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 $T_{\rm SDW}$ $\approx$ 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 $μ$J/cm$^2$, while the SDW is remarkably robust, persisting to 200 $μ$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.

Figures (4)

• Figure 1: (a) Transient reflectivity ($\Delta R/R$) as a function of delay time at various temperatures, measured at a pump fluence of $\sim$9.9 $\mu$J/cm$^2$. The experiential data can be well fitted by two-exponential decays. The solid lines are the fitting curves. (b) 2D false-color image of $\Delta R/R$ as a function of temperature and delay time. Red and green dashed lines indicate $T_{DW}$ and $T_{\rm SDW}$, respectively. (c), (d) Temperature dependence of amplitude ($A_s$) and relaxation time ($\tau_s$), respectively. The red solid lines are the RT model fitting curves. (e), (f) Temperature dependence of amplitude ($A_f$) and relaxation time ($\tau_f$), respectively. Error bars are the standard error in the exponential fitting.
• Figure 2: (a) Transient ellipticity ($\Delta\eta$) as a function of delay time, induced by left ($\sigma^+$) and right ($\sigma^-$) circularly polarized light at selected temperatures. (b) 2D false-color imag of [$\Delta\eta_{\sigma^+}$ - $\Delta\eta_{\sigma^-}$] as a function of temperature and delay time. (c) Temperature dependence of $\tau_{spin}$ extracted using an exponential decay model. Error bars are the standard error in the exponential fitting.
• Figure 3: 2D false color image of $\Delta R/R$ as a function of temperature and delay time at various pump fluences: (a) 9.9 $\mu$J/cm$^2$, (b) 19.9 $\mu$J/cm$^2$, (c) 29.8 $\mu$J/cm$^2$, (d) 39.8 $\mu$J/cm$^2$, (e) 59.7 $\mu$J/cm$^2$, (f) 99.5 $\mu$J/cm$^2$, (g) 139.3 $\mu$J/cm$^2$, and (h) 198.9 $\mu$J/cm$^2$. The green scatter points in the top panel represent the extracted $\tau_f$, and the purple scatter points in all figures represent the extracted $\tau_s$. The phonon bottleneck effect is observed at all fluences. Error bars are the standard error in the exponential fitting.
• Figure 4: Temperature-fluence phase diagram of La$_3$Ni$_2$O$_7$. The upper panel summarizes the SDW gap magnitudes and the ratio of 2$\Delta_{\rm SDW}/T_{\rm SDW}$ as a function of pump fluence, extracted using the RT model. The lower panel presents the onset temperatures of the two DW orders identified in this study. Solid lines serve as guides to the eye.