Jahn-Teller Distortion on Strained La_3Ni_2O_7 Thin Films

We present a systematic study of the electronic structure of strained La_3Ni_2O_7 thin films. We show that biaxial compressive strain mainly elongates the outer apical Ni-O bond while leaving the inner apical Ni-O bond nearly unchanged. As a result, the Jahn-Teller splitting Δ_JT is strongly enhanced, whereas the interlayer d_z^2 hopping t_⊥^z changes only weakly. Since superconductivity is widely believed to emerge only below a critical in-plane lattice constant, our results identify the strain-enhanced Δ_JT as the relevant microscopic tuning parameter. Consistently, the calculated Fermi surfaces and Hall response for LaAlO_3 and SrLaAlO_4 substrates agree with ARPES and Hall measurements. Our results identify Jahn-Teller distortion as a key tuning parameter in strained La_3Ni_2O_7 and support its central role in optimizing superconductivity in bilayer nickelates.