Abstract
We review the electronic structure of nickelate superconductors with and without effects of electronic correlations. As a minimal model, we identify the one-band Hubbard model for the Ni 3dx2−y2 orbital plus a pocket around the A-momentum. The latter, however, merely acts as a decoupled electron reservoir. This reservoir makes a careful translation from nominal Sr-doping to the doping of the one-band Hubbard model mandatory. Our dynamical mean-field theory calculations, in part already supported by the experiment, indicate that the Γ pocket, Nd 4f orbitals, oxygen 2p, and the other Ni 3d orbitals are not relevant in the superconducting doping regime. The physics is completely different if topotactic hydrogen is present or the oxygen reduction is incomplete. Then, a two-band physics hosted by the Ni 3dx2−y2 and 3d3z2−r2orbitals emerges. Based on our minimal modeling, we calculated the superconducting Tc vs. Sr-doping x phase diagram prior to the experiment using the dynamical vertex approximation. For such a notoriously difficult to determine quantity as Tc, the agreement with the experiment is astonishingly good. The prediction that Tc is enhanced with pressure or compressive strain has been confirmed experimentally as well. This supports that the one-band Hubbard model plus an electron reservoir is the appropriate minimal model.
Highlights
Twenty years ago, Anisimov, Bukhvalov, and Rice [1] suggested high-temperature (Tc) superconductivity in nickelates based on material calculations that showed apparent similarities to cuprates
The devil is in the details, and here cuprates and nickelates differ. For revealing such material-specific differences, band-structure calculations based on density functional theory (DFT) are the method of choice. They serve as a starting point for understanding the electronic structure and, subsequently, the phase diagram of nickelate superconductors
We review the electronic structure of nickelates in comparison to that of cuprates and the arguments for a simpler description of nickelate superconductors, namely, a Hubbard model for the Ni 3dx2−y2 band plus a largely decoupled reservoir corresponding to the A pocket
Summary
Anisimov, Bukhvalov, and Rice [1] suggested high-temperature (Tc) superconductivity in nickelates based on material calculations that showed apparent similarities to cuprates. We review the electronic structure of nickelates in comparison to that of cuprates and the arguments for a simpler description of nickelate superconductors, namely, a Hubbard model for the Ni 3dx2−y2 band plus a largely decoupled reservoir corresponding to the A pocket This A pocket is part of the Nd 5dxy band which has, a major admixture of Ni 3dxz/yz and O 2pz states around the momentum A. The simple picture of a one-band Hubbard model, whose doping needs to be carefully calculated since part of the holes in Sr-doped SrxNd1−xNdO2 go to the A pocket, allowed us [33] to calculate Tc, see Figure 5, at a time when only the Tc for a single doping x 20% was experimentally available To this end, state-of-the-art dynamical vertex approximation (DΓA) [36,37,38], a Feynman diagrammatic extension of dynamical mean-field theory (DMFT) [39,40,41,42] has been used.
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