Electronic structure and superconductivity in bilayer La_3Ni_2O_7

We show that the crystal field splitting, the bilayer coupling, and electron-electron correlations all play a crucial role in creating an electronic environment similar to hole-doped cuprates for high temperature superconductivity in bilayer La_3Ni_2O_7 under high pressure. The previous density functional theory calculations highly underestimated the crystal field splitting and bilayer coupling. Employing the hybrid exchange-correlation functional, we show that the exchange-correlation pushes the antibonding d_z^2 bands below the Fermi level to be fully occupied in both the low-pressure (LP) non-superconducting phase and the high-pressure (HP) phase exhibiting superconductivity. In the LP phase, the calculated Fermi surfaces and the correlation normalized band structure match well with the experimental findings at ambient pressure. Moreover, the electronic susceptibility calculated for this new band structure features nesting-induced peaks near the wave vector Q=(π/2, π/2), suggesting a possible density wave instability in agreement with recent experiments. In the HP phase, an inversion symmetry between the bilayer emerges and produces a very different band structure and Fermi surface, which can be described by low-energy effective models for the inversion antisymmetric β and symmetric α bands. The β band turns out to be close to half-filling while the α band is highly overdoped. Considering the strong local Coulomb repulsion, we find a dominant pairing with B_1g symmetry in the β band that, upon Josephson coupling to the α band, drives a superconducting ground state with a congruent d-wave symmetry. Our results capture the essential physics of La_3Ni_2O_7 in both the LP and HP phases and reveal its similarity and difference from the hole-doped cuprate high-temperature superconductors.