Origin of metal-insulator transition in rare-earth Nickelates

TL;DR

The paper tackles the origin of the metal-insulator transition in rare-earth nickelates (RNiO3) and its entanglement with magnetism and lattice distortions. It employs a parameter-free, first-principles MBPT approach—Quasiparticle Self-Consistent GW (QSGW) in the Questaal package—across seven RNiO3 to separate the effects of structural symmetry breaking and magnetism. It finds a metallic Pbnm phase and an insulating P2_1/n phase across all compounds, with MIT driven by spin-disproportionation between two inequivalent Ni sites and accompanying charge/orbital redistribution, including an orbital-selective gap and a midgap state from oxygen, stabilized by the breathing-mode distortion. The results highlight the essential role of non-local correlations and real-space spin multiplets in RNiO3, argue for minimal models that include at least two Ni sites, and offer a parameter-free route to disentangle magnetic and lattice contributions with implications for nickelate-based devices.

Abstract

Rare-earth nickelates RNiO3 (R=rare-earth element) exhibit three kinds of phase transitions with decreasing temperature: a structural transition from a pseudo-cubic to a monoclinic phase, a metal- insulator transition (MIT), and a magnetic transition from a paramagnetic state to an ordered one. The first two occur at the same temperature, which has led to a consensus that the MIT is driven by lattice distortions. We show here that the primary driving force for the MIT is magnetic; however because of the unusual d7 configuration of Ni, additional flexibility in spin configurations are also needed which symmetry-lowing structural deformations make possible. The latter enable Ni to disproportionate into two kinds: a high-spin and a low-spin configuration, which allow the system to reduce its unfavorable orbital moment and also open a gap.

Figures (5)

• Figure 1: Nature of the pseudo-cubic distortion in RNiO$_3$: In RNiO$_3$ compounds, the pseudo-cubic distortion causes the B-O-B bond angle (B is Ni in this case) to deviate from the ideal 180$^\circ$. This distortion is influenced by the size of the rare-earth ion: ions with less-filled $f$-orbitals tend to have a larger ionic radius. As the rare-earth ionic radius increases, the degree of pseudo-cubic distortion decreases. Although there is a correlation between the metal-insulator transition (MIT) temperature and the extent of pseudo-cubic distortion, the orthorhombic Pbnm phase alone is not sufficient to support an insulating state. Achieving insulation requires an additional reduction in crystal symmetry beyond the Pbnm structure.
• Figure 2: NdNiO$_{3}$ in the pseudocubic Pbnm phase. (a) energy bands in the nonmagnetic case. The $\Gamma$-S-Y lines deviate from mirror images of the $\Gamma$-Z-U lines because of deviations from cubic symmetry. Colors represent the following projections: Ni-$t_{2g}$ (red); Ni-$e_{g}$ (green); O-$p$ (blue); thus the $e_{g}$ fall above the $t_{2g}$. (b,c) energy bands in the ferromagnetic case for (majority, minority) spins. The bands are similar except $t_{2g}$ and $e_{g}$ are spin split, enabling a gap to form in the minority channel. (d) Nonmagnetic density-of-states, showing the non-negligible amount of O character around $E_{F}$=0. (e) FM density-of-states, showing how the majority and minority Ni d states are spin split by $~\sim$1 eV. To distinguish spins, colors are modified to: $t_{2g}$ (rust=majority, light-red=minority); $e_{g}$ (green=majority, olive=minority); O-$p$ (cornflower blue).
• Figure 3: Local magnetic moments (a) and bandgaps (b) of Pbnm NdNiO$_{3}$ in the presence of a site-local magnetic field $\pm{\textit{B}}$. Right panels show energy bands for ${\textit{B}}=0.04$ Ry.
• Figure 4: Schematic representation of cubic, pseudo-cubic Pbnm and P2$_{1}$/n distortions: NiO appears in cubic crystal field and is insulating in both the paramagnetic and antiferromagnetic phases. NdNiO$_{3}$ is a paramagnetic metal in the Pbnm phase and only becomes insulating in the P2$_{1}$/n phase with a concomitant charge, spin and bond disproportionation. Note that the Ni-O-Ni bond angle does not change on average due to the Pbnm to P2$_{1}$/n distortion. The corresponding crystal field diagrams are also shown for all cases. The insulating gap in P2$_{1}$/n phase opens between the e$_{g}$ states of the two distinct Ni sites, while in NiO the band gaps opens at a single Ni site.
• Figure 5: AFM NdNiO$_{3}$ in P2$_{1}$/n phase: (a) Energy bands in AFM phase, and (b) the AFM phase with additional constraining fields as described in the text. (In both cases up- and down- bands are nearly equivalent.) Colors represent the following projections: Ni$_{1}$ (red); Ni$_{2}$ (green); O-$p$ (blue). In the unconstrained case, Ni$_{1}$ and Ni$_{2}$ have local moments ${\pm}1.22\,\mu_B$ and 0, respectively. In the constrained case, Ni$_{1}$ and Ni$_{2}$ all have approximate moments ${\pm}1.2\,\mu_B$. (c,d) corresponding DOS. Colors represent the following projections: $e_{g}$ (rust=Ni$_{1}$, bright-red=Ni$_{2}$); $t_{2g}$ (olive=Ni$_{1}$, bright-green=Ni$_{2}$); O-$p$ (cornflower blue).