Phase Diagram of Nickelate Superconductors Calculated by Dynamical Vertex Approximation

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.

Strongly correlated superconductivity with long-range spatial fluctuations

“…The theoretical (DΓA) result (yellow circles) is roughly consistent with the experimental value of 15K at 20%-doping (red diamond) [5]. Furthermore, the T c dome structure around 20%-doping is also compatible with subsequent experiments [103,104,105] (please see the review article [106] for details of nickelate calculations). We note that the ignored d-wave pairing fluctuation effect and the interlayer coupling will somewhat decrease T c while we expect these are secondary (minor) effects as mentioned in Sec.…”

Magnetic Properties and Pseudogap Formation in Infinite-Layer Nickelates: Insights From the Single-Band Hubbard Model

“…Such models has already been successfully applied in the description of the superconducting phase in NdNiO 2 [28][29][30] and the non-Curie-Weiss behavior of the magnetic susceptibility in LaNiO 2 [18]. The material realistic hopping parameters, resulting from a Wannier-and tight-binding projection are t = 395meV, t′ = − 0.25t = − 95 meV, and t″ = 0.12t = 47 meV as well as the local Hubbard interaction U = 8t = 3.16 eV from a cRPA calculation [28].…”

Fingerprints of Topotactic Hydrogen in Nickelate Superconductors

“…For undoped LaNiO 2 , the GGA-PBEsol relaxations predict its in-plane lattice constant as 3.890 Å whichis close to that of the STO substrate: 3.905 Å. The computations for La 1−x Ca x NiO 2 and LaCoO 2 , LaCuO 2 , SrCoO 2 and SrNiO 2 are performed without spin-polarization and a DFT+U treatment [79], as the inclusion of Coulomb U and spin-polarization only slightly decreases the E b by ∼5% for LaNiO 2 [66] . For NdNiO 2 , an inevitably computational issue are the localized Nd-4 f orbitals.…”

“…Besides avoiding topotactic H, compressive strain is also predicted as an effectively way to enhance T c . Previous dynamical vertex approximation calculations [27,66] reveal the key to enhance T c in nickelates is to enhance the bandwidth W and to reduce the ratio of Coulomb interaction U to W. Based on this prediction, we have proposed [27,66] three experimental ways to enhance T c in nickelates: (1) in-plane compressive strain, which can indeed be achieved by using other substrates having a smaller lattice than STO, such as LSAT (3.868 Å), LaAlO 3 (3.80 Å) or SrLaAlO 4 (3.75 Å). The smaller in-plane lattice shrinks the distance between Ni atoms thus increases their orbital overlap, leading to a larger W and a smaller U/W.…”