Bcc $^4$He as a Coherent Quantum Solid

TL;DR

This paper explores the quantum properties of bcc $^{4}$He, revealing it as a coherent quantum solid with unique dipolar order, fermionic excitations, and implications for its stability and phase behavior.

Contribution

It introduces the concept of a coherent dipolar quantum solid in bcc $^{4}$He, highlighting its unique order parameter and fermionic excitations, which are novel insights compared to prior models.

Findings

bcc $^{4}$He exhibits off-diagonal long-range order of electric dipoles.

The dipolar ground state stabilizes the bcc phase over hcp at finite temperatures.

Fermionic excitations form an optical-like branch in the (110) direction.

Abstract

In this work we investigate implications of the quantum nature of bcc $^{4}$% He. We show that it is a unique solid phase with both a lattice structure and an Off-Diagonal Long Range Order of coherently oscillating local electric dipole moments. These dipoles arise from the local motion of the atoms in the crystal potential well, and oscillate in synchrony to reduce the dipolar interaction energy. The dipolar ground-state is therefore found to be a coherent state with a well defined global phase and a three-component complex order parameter. The condensation energy of the dipoles in the bcc phase stabilizes it over the hcp phase at finite temperatures. We further show that there can be fermionic excitations of this ground-state and predict that they form an optical-like branch in the (110) direction. A comparison with 'super-solid' models is also discussed.