Ice phonon spectra and Bayes inference: A gateway to a new understanding of terahertz sound propagation in water

Understanding how molecules engage in collective motions in a liquid where a network of bonds exists has both fundamental and applied relevance. On the one hand, it can elucidate the ordering role of long-range correlations and inspire new avenues to control such order to implement sound manipulation. Water represents an ideal investigation case to unfold these general aspects, and, across the decades, it has been the focus of thorough scrutiny. Despite this investigative effort, the spectrum of terahertz density fluctuations of water largely remains a puzzle for condensed matter physicists. To unravel it, we compare previous scattering measurements of water spectra with new ones on ice. Owing to the unique asset of Bayesian inference, we draw a more detailed portrayal of the phonon response of ice. The comparison with the one of liquid water challenges the current understanding of density fluctuations in water, or more in general, of any networked liquid.

Automated summary

Understanding collective motions in liquids with a network of bonds has both fundamental and applied relevance. Water is an ideal case to study these aspects, but the terahertz density fluctuations of water remain puzzling. By comparing scattering measurements of water and ice, we challenge the current understanding of density fluctuations in water and networked liquids in general, using Bayesian inference to provide a more detailed portrayal of ice's phonon response.