Supersolid symmetry breaking from compressional oscillations in a dipolar quantum gas

TL;DR

This paper experimentally demonstrates supersolid symmetry breaking in a dipolar quantum gas by observing two distinct compressional oscillation modes, confirming the presence of Goldstone excitations linked to simultaneous breaking of phase and translational symmetries.

Contribution

It provides the first experimental observation of Goldstone modes in a supersolid, revealing two distinct oscillation modes associated with symmetry breaking in a dipolar Bose-Einstein condensate.

Findings

Observation of two gapless compressional modes in a dipolar BEC

Different natures of the modes linked to lattice periodicity and superfluidity

Potential to explore phase transitions between superfluid, supersolid, and solid states

Abstract

The existence of a paradoxical supersolid phase of matter, possessing the apparently incompatible properties of crystalline order and superfluidity, was predicted 50 years ago. Solid helium was the natural candidate, but there supersolidity has not been observed yet, despite numerous attempts. Ultracold quantum gases have recently shown the appearance of the periodic order typical of a crystal, due to various types of controllable interactions. A crucial feature of a D-dimensional supersolid is the occurrence of up to D+1 gapless excitations reflecting the Goldstone modes associated with the spontaneous breaking of two continuous symmetries: the breaking of phase invariance, corresponding to the locking of the phase of the atomic wave functions at the origin of superfluid phenomena, and the breaking of translational invariance due to the lattice structure of the system. The occurrence of such modes has been the object of intense theoretical investigations, but their experimental observation is still missing. Here we demonstrate the supersolid symmetry breaking through the appearance of two distinct compressional oscillation modes in a harmonically trapped dipolar Bose-Einstein condensate, reflecting the gapless Goldstone excitations of the homogeneous system. We observe that the two modes have different natures, with the higher frequency mode associated with an oscillation of the periodicity of the emergent lattice and the lower one characterizing the superfluid oscillations. Our work paves the way to explore the two quantum phase transitions between the superfluid, supersolid and solid-like configurations that can be accessed by tuning a single interaction parameter.