Type-II t−J model and shared superexchange coupling from Hund's rule in superconducting La3Ni2O7

Abstract

Recently, an 80 K superconductor was discovered in La3Ni2O7 under high pressure. Density function theory calculations identify dx2−y2,dz2 as the active orbitals on the bilayer square lattice with a d8−x configuration of Ni per site. Here, x is the hole doping level. One naive expectation is to describe this system in terms of a two-orbital t−J model. However, we emphasize the importance of Hund's coupling JH and the x=0 limit should be viewed as a spin-one Mott insulator. Especially, the significant Hund's coupling shares the interlayer superexchange J⊥ of the dz2 orbital to the dx2−y2 orbital, an effect that cannot be captured by conventional perturbation or mean-field approaches. This study first explores the limit where the dz2 orbital is Mott localized, dealing with a one-orbital bilayer t−J model focused on the dx2−y2 orbital. Notably, we find that strong interlayer pairing survives up to x=0.5 hole doping driven by the transmitted J⊥, which explains the existence of a high Tc superconductor in the experiment at this doping level. Next, we uncover the more realistic situation where the dz2 orbital is slightly hole-doped and cannot be simply integrated out. We take the JH→+∞ limit and propose a type II t−J model with four spin-half singlon (d7) states and three spin-one doublon (d8) states. Employing a parton mean-field approach, we recover similar results as in the one-orbital t−J model, but now with the effect of the J⊥ automatically generated.

Source-reported materials — not catalogue approval

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