Charge and spin instabilities in superconducting La$_3$Ni$_2$O$_7$

TL;DR

This paper investigates the charge and spin instabilities in La$_3$Ni$_2$O$_7$ and their suppression under pressure and doping, providing insights into the emergence of superconductivity in this material.

Contribution

It combines model analysis and first-principles calculations to identify charge and spin instabilities and their suppression mechanisms in La$_3$Ni$_2$O$_7$.

Findings

Charge instability characterized by oxygen hole fluctuations

Strong antiferromagnetic tendencies revealed by DFT+U calculations

External pressure and doping suppress instabilities, facilitating superconductivity

Abstract

Motivated by the recent discovery of superconductivity in La$_3$Ni$_2$O$_7$ under high pressure, we explore its potential charge and spin instabilities through combined model analysis and first-principles calculations. Taking into account the small charge-transfer nature of high valence nickel, a fully correlated two-cluster model identifies a lattice-coupled charge instability characterized by substantial short-range fluctuations of oxygen holes. This instability is corroborated by density-functional-theory plus $U$ calculations that also reveal a strong tendency towards concurrent antiferromagnetic ordering. The charge, spin, and associated lattice instabilities are significantly suppressed with increasing external pressure, contributing to the emergence of superconductivity in pressurized La$_3$Ni$_2$O$_7$. Carrier doping is found to effectively suppress these instabilities, suggesting a viable strategy to stabilize a superconducting phase under ambient pressure.