Probing supersolidity through excitations in a spin-orbit–coupled Bose-Einstein condensate.

Spin-orbit–coupled Bose-Einstein condensates are a flexible experimental platform to engineer synthetic quantum many-body systems. In particular, they host the so-called stripe phase, an instance of a supersolid state of matter. The peculiar excitation spectrum of the stripe phase, a definite footprint of its supersolidity, has been difficult to measure experimentally. In this work, we performed in situ imaging of the stripes and directly observed both superfluid and crystal excitations. We investigated superfluid hydrodynamics and revealed a stripe compression mode, thus demonstrating that the system possesses a compressible crystalline structure. Through the frequency softening of this mode, we located the supersolid transition point. Our results establish spin-orbit–coupled supersolids as ideal systems to investigate supersolidity and its rich dynamics. Editor's summary: Supersolidity is a peculiar state of matter that combines the properties of a solid with those of a (super)fluid. Initial evidence for supersolidity in helium was later found to have been misinterpreted, but more recently, robust findings have been reported in cold atomic gases. Chisholm et al. used a gas of bosonic potassium atoms with induced spin-orbit coupling to directly observe the formation of a phase with striped density modulations. The stripes reflected the breaking of translational symmetry characteristic of a solid. The researchers also showed that its crystal structure was compressible, strengthening the evidence for supersolidity. —Jelena Stajic [ABSTRACT FROM AUTHOR]