Observation of rapid atomic motion in solid³He-⁴He mixtures using NMR

Abstract

We compared liquid and solid 3 He-4 He mixtures at coexistence using NMR imaging of the density of 3 He and of the T2 relaxation time in the two phases. We find that above 1 K, 3 He atoms in a bcc solid are more mobile than in the superfluid liquid. This rapid motion, induced by vacancies, takes place on a microscopic scale and can be explained in terms of 3 He-vacancy attraction of quantum origin. We relate our observations to similar effects observed in Sn-Pb alloys.

bcc4Heas a coherent quantum solid

“…We show that the bcc 4 He is a unique phase having both Diagonal Long Range Order (DLRO) of the solid lattice and Off-Diagonal Long Range Order (ODLRO) of the local dipoles. There is therefore a complex threecomponent order parameter which describes the coherently oscillating dipoles in each of the three orthogonal directions in the lattice.…”

“…The bcc phase of 4 He has a pronounced quantum nature due to the relatively open structure of the lattice. Quantum effects are manifested in strong anharmonicity of some phonon modes and in the large zero-point kinetic energy of the atoms 1 .…”

“…For the sake of clarity we have reproduced here some of the arguments and calculations already given in 2 . We propose that in the bcc 4 He phase the local excitations of the atoms in their potential wells, result in oscillating local electric dipoles. The ground-state of these dipoles has the dipoles oscillating in synchrony, thereby reducing the dipolar interaction energy between them.…”

“…In the following we further explore the nature of the coherent ground-state of the local-modes in the bcc 4 He. We show that the bcc 4 He is a unique phase having both Diagonal Long Range Order (DLRO) of the solid lattice and Off-Diagonal Long Range Order (ODLRO) of the local dipoles.…”

“…The usual treatment of the ground-state and energy of bcc 4 He employs variational wavefunctions that aim to account for the short-range correlations between the atoms 1 . These correlations arise mainly due to the hardcore repulsion between the atoms.…”