Abstract
In a classical cubic crystal with one atom per unit cell, the only allowed excitations are three acoustic phonon branches. Recently, it was predicted that in a quantum crystal another optic-like excitation branch can exist. This additional excitation arises from the assumption that atomic zero-point motion in the crystal can be correlated. Here, we describe a neutron scattering experiment aiming to detect this excitation in BCC 4 He . In addition to the three phonon branches, we indeed observed a weakly dispersive, optic-like excitation branch along the [1 1 0] direction. One possible interpretation is that the new excitation is a quantum analogue of a point defect.
Published Version
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