Anisotropic Electronic Correlations in the Spin Density Wave State of La$_3$Ni$_2$O$_7$

TL;DR

This study uses polarization-resolved Raman scattering to reveal anisotropic, momentum-dependent spin-density-wave gaps in La$_3$Ni$_2$O$_7$, highlighting the role of electronic correlations in its magnetic and superconducting properties.

Contribution

It provides the first detailed microscopic characterization of the SDW state in La$_3$Ni$_2$O$_7$, emphasizing anisotropic electronic correlations as a key factor.

Findings

Observation of symmetry-dependent spectral weight redistribution below 150 K.

Identification of momentum-selective SDW gap amplitudes.

Evidence for an unconventional SDW driven by anisotropic correlations.

Abstract

The bilayer nickelate superconductor La$_3$Ni$_2$O$_7$ undergoes a density wave transition near 150 K that has attracted intensive scrutiny, yet its microscopic origin remains elusive. Here we report polarization-resolved electronic Raman scattering measurements on high-quality single crystals of La$_3$Ni$_2$O$_7$. Below 150\,K, we observe a pronounced, symmetry-dependent redistribution of spectral weight in B$_{1g}$ and B$_{2g}$ channels, consistent with the formation of spin-density-wave (SDW) gaps. Quantitative analysis reveals momentum-selective SDW gap amplitudes, with intermediate-to-strong coupling near X/Y points of the Brillouin zone and weaker coupling along the diagonal direction, indicating an unconventional SDW driven by anisotropic electronic correlations. Our results establish the electronic character of the SDW in La$_3$Ni$_2$O$_7$, and provide a microscopic foundation for understanding the emergence of high-temperature superconductivity under pressure in nickelates.