Lattice-Entangled Density Wave Instability and Nonthermal Melting in La$_4$Ni$_3$O$_{10}$

Abstract

The recent discovery of high-temperature superconductivity in pressurized nickelates has renewed interest in the broken-symmetry states of their ambient-pressure parent phases, where a density-wave (DW) order emerges and competes with superconductivity, but its microscopic origin remains unresolved. Using ultrafast optical spectroscopy, we track quasiparticle relaxation dynamics across the DW transition at $T_{\rm DW} \approx$ 136 K in trilayer nickelate La$_4$Ni$_3$O$_{10}$ single crystals, revealing the opening of an energy gap of $\sim$ 52 meV. Multiple coherent phonons, including $A_g$ modes near 3.88, 5.28, and 2.09 THz, display pronounced mode-selective anomalies across the transition, demonstrating that the DW is coupled with lattice degree of freedom stabilized through electron-phonon coupling. At higher excitation densities, the DW is nonthermally suppressed, producing a temperature-fluence phase diagram that parallels pressure-tuned behavior. These results establish the DW in La$_4$Ni$_3$O$_{10}$ as a lattice-entangled instability involving multiple phonon modes, and highlight ultrafast optical excitation as a powerful tool to manipulate competing orders in nickelates.

Figures (4)

• Figure 1: Quasiparticle dynamics across the DW transition. (a) Transient reflectivity $\Delta R(t)/R$ as a function of temperature and time delay, measured at a low pump fluence of $\sim$10.2 $\mu$J/cm$^2$. The sharp change at $T_{\rm DW}$ marks the phase transition. (b) Representative $\Delta R(t)/R$ traces at different temperatures. Black lines are fits using a bi-exponential model. (c, d) Temperature dependence of the amplitude ($A_2$) and lifetime ($\tau_2$) of the slow relaxation component. Dashed red curves are fits to the RT model.
• Figure 2: Anomalous phonon dynamics at low pump fluence. (a) Coherent oscillations at select temperatures, isolated by subtracting the quasiparticle background. Traces are offset for clarity. (b) Corresponding FFT map, showing the temperature evolution of the two most prominent phonon modes, $\omega_1$ and $\omega_2$. (c) Temperature dependence of the $\omega_1$ mode frequency. The solid green line is a fit to a standard anharmonic decay model, which the data clearly deviates from below $T_{\rm DW}$. Dashed curves are power-law fits ($T^2$ above $T_{\rm DW}$, $T^{5.2}$ below), and the right axis shows the data on a log-log scale to highlight the change in slope at the transition. (d) Temperature dependence of the $\omega_2$ mode frequency.
• Figure 3: Full phonon spectrum and mode-selective coupling at high pump fluence. (a) raw $\Delta R(t)/R$ map measured at a high pump fluence of $\sim$130 $\mu$J/cm$^2$, which suppresses the transition to $T_{\rm DW} \approx$ 95 K. (b) Corresponding FFT map revealing six distinct coherent phonon modes ( $\omega_1$ to $\omega_6$). (c-h) Temperature dependence of the frequency for each of the six modes. Vertical dashed lines indicate the suppressed $T_{\rm DW}$. Solid green lines are fits to the anharmonic decay model, highlighting the anomalous behavior of modes $\omega_1$ and $\omega_4$ below the transition.
• Figure 4: Non-thermal melting of the DW order. (a1)-(a6) Normalized $\Delta R(t)/R$ maps as a function of pump fluence at several fixed temperatures. The black circles trace the slow relaxation time $\tau_2$, which peaks at the critical fluence $F_C$ required to melt the DW order. (b) The resulting Temperature-Fluence phase diagram for La$_4$Ni$_3$O$_{10}$. The phase boundary is determined by the $F_C(T)$ values, extracted from the peaks in $\tau_2$ shown in panel (a), together with data from Figs. 1 (black symbol) and 3 (green symbol).