Discrete time crystals (DTCs) are nonequilibrium phases of matter that emerge due to the breaking of time translational symmetry of periodically driven many-body systems. A nonergodic, rigid subharmonic response of a many-body system is a typical signature of the existence of a time crystal. Here, we report experimental observation of multiple alternative DTC phases, including a subharmonic response, in a classical nanoelectromechanical system (NEMS) based on coupled graphene and silicon nitride membranes. Unlike prethermal-time-crystalline phases of a closed system, stability of the phases is ensured by the presence of damping and nonlinearity in the system, and the phases are stable in the true sense. Furthermore, we employ controlled mechanical strain to drive the transitions between different phases and show a phase diagram. Finally, we model the system theoretically using a mean-field description, which provides crucial insights into the dynamical nature of these phases. However, its failure at the time-crystalline phase boundary confirms the many-body nature of the emergent DTC phases.

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• Nonequilibrium & irreversible thermodynamics
• Time crystals
• Collective dynamics
• Coupled oscillators
• Floquet systems
• Micromechanical & nanomechanical oscillators
• Nanomechanical devices
• Stochastic dynamical systems