Comparison of coherent phonon generation by electronic and ionic Raman scattering in LaAlO$_3$

TL;DR

This study compares ionic and electronic Raman scattering in LaAlO₃, revealing ionic Raman scattering can dominate in wide band-gap insulators and identifying resonance conditions and competing channels affecting phonon coherence.

Contribution

It provides the first direct comparison of ionic and electronic Raman scattering mechanisms in LaAlO₃ using time-resolved spectroscopy, highlighting the dominance of ionic Raman scattering under certain conditions.

Findings

Resonance peak in phonon amplitude when pump matches infrared-active mode.

Ionic Raman scattering can dominate over electronic scattering in wide band-gap insulators.

Evidence of competing scattering channels at high fluences affecting phonon amplitude.

Abstract

In ionic Raman scattering, infrared-active phonons mediate a scattering process that results in the creation or destruction of a Raman-active phonon. This mechanism relies on nonlinear interactions between phonons and has in recent years been associated with a variety of emergent lattice-driven phenomena in complex transition-metal oxides, but the underlying mechanism is often obscured by the presence of multiple coupled order parameters in play. Here, we use time-resolved spectroscopy to compare coherent phonons generated by ionic Raman scattering with those created by more conventional electronic Raman scattering on the nonmagnetic and non-strongly-correlated wide band-gap insulator LaAlO$_3$. We find that the oscillatory amplitude of the low-frequency Raman-active $E_g$ mode exhibits a sharp peak when we tune our pump frequency into resonance with the high-frequency infrared-active $E_u$ mode, consistent with first-principles calculations. Our results suggest that ionic Raman scattering can strongly dominate electronic Raman scattering in wide band-gap insulating materials. We also see evidence of competing scattering channels at fluences above 28~mJ/cm$^2$ that alter the measured amplitude of the coherent phonon response.