Orbital dimerization-induced first-order structural phase transition: a case study in La$_3$Ni$_2$O$_7$

Abstract

First-order structural phase transition is a common phenomenon in materials that qualitatively alters their physical properties. Yet, the abrupt first-order nature is usually unexplained by realistic computations, implying an omission of important physics in describing the electronic structure of the nearby stable phases. Using the recently discovered nickelate superconductors La$_3$Ni$_2$O$_7$ as a prototypical example, we demonstrate that such first-order nature is typically beyond intra-atomic correlation considered in state-of-the-art material computations. Instead, a full many-body treatment of low-energy active orbitals reveals a generic inter-atomic "orbital dimerization" mechanism of first-order structural phase transition, corresponding to abrupt energy reduction upon a spin-singlet bond formation. Such an inter-atomic correlation qualitatively changes not only the essential lattice bonding but also the characteristics of low-energy electronic properties across the transition. This strong mechanism and the developed computational framework are generally applicable to a wide variety of ionic materials, to produce valuable insights into atomic and electronic structures essential for their physical properties and functionalities.

Figures (3)

• Figure 1: Lack of first-order phase transition in standard treatments - System total energies against octahedral tile angle $\theta$ at low (a)(b), intermediate (c)(d), and high (e)(f) pressures, computed via standard LDA, LDA+DMFT, and LDA+$U$ treatments. The lattice structures for each constrained octahedral tile angle are optimized within each treatment with the same $U=6~{\rm eV},\ J=0.9~{\rm eV}$. The optimal tilt angels ($\theta_\mathrm{min}$, indicated by the red arrows) smoothly approach $180^\degree$ at higher pressure, displaying a continuous structure transition in contradiction to experimental observations.
• Figure 2: First-order phase transition via inter-atomic orbital dimerization - (a)(c)(e) System total energy with inter-atomic correlation display two stable structures with a tilted ($\theta_\mathrm{t}\sim 170^\degree$) and untilded ($\theta_\mathrm{u}=180^\degree$) octahedra. Around 12 GPa, the energy minimum jumps from the tildted octahedra to the untilted ones, indicating a first-order structural transition in agreement with experiments. (b)(d)(f) The essential double-minimum structures originate from a strong energy reduction, $\Delta E$, from the LDA+$U$ energy, $E_\mathrm{DFT}$, near $\theta\sim 180^\degree$ due to inter-atomic orbital dimerization associated with an enhanced spin-singlet bond.
• Figure 3: Distinct low-energy electronic structures - The eV-scale eigen-energy structures of $H$ correspond to dynamics of 4 dressed 1/2-spins in the upper (u) and lower (l) layers with competing couplings. The low-pressure case is dominated by the renormalized intra-atomic Hund's coupling $\tilde{J}_\mathrm{H}$ that results in typical spin-1 Ni$^{2+}$ ions in the sub-eV dynamics (in blue). In contrast, the high-pressure case is dominated by the inter-atomic antiferromagnetic $\mathbf{S}^{z^2}_u$-$\mathbf{S}^{z^2}_l$ superexchange that effectively fractionalizes the Ni$^{2+}$ ionic spins from 1 to 1/2 in the sub-eV scale (in red).