Phonon frequency comb close to an isolated Einstein mode in InSiTe3

TL;DR

This paper reports the observation of a phonon frequency comb in InSiTe3, a layered Van der Waals material, revealing strong anharmonicity and collective vibrational states near an Einstein mode, with potential for exploring novel vibrational phenomena.

Contribution

It demonstrates the formation of a phonon frequency comb in InSiTe3 and highlights its strong anharmonicity and collective lattice excitations as a new platform for vibrational physics.

Findings

Pronounced anharmonicity in symmetry-predicted phonon modes.

Formation of a self-organized frequency domain structure near 500 cm$^{-1}$.

Anomalous temperature dependence around 200 K indicating strong phonon-phonon coupling.

Abstract

The emergence of phonon frequency combs exemplifies a rare and intriguing phenomenon in quantum solids. Materials with distinctive phonon band structures are especially promising for hosting such states, as their vibrational dispersion landscape across the Brillouin zone can facilitate the formation of long-lived, collective lattice excitations. In the layered Van der Waals compound InSiTe$_3$, polarization-resolved Raman spectroscopy reveals a pronounced anharmonicity in symmetry-predicted modes and the formation of a self-organized frequency domain structure (coherent-like state), in the range of a localized highenergy A$_{1g}$ phonon mode near 500 cm$^{-1}$. This strong phonon-phonon coupling manifests itself as an anomalous temperature dependence around 200 K, coinciding with the appearance of higher-order excitations within the phonon density of states gap. These findings position InSiTe$_3$ as an unconventional platform where intrinsic highly structured phonon spectral correlations and unusually strong anharmonic effects coexist, opening new avenues for exploring emergent vibrational phenomena in low-dimensional materials.