Bcc ^4He as a Coherent Quantum Solid: "Super-Solid" ?

Abstract

In this work we investigate the quantum nature of bcc $^{4}$He. We show that it is a solid phase with an Off-Diagonal Long Range Order of coherently oscillating local electric dipole moments. These dipoles arise from the correlated zero-point motion of the atoms in the crystal potential, which oscillate in synchrony so that the dipolar interaction energy is minimized. This coherent state has a three-component complex order parameter. The condensation energy of these dipoles in the bcc phase further stabilizes it over the hcp phase at finite temperatures. This condensation of the dipoles is not a 'super-solid'. We further show that there can be fermionic excitations of this ground-state and predict that they form an optic-like branch in the (110) direction.