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 Tc ∼ 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 Ni-d_3z^2-r^2 and O-pz orbitals demonstrates strong antiferromagnetic correlations and narrow effective bandwidth, the in-plane singlet band consisting of the Ni-d_x^2-y^2 and O-px/py orbitals is in general more itinerant. Over a broad range of hole doping, the doped holes occupy primarily the d_x^2-y^2 and px/py orbitals, whereas the d_3z^2-r^2 and pz 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.