Ordering and multiple phase transitions in ultrathin nickelate superlattices

We interpret via advanced ab initio calculations the multiple phase transitions observed recently in ultrathin LaNiO${}_3$/LaAlO${}_3$ superlattices. The ground state is insulating, weakly charge ordered, and antiferromagnetic due to concurrent structural distortion and weak valency disproportionation. We infer distinct transitions around 50 K and 110 K, respectively, from antiferromagnetic order to moment disorder, and from a structurally dimerized insulator to an undistorted metallic Pauli paramagnet (exhibiting a cupratelike Fermi surface). The results are in satisfactory agreement with experimental results.

Figure 1

Magnetic pattern of the LNO layer of LNO/LAO (1+1). Expanded and contracted octahedra around Ni${}_{\mathrm{II}}$ and Ni${}_{\mathrm{IV}}$, respectively, are red (solid) and blue (shaded). Oxygens (not shown) sit at the shared vertexes of the octahedra. Dashed: Supercell lattice vectors. Drawing is approximately to scale.

Figure 2

Right panel: Total density of states of the AFD (orange, shaded) and Pauli-PM (solid line) states (zero is the Fermi energy). Left panel: Fermi surface of the Pauli-PM phase in the $1\times 1$ Brillouin zone.

Figure 3

Calculated conductivity vs $T$ for the metallic PM phase (dashed) and the $n$-doped AFD phase (dash-dot). Solid line: Linear interpolation of the metal and insulator curves. The value of $\mu$ is fixed at the vertical dashed line in Fig. 4.

Figure 4

Conductivity vs chemical potential and $T$ from 10 K, thickest blue (black) line, to 290 K, thickest red (gray) line (20-K steps). Inset: DOS near conduction edge. The dashed (solid) vertical line marks the intermediate regime (metallic crossover). Chemical potential zero at midgap.