Superexchanges and Charge Transfer in La₃Ni₂O₇ Thin Films

The recent discovery of ambient-pressure superconductivity with a critical temperature T_\rm c exceeding 40 K in La_3Ni_2O_7 thin films represents a significant advancement in the field of nickelate superconductivity. Motivated by recent experimental reports, we investigate an 11-band d-p Hubbard model with tight-binding parameters derived from ab initio calculations, employing large-scale determinant quantum Monte Carlo and cellular dynamical mean-field theory. Our results show that the dominant superexchange couplings in the La_3Ni_2O_7 thin films are substantially weaker than those in the bulk material at 29.5 GPa. Specifically, the out-of-plane antiferromagnetic correlation between the Ni-d_3z^2-r^2 orbitals is reduced by approximately 27% in the film, whereas the in-plane magnetic correlations remain largely unaffected. We further evaluate the corresponding antiferromagnetic coupling constants, J_\perp and J_\parallel, within a perturbative framework. Regarding charge-transfer properties, we find that biaxial compressive strain in the films reduces the charge-transfer gap. We further resolve the orbital distribution of doped holes and electrons between the IP (Ni-d_x^2-y^2 and O-p_x/p_y) and OP (Ni-d_3z^2-r^2 and O-p_z) orbitals, revealing a pronounced particle-hole asymmetry. These findings lay the groundwork for constructing a low-energy t-J model for La_3Ni_2O_7 films and provide key insights into the physical distinctions between thin-film and bulk bilayer nickelates.