Lattice dynamics of the infinite-layer nickelate LaNiO$_2$

TL;DR

This work characterizes the lattice dynamics of the infinite-layer nickelate LaNiO2 using time-of-flight inelastic neutron scattering on a large array of co-aligned bulk crystals and density-functional perturbation theory. The measured phonon spectrum below ~38 meV agrees with DFPT predictions, enabling assignments of key modes such as the X-point half-breathing and M-point full-breathing vibrations, while no spin excitations are resolved at the M point within the current experimental sensitivity. Simulations with Euphonic using DFPT inputs reproduce the observed intensity patterns and the absence of high-energy phonons, establishing a reference framework for lattice dynamics and electron-phonon studies in IL nickelates. These results lay the groundwork for future high-resolution investigations into EPC, charge fluctuations, and their possible connections to superconductivity in this material class.

Abstract

Infinite-layer (IL) nickelates have rapidly emerged as a new class of superconductors. However, due to the technical challenges of their topotactic synthesis, they have so far been realized primarily as thin films or polycrystalline powder samples, limiting comprehensive investigations of fundamental physical properties such as the lattice dynamics. Here, we present a time-of-flight inelastic neutron scattering study on a sample composed of a large number of co-aligned bulk crystals of the IL nickelate LaNiO$_2$. We observe several dispersive phonon branches, which are in good agreement with lattice dynamical calculations based on density-functional perturbation theory. In addition, we compare the characteristics of selected LaNiO$_2$ phonon modes to those of isostructural cuprate superconductors. Our findings provide a reference point for future experimental and theoretical efforts aimed at understanding the interplay between lattice dynamics and electronic properties in IL nickelates.

Figures (4)

• Figure 1: (a) Sample array with co-aligned LaNiO$_{2}$ crystals on two sides of an Al grid. (b) Representative XRD patterns from the surface of individual LaNiO$_{2}$ crystals, acquired with Cu $K_\alpha$ radiation at 300 K. The Bragg peaks are indexed. The inset shows the tetragonal $P4/mmm$ unit cell of LaNiO$_{2}$.
• Figure 2: (a)--(d) Constant energy slices of the INS spectra in the $(H,K,0)$ plane for incident neutron energy $E_{\rm i}=76$ meV. The transferred energies are integrated over the following ranges: (a) $-2\leq E \leq 2$ meV , (b) $6\leq E \leq 10$ meV, (c) $20\leq E \leq 25$ meV, and (d) $50\leq E \leq 70$ meV. The integration range along the out-of-plane direction is $\pm0.37$ Å$^{-1}$. The inset in panel (a) highlights two satellite peaks from $P4/mmm$ twin domains around the $(-2,2,0)$ Bragg peak.
• Figure 3: (a),(b) Constant energy slices of folded INS spectra in the $(H,K,0)$ plane for incident energy $E_{\rm i}=76$ meV. The transferred energies are integrated over the following ranges: (a) $-2\leq E \leq 2$ meV and (b) $15\leq E \leq 25$ meV. A path along high-symmetry points ($\Gamma$-$X$-$M$-$\Gamma$) is illustrated in red color around the (060) Bragg reflection in panel (a). (c) Schematic of a three-dimensional path moving along the high-symmetry points $\Gamma$-$X$-$M$-$\Gamma$-$Z$-$R$-$A$-$Z$ according to the tetragonal P4/mmm unit cell of LaNiO$_2$. (d) INS map along a high-symmetry path from folded data around the (060) and (600) Bragg peaks, acquired with $E_{\rm i}=76$ meV. The integration range along the two orthogonal directions relative to the path is $\pm0.2$ Å$^{-1}$. The DFPT computed phonon dispersions for the three twin domains of LaNiO$_2$ are superimposed as solid (domain 1), dashed (domain 2), and dotted lines (domain 3). The colored circles on the dispersion curves of domain 1 indicate specific phonon modes discussed in the text. (e) Phonon intensities calculated with the Euphonic software package, using the DFPT phonons as input. The intensities are averaged over the three domains, while the indexing of the high-symmetry path refers to domain 1. In addition, Gaussian broadening is applied along both the momentum ($\Delta Q=0.92$ Å$^{-1}$) and energy ($\Delta E=4.9$ meV) transfer directions. Details about the domain averaging and broadening are given in the Supplementary Information SM.
• Figure 4: (a) Atomic displacement patterns of selected phonons of LaNiO$_2$ at the $\Gamma$ point together with the computed phonon energies in units of meV. La atoms are shown in green, Ni in blue, and oxygen in red. The yellow arrows indicate the directions and amplitudes of the atomic vibrations. For clarity, the La, Ni, and O atoms are displayed with reduced atomic radii. (b)-(d) Displacement patterns of phonons at the $X$, $M$, and $Z$ points, respectively.