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.
Categories: cond-mat.str-el cond-mat.supr-con