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

TL;DR

This study uncovers a lattice-entangled density wave in La4Ni3O10, showing its coupling with phonons and nonthermal suppression via ultrafast optical excitation, providing insights into nickelate phase competition.

Contribution

It reveals the microscopic origin of the density wave in La4Ni3O10 as a lattice-entangled instability involving multiple phonon modes, demonstrated through ultrafast spectroscopy.

Findings

Density wave opens a 52 meV gap at 136 K.

Multiple phonon modes show anomalies across the transition.

High excitation densities nonthermally suppress the density wave.

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.