New excitations in bcc $^{4}$He - an inelastic neutron scattering study

TL;DR

This study uses inelastic neutron scattering to explore phonon and novel 'optic-like' excitations in bcc solid helium-4, revealing unexpected dispersionless modes that suggest unique localized quantum excitations.

Contribution

It reports the discovery of a new dispersionless 'optic-like' excitation branch in bcc helium-4, indicating novel localized quantum excitations not previously observed.

Findings

Discovered a dispersionless 'optic-like' branch at ~1 meV

Identified different properties of two 'optic-like' branches

Provided evidence for localized excitations in quantum solid helium

Abstract

We report neutron scattering measurements on bcc solid $^{4}$% He. We studied the phonon branches and the recently discovered ''optic-like'' branch along the main crystalline directions. In addition, we discovered another, dispersionless "optic-like'' branch at an energy around 1 meV ($\sim$~11K). The properties of the two "optic-like" branches seem different. Since one expects only 3 acoustic phonon branches in a monoatomic cubic crystal, these new branches must represent different type of excitations. One possible interpretation involves localized excitations unique to a quantum solid.