Model Construction and a Possibility of Cupratelike Pairing in a New $d^9$ Nickelate Superconductor $(\mathrm{Nd},\mathrm{Sr}){\mathrm{NiO}}_2$

Effective models are constructed for a newly discovered superconductor $(\mathrm{Nd},\mathrm{Sr})\mathrm{Ni}{\mathrm{O}}_2$, which has been considered as a possible nickelate analog of the cuprates. Estimation of the effective interaction, which turns out to require a multiorbital model that takes account of all the orbitals involved on the Fermi surface, shows that the effective interactions are significantly larger than in the cuprates. A fluctuation exchange study suggests occurrence of $d_{x^2-y^2}$-wave superconductivity, where the transition temperature should be lowered from the cuprates due to the larger interaction.

Figure 1

First-principles band structure of ${\mathrm{LaNiO}}_2$ (solid lines). The band structure of the seven-orbital model is superposed, where the Wannier-orbital weight is represented by the thickness of lines with color-coded orbital characters. Top right-hand panels display cross sections of the Fermi surface at $k_z=0$ (left) and $k_z=\pi$ (right), where the red and blue lines depict Ni- and La-originated Fermi surfaces, respectively. See the Supplemental Material for the 3D plot of the Fermi surface [36].

Figure 2

The band structure of the doped nickelate with $p=0.2$ in (a) the seven-orbital model, (b) the six-orbital model (see text), and (c) the two-orbital model, with color-coded orbital characters, superposed with the first-principles band structure (black lines). Cross sections of the Fermi surfaces are depicted in top right as in Fig. 1.

Figure 3

Temperature dependence of the $d_{x^2-y^2}$-wave eigenvalue $\lambda$ of the Eliashberg equation (a) and the Stoner factor ${\alpha }_S$ (b) for the six- and two-orbital models for $p=0.2$. Also shown is the result for the two-orbital model with the interaction parameters taken to be as in the seven-orbital model (denoted as $U=“\mathrm{seven}\text{−}\mathrm{orbital}”$). For comparison, result for ${\mathrm{HgBa}}_2{\mathrm{CuO}}_4$ in the five-orbital model (with the same $n_{3d{x}^2-y^2}$ as in the $p=0.2$ nickelate) is also shown. The insets in (a) are a log-log plot of $\lambda$ versus $T$ (bottom left), and the eigenfunction of the Eliashberg equation at $k_z=0$ (top right; see the Supplemental Material [36] for other $k_z$ cuts) for the six-orbital model of the nickelate ($p=0.2$) at $T=0.005\mathrm{eV}$.