Spontaneous decay of a soft optical phonon in the relaxor ferroelectric PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$

TL;DR

This study reveals that a soft optical phonon in the relaxor ferroelectric PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ spontaneously decays into two acoustic phonons, with short lifetimes observed via neutron spectroscopy, independent of ferroelectric order.

Contribution

It demonstrates the spontaneous decay of a soft optical phonon in a relaxor ferroelectric, linking the relaxor waterfall effect to quasiparticle decay phenomena.

Findings

Phonon lifetimes are anomalously short within specific energy-momentum ranges.

Decay occurs when optical phonons can kinematically decay into two acoustic phonons.

The relaxor waterfall effect is analogous to quasiparticle decay in quantum liquids.

Abstract

We report the spontaneous decay of a soft, optical phonon in a solid. Using neutron spectroscopy, we find that specific phonon lifetimes in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_{3}$ are anomalously short within well-defined ranges of energy and momentum. This behavior is independent of ferroelectric order and occurs when the optical phonon with a specific energy and momentum can kinematically decay into two acoustic phonons with lower phase velocity. We interpret the well-known relaxor "waterfall" effect as a form of quasiparticle decay analogous to that previously reported in quantum spin liquids and quantum fluids.