Experimental signals of the Gardner transition in a quasi-thermal pressure-controlled granular glass

The Gardner transition in glasses is predicted to occur when metabasins in the energy landscape break up into a hierarchy of sub-basins, leading to the emergence of marginal stability. We experimentally search for signals of the Gardner transition in a quasi-thermal granular glass consisting of a monolayer of star-shaped granular particles under sub-levitating air fluidization. The pressure of the system is controlled by adjusting the tension exerted on an enclosing flexible boundary. Velocity distributions of the internal particles and the scaling of the pressure, density, effective-temperature, and relaxation time demonstrate that the system has important features of a thermal system. A pressure-based quenching protocol was developed to bring the system into deeper glassy states, and signals of the Gardner transition are detected via cage size and separation order parameters for both particle positions and orientations. This gives strong experimental evidence of the Gardner transition for a quasi-thermal system at low dimensions.