Low-energy plasmon excitations in infinite-layer nickelates

TL;DR

This paper investigates the charge excitation spectrum in infinite-layer nickelates, analyzing plasmon excitations through two theoretical approaches, and compares findings with experimental data to understand their similarities to cuprates.

Contribution

It provides a comparative analysis of plasmon excitations in nickelates using both strongly correlated and RPA models, offering insights into their charge dynamics and relation to cuprates.

Findings

Plasmon excitations occur around momentum transfer (0,0,q_z) in both models.

Damping of plasmons is stronger in RPA due to larger interlayer hopping.

Low-energy plasmons with gaps of ~360 and ~560 meV are predicted for finite q_z.

Abstract

The discovery of superconductivity in infinite-layer nickelates is presently an important topic in condensed-matter physics, and potential similarities to and differences from cuprates are under intense debate. We determine general features of the charge excitation spectrum in nickelates from two opposite viewpoints: (i) Nickelates are regarded as strongly correlated electron systems like cuprate superconductors and thus can be described by the $t$-$J$ model, and (ii) electron correlation effects are not as strong as in cuprates, and thus, random-phase approximation (RPA) calculations may capture the essential physics. We find that in both cases, plasmon excitations are realized around the momentum transfer $\vq=(0,0,q_z)$, although they tend to be damped more strongly in the RPA. In particular, this damping is enhanced by the relatively large interlayer hopping expected in nickelates. Besides reproducing the optical plasmon at $\vq=(0,0,0)$ observed in Nd$_{0.8}$Sr$_{0.2}$NiO$_2$, we obtain low-energy plasmons with gaps of $\sim 360$ and $\sim 560$ meV at $\vq=(0,0,q_z)$ for finite $q_z$ in cases (i) and (ii), respectively. The present work offers a possible theoretical hint to answer whether nickelates are cupratelike or not and contributes to the general understanding of the charge dynamics in nickelates.