Group seminar at MPQ and Zoom: Observation of supersolid-like sound modes in a driven quantum gas

Nikolas Liebster, University of Heidelberg, Germany
Group seminar at MPQ lecture hall and Zoom
Tuesday, 20 May, 09:00 am (MEZ)

Systems far from equilibrium can have radically different properties from the same system at equilibrium. A certain class of out-of-equilibrium systems are those where parameters are periodically driven in time. Though driving generically leads to heating, in certain cases it can produce ordered stationary states, enabling the successful application of mathematical descriptions developed for equilibrium scenarios. Bosonic quantum gases, for instance, have been shown to spontaneously develop self-stabilized, periodic density modulations when the two-particle interaction strength is driven in time. These patterned states share key physical properties to a seemingly different equilibrium physical system, namely supersolids. In this work, we probe the excitation spectrum of such a patterned state in a driven superfluid, finding that its response is identical to that of a one-dimensional supersolid. By imprinting and probing wavepacket dynamics as well as preparing specific collective modes, we identify two distinct sound modes associated with spontaneously broken U(1) and translational symmetries. Consistent with the hydrodynamic description of superfluid smectics, longitudinal excitations propagate with finite velocities, while transverse perturbations exhibit diffusive behavior. These results demonstrate how the conceptual framework of supersolidity can be used to characterize dynamic and far-from-equilibrium states.