Charge Transfer and Zhang-Rice Singlet Bands in the Nickelate Superconductor La_3Ni_2O_7 under Pressure

Recently, a bulk nickelate superconductor La_3Ni_2O_7 is discovered at pressures with a remarkable high transition temperature T_c ∼ 80K. Here, we study a Hubbard model with tight-binding parameters derived from ab initio calculations of La_3Ni_2O_7, 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 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-p_z 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-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 and p_x / p_y orbitals, whereas the 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 La_3Ni_2O_7 superconductivity.