Prediction and Observation of Intermodulation Sidebands from Anharmonic Phonons

TL;DR

This paper predicts and observes intermodulation phonon sidebands in NaBr, revealing anharmonic phonon interactions and quantum noise effects through a quantum Langevin model, ab initio calculations, and neutron scattering experiments.

Contribution

It introduces a quantum Langevin model to predict intermodulation phonon sidebands and confirms these features experimentally in NaBr, highlighting their origin from anharmonic phonon interactions.

Findings

Observation of diffuse high-energy spectral features consistent with IPS

Identification of the lower IPS as an intrinsic localized mode

Spectral broadening due to thermal bath interactions

Abstract

A quantum Langevin model, similar to models used recently for optomechanics, was used to predict intermodulation phonon sidebands (IPS). Ab initio calculations of anharmonic phonons in rocksalt NaBr showed these spectral features as "many-body effects". Modern inelastic neutron scattering measurements on a crystal of NaBr at 300K revealed diffuse intensity at high phonon energy from a predicted upper IPS. The transverse optical (TO) part of the new features originates from phonon intermodulation between the transverse acoustic (TA) and TO phonons. The longitudinal optical (LO) spectral features originate from three-phonon coupling between the TA modes and the TO lattice modes. The partner lower IPS proves to be an "intrinsic localized mode". Interactions with the thermal bath broaden and redistribute the spectral weight of the IPS pair. These sidebands are a probe of anharmonicity and quantum noise, which originate from interactions between the phonons in NaBr.