Superexchange and charge transfer in the nickelate superconductor La₃Ni₂O₇ under pressure

Recently, a bulk nickelate superconductor La₃Ni₂O₇ is discovered at pressures with a remarkable high transition temperature T_c ∼ 80 K. Here, we study a Hubbard model with tight-binding parameters derived from ab initio calculations of La₃Ni₂O₇, by employing large scale determinant quantum Monte Carlo and cellular dynamical mean-field theory. Our result suggests that the superexchange couplings in this system are comparable to that of cuprates. The system is a charge transfer insulator as the hole concentration becomes four per site at large Hubbard U. Upon hole doping, two low-energy spin-singlet bands emerge in the system exhibiting distinct correlation properties: while the one composed of the out-of-plane \(\text{Ni-}d_{3z^{2}-r^{2}}\) $$\text{Ni-}{d}_{3z^2-r^2}$$ and O-p_z orbitals demonstrates strong antiferromagnetic correlations and narrow effective bandwidth, the in-plane singlet band consisting of the \(\text{Ni-}d_{x^{2}-y^{2}}\) $$\text{Ni-}{d}_{x^2-y^2}$$ and O-p_x/p_y orbitals is in general more itinerant. Over a broad range of hole doping, the doped holes occupy primarily the \(d_{x^{2}-y^{2}}\) $$d_{x^2-y^2}$$ and p_x/p_y orbitals, whereas the \(d_{3z^{2}-r^{2}}\) $$d_{3z^2-r^2}$$ and p_z orbitals retain underdoped. We propose an effective t-J model to capture the relevant physics and discuss the implications of our result for comprehending the La₃Ni₂O₇ superconductivity.