Electronic structure, magnetic correlations, and superconducting pairing in the reduced Ruddlesden-Popper bilayer La3Ni2O6 under pressure: Different role of d3z2-r2 orbital compared with La3Ni2O7

The recent discovery of superconductivity in bilayer La3Ni2O7 (327-LNO) under pressure stimulated much interest in layered nickelates. However, superconductivity was not found in another bilayer nickelate system, La3Ni2O6 (326-LNO), even under pressure. To understand the similarities and differences between 326-LNO and 327-LNO, using density functional theory and the random phase approximation (RPA), we systematically investigate 326-LNO under pressure. The large crystal-field splitting between the eg orbitals caused by the missing apical oxygen moves the d3z2-r2 orbital farther away from the Fermi level, implying that the d3z2-r2 orbital plays a less important role in 326-LNO than in 327-LNO. This also results in a smaller bandwidth for the dx2-y2 orbital and a reduced energy gap for the bonding-antibonding splitting of the d3z2-r2 orbital in 326-LNO, as compared to 327-LNO. Moreover, the in-plane hybridization between the dx2-y2 and d3z2-r2 orbitals is found to be small in 326-LNO, while it is much stronger in 327-LNO. Furthermore, the low-spin ferromagnetic state is found to be the likely ground state in 326-LNO under high pressure. The weak interlayer coupling suggests that s +/--wave pairing is unlikely in 326-LNO. The robust in-plane ferromagnetic coupling also suggests that d-wave superconductivity, which is usually caused by antiferromagnetic fluctuations of the dx2-y2 orbital, is also unlikely in 326-LNO. These conclusions are supported by our many-body RPA calculations of the pairing behavior. In addition, for the bilayer cuprate HgBa2CaCu2O6, we find a strong self-doping effect of the dx2-y2 orbital under pressure, with the charge of Cu being reduced by approximately 0.13 electrons from 0 GPa to 25 GPa. In contrast, we do not observe such a change in the electronic density in 326-LNO under pressure, establishing another important difference between the nickelates and the cuprates.