Ultrafast Optical Evidence of Coexisting Density Waves in Bilayer Nickelate La$_3$Ni$_2$O$_7$

TL;DR

This study uses ultrafast optical techniques to reveal and control coexisting density waves in La$_3$Ni$_2$O$_7$, distinguishing their stability and interactions in a quantum material.

Contribution

It demonstrates selective optical manipulation of coexisting density waves, revealing their hierarchy and providing insights into their interplay in bilayer nickelates.

Findings

Identification of a spin-density wave with nematicity below 140 K

Discovery of a nonmagnetic charge order below 115 K

Charge order is fragile and suppressed at low pump fluence, SDW is robust

Abstract

Utilizing ultrafast optical pump-probe spectroscopy, we investigate the coexistence and competition of electronic orders in the bilayer nickelate La$_3$Ni$_2$O$_7$. Our results reveal two coexisting density waves that can be selectively manipulated with light. We directly identify a spin-density wave (SDW) with electronic nematicity emerging below $T_{\rm SDW}$ $\approx$ 140 K by measuring its spin dynamics, and discover a distinct, nonmagnetic charge order appearing below $T_{\rm DW}$ $\approx$ 115 K. The central finding is the demonstration of differential optical control: the charge order is fragile, completely suppressed by a pump fluence of approximately 40 $\mu$J/cm$^2$, while the SDW is remarkably robust, persisting to 200 $\mu$J/cm$^2$. This work establishes a clear hierarchy in the stability of competing electronic orders and provides a powerful method for disentangling their interplay in quantum materials.