Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate

TL;DR

This study uses ultrafast spectroscopy to observe how vibrational symmetry breaking occurs over time in a charge-ordered nickelate, revealing the dynamics of stripe phase melting and formation.

Contribution

It provides the first direct time-domain observation of vibrational symmetry breaking in a stripe-ordered nickelate using multi-THz response measurements.

Findings

Electronic carriers delocalize immediately after excitation.

Lattice symmetry remains initially frozen despite electronic changes.

Different vibrational modes respond at different speeds, indicating directional interactions.

Abstract

The ability to probe symmetry breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-THz response of the model stripe compound La$_{1.75}$Sr$_{0.25}$NiO$_{4}$, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen - as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry breaking dynamics in solids.