Observation of two broken symmetries in a supersolid


The paradoxical supersolid phase of matter has the apparently incompatible properties of crystalline order and superfluidity. A crucial feature of a one-dimensional supersolid is the occurrence of two 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. We demonstrate the supersolid nature of the coherent stripe regime we discovered in dipolar Bose-Einstein condensates. In our trapped system, the symmetry breaking appears as two distinct compressional oscillation modes, 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 crystal-like configurations that can be accessed by tuning a single interaction parameter.

L. Tanzi, et al.
Supersolid symmetry breaking from compressional oscillations in a dipolar quantum gas
Nature 574, 382 (2019)

See also the Nature News and Views by S. M. Mossman:

S. M. Mossman, Sounds of a supersolid detected in dipolar atomic gases for the first time
Nature 574, 341 (2019)

and the Nature Physics research highligh by Y. Li:

Y. Li, The buried trace
Nature Physics 15, 986 (2019)