Fermiology and Cooper-pairing in Bilayer Nickelate Superconductors: From Bulk to Strained Thin Films

The recently discovered Ruddlesden-Popper bilayer superconductor La3Ni2O7 has attracted widespread attention due to its high Tc~80 K under a pressure. A new breakthrough is that ambient-pressure superconductivity with Tc over 40 K has been realized in La3Ni2O7 and (La,Pr)3Ni2O7 thin films by using the strain from substrates. Our density functional theory calculations indicate that the 3dx2−y2 and 3dz2 orbitals of Ni cations mixing with oxygen 2p orbitals play critical role in the emergence of superconductivity in the bilayer nickelate superconductors for bulk and thin films. We propose the bilayer two-orbital models for La3Ni2O7, primarily based on the 3dx2−y2 and 3dz2 orbitals of Ni. Through Wannier downfolding and symmetry analysis, we obtain parameters such as electron hoppings and site-energies, which provide excellent description of the electronic band structures and Fermi surfaces. To explicitly consider the physics of O-p orbitals, we further introduce the higher energy models. Based on these models, we study the charge transfer, Zhang-Rice singlet bands, pairing symmetry, and superconducting transition temperature in La3Ni2O7 and related nickelate superconductors. Superexchanges and charge transfer were studied for the bilayer nickelate superconductors from bulk to thin films. Our result suggests that the superexchange couplings in this system are comparable to that of cuprates. We propose the effective two-orbital t-J models to capture the relevant physics for the bilayer nickelates. By using the renormalized mean-field theory, we find a robust s±-wave pairing driven by the inter-layer dz2 magnetic coupling, which exhibits Tc within the same order of magnitude as the experimentally observed values. We obtain a comprehensive superconducting phase diagram in the doping plane and find that the La3Ni2O7 under pressure is situated roughly in the optimal doping regime of the phase diagram. The decrease of Tc in thin films compared with La3Ni2O7 bulks can be attributed to the decrease of interlayer superexchange coupling. Also, we find the nodeless superconducting gap at the β pocket, which is consistent with the recent ARPES measurement. Our exposition of electronic reconstructions, multiorbital models and superconductivity shed light on theoretical electronic correlation study and experimental exploration of superconductors in the RP series.