Lattice-Charge Coupling in a Trilayer Nickelate with Intertwined Density Wave Order

Intertwined charge and spin correlations are ubiquitous in a wide range of transition metal oxides and are often perceived as intimately related to unconventional superconductivity. Theoretically envisioned as driven by strong electronic correlations, the intertwined order is usually found to be strongly coupled to the lattice as signaled by pronounced phonon softening. Recently, both charge and spin density waves (CDW and SDW) and superconductivity have been discovered in several Ruddlesden-Popper (RP) nickelates, in particular trilayer nickelates <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>R</a:mi> <a:msub> <a:mrow> <a:mi>E</a:mi> </a:mrow> <a:mrow> <a:mn>4</a:mn> </a:mrow> </a:msub> <a:msub> <a:mrow> <a:mi>Ni</a:mi> </a:mrow> <a:mrow> <a:mn>3</a:mn> </a:mrow> </a:msub> <a:msub> <a:mrow> <a:mi mathvariant="normal">O</a:mi> </a:mrow> <a:mrow> <a:mn>10</a:mn> </a:mrow> </a:msub> </a:mrow> </a:math> ( <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"> <d:mrow> <d:mi>R</d:mi> <d:mi>E</d:mi> <d:mo>=</d:mo> <d:mi>Pr</d:mi> </d:mrow> </d:math> , La). The nature of the intertwined order and the role of lattice-charge coupling are at the heart of the debate about these materials. Using inelastic x-ray scattering, we mapped the low-energy phonon dispersions in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mrow> <f:mi>R</f:mi> <f:msub> <f:mrow> <f:mi>E</f:mi> </f:mrow> <f:mrow> <f:mn>4</f:mn> </f:mrow> </f:msub> <f:msub> <f:mrow> <f:mi>Ni</f:mi> </f:mrow> <f:mrow> <f:mn>3</f:mn> </f:mrow> </f:msub> <f:msub> <f:mrow> <f:mi mathvariant="normal">O</f:mi> </f:mrow> <f:mrow> <f:mn>10</f:mn> </f:mrow> </f:msub> </f:mrow> </f:math> and found no evidence of softening near the CDW wave vector over a wide temperature range, which contrasts with the pronounced anomalies frequently observed in cuprate superconductors. Calculations of the electronic susceptibility revealed a peak at the observed SDW ordering vector but not at the CDW wave vector. Our experimental and theoretical findings highlight the crucial role of the spin degree of freedom and establish a foundation for understanding the interplay between superconductivity and density-wave transitions in RP nickelate superconductors and beyond.