Spontaneously formed phonon frequency combs in van der Waals solid CrXTe$_3$ (X=Ge,Si)

TL;DR

This study reports the discovery of long-lived, spontaneously formed phonon frequency combs in van der Waals CrXTe3 materials, achieved through high-resolution Raman scattering, revealing new possibilities for ultrafast material control and phonon-based technologies.

Contribution

It demonstrates the formation of persistent phonon frequency combs in CrXTe3, a phenomenon previously unobserved, and models this behavior with a nonlinear oscillator approach.

Findings

Frequency combs persist for hundreds of oscillations.

Comb modes survive up to 100K temperature.

Localized oscillations behave as independent quantum oscillators.

Abstract

Optical phonon engineering through nonlinear effects has been utilized in ultrafast control of material properties. However, nonlinear optical phonons typically exhibit rapid decay due to strong mode-mode couplings, limiting their effectiveness in temperature or frequency sensitive applications. In this study, we report the observation of long-lived nonlinear optical phonons through the spontaneous formation of phonon frequency combs in the van der Waals material CrXTe$_3$ (X=Ge, Si) using high-resolution Raman scattering. Unlike conventional optical phonons, the highest $A_g$ mode in CrGeTe$_3$ splits into equidistant, sharp peaks forming a frequency comb that persists for hundreds of oscillations and survives up to 100K before decaying. These modes correspond to localized oscillations of Ge$_2$Te$_6$ clusters, isolated from Cr hexagons, behaving as independent quantum oscillators. Introducing a cubic nonlinear term to the harmonic oscillator model, we simulate the phonon time evolution and successfully replicate the observed comb structure. Similar frequency comb behavior is observed in CrSiTe$_3$, demonstrating the generalizability of this phenomenon. Our findings reveal that Raman scattering effectively probes high-frequency nonlinear phonon modes, providing new insight into generating long-lived, tunable phonon frequency combs with applications in ultrafast material control and phonon-based technologies.