Supersolid symmetry breaking from compressional oscillations in a dipolar quantum gas

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 1-3 . Solid helium was the natural candidate, but there supersolidity has not been observed yet, despite numerous attempts 4-7 . Ultracold quantum gases have recently shown the appearance of the periodic order typical of a crystal, due to various types of controllable interactions 8-12 . A crucial feature of a Ddimensional 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 1,13-17 , 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.

Three-body interaction near a narrow two-body zero crossing

“…Two-body interactions of different kinds (contact and dipole-dipole in the dipolar case and interspecies and intraspecies in the mixture case) are tuned to compete with each other such that the resulting weak overall attraction gets compensated by a higher-order Lee-Huang-Yang (LHY) term [8][9][10]. An impressive experimental progress has been made in the dipolar case on pursuing supersolidity through the formation of coherent arrays of quantum droplets [11][12][13][14][15].…”

The low-energy Goldstone mode in a trapped dipolar supersolid

“…Note: During the preparation of this manuscript, we became aware of a related complimentary study of higher lying collective modes, showing a bifurcation upon the transition from a regular BEC to the supersolid droplet array 41 .…”

Saga of Superfluid Solids

“…In experiments, dipolar quantum droplets arranged in periodic arrays have been observed in trapped dipolar Bose-Einstein condensates of 162 Dy, 164 Dy and 166 Er [202][203][204][205]. The existence of periodic droplet structures of quantum gases with superfluid properties is confirmed by the observation of the low-energy Goldstone mode [204,205].…”