Oxygen-isotope effect on density wave transitions in La$_3$Ni$_2$O$_{7}$

Abstract

TThe isotope effect is a powerful probe of electron-phonon interactions in solid-state systems, offering key insights into how atomic mass influences emergent quantum states. Here, the impact of oxygen isotope substitution ($^{16}{\rm O}\rightarrow \; ^{18}{\rm O}$) on charge- and spin-density wave (CDW and SDW) transitions in the double-layer Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_7$ is investigated. A clear isotope effect is observed in the CDW transition: the transition temperature ($T_{\rm CDW}$) increases upon $^{18}$O substitution. In contrast, the SDW transition temperature remains unaffected within experimental uncertainty. These findings point to a strong involvement of lattice vibrations in the formation of charge order, while spin order appears to be predominantly of electronic origin. The results suggest that electron-phonon coupling, manifested through the CDW response to isotope substitution, may be relevant to the superconducting pairing mechanism in Ruddlesden-Popper nickelates.

Figures (3)

• Figure 1: (a) Room-temperature Raman spectra of oxygen-isotope substituted La$_3$Ni$_2$O$_7$ samples. Solid lines represent fits using seven Lorentzian functions. Dashed lines indicate the peak positions. (b) The oxygen participation parameter $f_{\rm O}$ as a function of the Raman wavenumber $^{16}\nu$.
• Figure 2: (a) Raw resistivity curves of the $^{16}$O/$^{18}$O-substituted La$_3$Ni$_2$O$_7$ samples. (b) Extended view of the first derivatives of the resistivity data in the vicinity of the CDW transitions. The red and blue numbers correspond to the CDW transition temperatures of La$_3$Ni$_2\,^{16}$O$_7$ and La$_3$Ni$_2\,^{18}$O$_7$, determined using different criteria (see text for details).
• Figure 3: (a) Temperature dependence of the magnetic volume fraction $f_{\rm m}$ for $^{16}$O- and $^{18}$O-substituted La$_3$Ni$_2$O$_7$ and La$_4$Ni$_3$O$_{10}$ samples. (b) Extended view of the $f_{\rm m}(T)$ curves in the vicinity of the SDW transitions. The vertical dashed lines and grey stripe indicate the CDW and SDW transition temperatures ($T_{\rm CDW}$ and $T_{\rm SDW}$), as reported in the literature.Chen_PRL_2024Khasanov_NatPhys_La327_2025Chen_arxiv_2024Zhang_NatCom_2020Khasanov_arxiv_La4310_2025Liu_NatCom_2024Wang_ChinPhysLett_2024Wang_InorgChem_2024Wu_PRB_2001Liu_SciChina_2023Seo_InorgChem_1996