Direct experimental evidence for the Gardner transition in a colloidal glass*

A major breakthrough in relating glass formation and jamming has been the identification of the Gardner transition. This phase transition occurs as glassy systems are compressed toward a jammed state and corresponds to a marginal phase where single energy basins split into a complex hierarchy of metastable subbasins. While the Gardner transition has been confirmed analytically for mean-field systems in infinite dimensions, evidence from simulations and experiments in three dimensions is limited. Here, we present experiments aimed at directly observing the predicted metastable states of the Gardner phase in a colloidal glass. We use a compression cell mounted on a confocal microscope to repeatedly compress a binary colloidal glass and induce shifts between metastable states. By precisely locating and tracking particles, we identify particle subcages for different metastable states of the same glass. These results will concretely establish whether the Gardner transition can be seen in a three-dimensional glass and repeated experiments determine the volume fraction at which the Gardner transition takes place. Our experiments help to lay the groundwork for further investigations into the rich phenomenology of the metastable glass.