bulk crystals has attracted keen attention due to its characteristic energy diagram of $e{g}$ orbitals, containing nearly half-filled $d_{3z^2 - r^2}$ and quarter-filled $d_{x^2 - y^2}$ orbitals. This finding provides valuable insights into the orbital contributions and interlayer interactions in double NiO${6}$ octahedra, offering opportunities to control the electronic structure via ligand field variations. Here, we demonstrate strain-tuning of $T\mathrm{c}$ over a range of 50 K in La${3}$Ni${2}$O${7}$ films grown on different oxide substrates under 20 GPa. As the $c/a$ ratio increases, the onset $T\mathrm{c}$ systematically rises from 10 K in the tensile-strained film on SrTiO${3}$ to a maximum of about 60 K in the compressively strained film on LaAlO${3}$. These systematic variations suggest that strain engineering is a promising strategy for expanding superconductivity in bilayer nickelates by tuning the orbital energy landscape toward high-$T_\mathrm{c}$ superconductivity.