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

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 $T_{\mathrm{SDW}}$ $\approx$ 140 K by measuring its spin dynamics, and discover a distinct, nonmagnetic charge order appearing below $T_{\mathrm{DW}}$ $\approx$ 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 $\hat{I}¼$J/cm${}^2$, while the SDW is remarkably robust, persisting to 200 $\hat{I}¼$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.

8 pages, 4 figures