Pressure and doping effects on the electronic structure and magnetism of the single-layer nickelate La$_2$NiO$_4$

Abstract

La$_2$NiO$_4$ is a prototypical member of the Ruddlesden-Popper nickelate series that offers a valuable reference point for elucidating the key ingredients behind the intriguing properties of these systems. However, the structural and electronic properties of La$_2$NiO$_4$ under pressure and doping remain surprisingly underexplored. Here, we investigate these properties using density-functional-theory calculations. We find that its tetragonal $I4/mmm$ structure can be stabilized, not only under pressure, but also at ambient pressure via the partial substitution of La with Ba. In both cases, we find a pronounced magnetostructural interplay that manifests, in particular, as anomalies in the lattice-parameter evolution with composition, deviating from Vegard's law. Moreover, we show that the combined effects of Ba substitution and pressure leads to qualitative changes in the electronic structure towards the formal $d^{7.5}$ configuration of the superconducting bilayer nickelates. Further, while La$_2$NiO$_4$ can undergo a insulator-metal transition with pressure retaining G-type antiferromagnetic order, La$_{1.5}$Ba$_{0.5}$NiO$_4$ exhibits metallic behavior with an enhanced competition between different magnetic states. Our results thus offer new insights into the interplay of structure, doping, and magnetism across the Ruddlesden-Popper nickelate series.

Figures (13)

• Figure 1: (a) Ball-and-stick model of the single-layer nickelates illustrating the orthorhombic ($Bmab$) and tetragonal ($I4/mmm$) structures---with and without NiO$_6$ octahedra tilts---characteristic of these systems.
• Figure 2: Calculated tilt angle $\theta$ and enthalpy $\Delta H$ as a function of pressure in La$_{2}$NiO$_4$. Empty and filled circles correspond to non-spin polarized calculations and G-AFM order respectively. In both cases, the supression of the tilt as a funcion of pressure scales as $\theta \propto |P-P_c|^{1/2}$ as indicated by the lines.
• Figure 3: (Top panel) Calculated tilt angle $\theta$ and Ni magnetic moment $\mu_\text{Ni}$ as a function of the Ba content $x$ in La$_{2-x}$Ba$_x$NiO$_4$ (G-AFM configuration). Both the suppression of the tilt angle and the quenching of the Ni magnetic moment scale as $|x-x_c|^{1/2}$ (blue line for $\theta$ and red dashed line for $\mu_\text{Ni}$). (Bottom panel) Calculated lattice parameters as a function of the Ba content $x$ in La$_{2-x}$Ba$_x$NiO$_4$. Circles represent "pseudo-tetragonal" or tetragonal lattice parameters ($a_\text{pseudo-t} = \sqrt{ab/2}$ or $a$ and $c$). Squares and triangles correspond to the orthorhombic $a/\sqrt{2}$ and $b/\sqrt{2}$ respectively, whose difference correlates with the emergence of the tilt.
• Figure 4: Electronic structures and Fermi surfaces of La$_2$NiO$_4$ in its $I4/mmm$ phase (left) and La$_{1.5}$Ba$_{0.5}$NiO$_4$ (right), both at ambient pressure. Note that, while the blue $d_{z^2}$ flat bands yield the blue Fermi surface sheets, there is a $k$-dependent mixing between the $d_{x^2-y^2}$ and $d_{z^2}$ states that yield the red Fermi surface sheet. In other words, there is no 100% correspondence between the colors of the Fermi-surface sheets and their orbital character.
• Figure 5: Density of states (DOS) projected on the Ni-$d_{z^2}$ (blue) and Ni-$d_{x^2-y^2}$ (red) orbitals in the G-AFM ground state of La$_{2}$NiO$_4$ at ambient pressure and 8 GPa ($U = 3.5$ eV). Spin up/down states are indicated with positive/negative values respectively. The gray curves correspond to the total DOS.