Spin correlations in La${}_3$Ni${}_2$O${}_7$ thin films

The discovery of ambient-pressure superconductivity with $T_{c,\text{onset}}>40$ K in La${}_3$Ni${}_2$O${}_7$ (LNO) thin films grown on the SrLaAlO${}_4$ (SLAO) substrate with compressive ($\epsilon \approx -2\mathrm{\%}$) epitaxial strain provides a unique platform for investigating the superconducting mechanism in nickelate superconductors. Here, we use resonant inelastic X-ray scattering (RIXS) to unveil the dispersive spin excitations in the LNO/SLAO thin film and establish the strain dependence of the electronic and spin excitations in LNO thin films with strain ranging from $\epsilon \approx -2\mathrm{\%}$ to $+1.9\mathrm{\%}$. Compared with bulk LNO, LNO/SLAO exhibits similar $dd$ excitations and spin dynamics, but with a larger spin-excitation bandwidth, whereas tensile-strained LNO/SrTiO${}_3$ exhibits a marked suppression of both the spin excitations and the Ni $3d_{z^2}$-derived $dd$ excitations. This evolution reflects a strain-tuned interlayer exchange interaction $J_z$ and Ni $3d_{z^2}$-O 2$p_z$ hybridization. Our results demonstrate how epitaxial strain modulates the interlayer magnetic coupling and are consistent with scenarios in which the interlayer antiferromagnetic superexchange interaction promotes interlayer pairing in bilayer nickelates.