Jahn-Teller distortion on strained La$_3$Ni$_2$O$_7$ thin films

Abstract

We present a systematic study of the electronic structure of strained La$_3$Ni$_2$O$_7$ thin films. We show that biaxial compressive strain mainly elongates the outer apical Ni-O bond while leaving the inner apical Ni-O bond nearly unchanged. As a result, the Jahn-Teller splitting $Δ_{JT}$ is strongly enhanced, whereas the interlayer $d_{z^2}$ hopping $t_\perp^z$ changes only weakly. Since superconductivity is widely believed to emerge only below a critical in-plane lattice constant, our results identify the strain-enhanced $Δ_{JT}$ as the relevant microscopic tuning parameter. Consistently, the calculated Fermi surfaces and Hall response for LaAlO$_3$ and SrLaAlO$_4$ substrates agree with ARPES and Hall measurements. Our results identify Jahn-Teller distortion as a key tuning parameter in strained La$_3$Ni$_2$O$_7$ and support its central role in optimizing superconductivity in bilayer nickelates.

Figures (5)

• Figure 1: (a) The Ni-O octahedron elongates in the out-of-plane direction when a thin-film material is under in-plane compressive strain effect. $z_t$ represents the bond length between the Ni atom and the outer-layer (top or bottom) oxygen atom O$_{t/b}$, while $z_m$ denotes the bond length between the Ni atom and the middle oxygen atom O$_m$. The height of the NiO$_6$ octahedron is $h=z_t+z_m$. (b) Orbital energy level diagram. The JT distortion $\Delta_{JT}$ is defined as the on-site energy difference between the two $e_g$ orbitals of the Ni atom. The interlayer coupling is characterized by $t_{\perp}^{z}$. (c) Relationship between $a_{p}$ and $T_c$ from experimental data ($a_p=\sqrt{a^2+b^2}/2$ is an alternative convention used in the literature). (d) Relationship between $c$ and $T_c$ from experimental data. In (c,d), open black symbols represent bulk materials, and solid colored symbols represent thin-film materials.
• Figure 2: (a) Variation of the bond lengths $z_{t}$ (between Ni and outer-layer O$_{t}$) and $z_{m}$ (between Ni and inner-layer O$_{m}$) with respect to the in-plane lattice constant $a_{p}$. (b) Variation of $\Delta_{JT}$ and $|t_{\perp}^{z}|$ with respect to $a_{p}$. As a reference, the vertical blue, orange and gray thick line represents the experimental lattice constants associated with SLAO substrate 2025Hwang, LAO substrate 2025Hwangb and the bulk material under ambient pressure 2023Wangc, respectively. (c) Variation of $\Delta_{JT}$ with the in-plane lattice parameter $a_{p}$ while keeping the lattice parameter along the $c$-axis fixed at $c=20.79\AA$. The data is fitted linearly, depicted in solid line, with the slope given by $-2.75$eV/$\AA$. (d) Variation of $\Delta_{JT}$ with the lattice parameter $c$ while keeping the in-plane lattice parameter fixed at $a_p=3.75\AA$. The data is fitted linearly, depicted in solid line, with the slope given by $0.38$eV/$\AA$.
• Figure 3: (a) and (b) show Fermi surface for SLAO and LAO substrate, respectively. The Fermi surface extracted from ARPES on SLAO 2025Shen is plotted as black dots in (a). The main contrast between the two substrates are the emergence of $\gamma$ pocket highlighted in LAO. (c) The Hall coefficient obtained from the two substrates as a function of quasiparticle scattering rate $\Gamma$. The dashed line show the Hall coefficient from experimental measurements at 160K 2025Hwangb. (d) Schematic illustration of the local electronic configuration for SLAO and LAO, where the JT distortion $\Delta_{JT}$ is labeled by arrows.
• Figure 4: (a) Variation of the bond lengths $z_{t}$ (between Ni and outer-layer O$_{t}$) and $z_{m}$ (between Ni and inner-layer O$_{m}$) with the in-plane lattice constant $a_{p}$ for the bulk materials under hydrostatic pressure. (b) Variation of $\Delta_{JT}$ and $|t_{\perp}^{z}|$ with the in-plane lattice parameter $a_{p}$ in bulk case. The regular pentagon represents the values calculated using the experimental lattice constants under ambient pressure 2023Wangc. The black arrows indicate larger pressure.
• Figure A1: (a) Variation of the bond lengths between Ni and outer-layer O$_{t}$ ($z_{t}$) and between Ni and inner-layer O$_{m}$ ($z_{m}$) with the in-plane lattice constant $a_{p}$ for the monolayer crystal structure under in-plane pressure. (b) Variation of $\Delta_{JT}$ and $|t_{\perp}^{z}|$ with the in-plane lattice parameter $a_{p}$ in monolayer case.