Direct Observation of the Gardner/Marginal Glass Transition within a Colloidal Glass*

It has recently been shown that in the infinite dimensional limit there exists a second phase transition, called the Gardner transition, within the glassy phase, dividing so-called stable glass and marginal glass. We have developed a technique to experimentally probe this phase of matter using a colloidal glass. We avoid the difficulties inherent in measuring the long time behavior of glasses by instead focusing on the very short time dynamics of the ballistic to caged transition. We track a single tracer particle within a sedimenting mixture and measure the resulting mean squared displacement (MSD). By analyzing the MSD we find two distinct transitions at different packing fractions: 1) the transition into the stable glass phase is marked by the appearance of a plateau whose magnitude shrinks with increasing density and 2) the transition into the marginal glass phase is marked by logarithmic growth towards that plateau. This provides the first experimental evidence for the existence of a marginal glass in 3d.