Intensity map of inelastic neutron scattering from the body-centered-cubic phase of solid 4He showing a new optical phonon mode (the horizontal yellow-green band around 1.3 meV). A classical monoatomic cubic crystal can have only three acoustic phonon branches; the existence of this new branch in a 4He crystal is thought to be due to the large zero-point motion of its atoms.
From the article:
Observation of a New Excitation in bcc by Inelastic Neutron ScatteringT. Markovich, E. Polturak, J. Bossy, and E. Farhi
Phys. Rev. Lett. 88, 195301 (2002)
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