  The replica theory of glasses predicts that in the infinite dimensional mean
field limit there exist two distinct glassy phases of matter: stable glass and
marginal glass. We have developed a technique to experimentally probe these
phases 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 slowly densifying glass and measure the resulting mean
squared displacement (MSD). By analyzing the MSD we find that upon
densification our colloidal system moves through several states of matter. At
lowest densities it is a sub-diffusive liquid. Next it behaves as a stable
glass, marked by the appearance of a plateau in the MSD whose magnitude shrinks
with increasing density. However, this shrinking plateau does not shrink to
zero, instead at higher densities the system behaves as a marginal glass,
marked by logarithmic growth in the MSD towards that previous plateau value.
Finally, at the highest experimental densities the system returns to the stable
glass phase. This provides direct experimental evidence for the existence of a
marginal glass in 3d.
