  Structural defects are ubiquitous in condensed matter, and not always a
nuisance. For example, they underlie phenomena such as Anderson localization
and hyperuniformity, and they are now being exploited to engineer novel
materials. Here, we show experimentally that the density of structural defects
in a 2D binary colloidal crystal can be engineered with a random potential. We
generate the random potential using an optical speckle pattern, whose induced
forces act strongly on one species of particles (strong particles) and weakly
on the other (weak particles). Thus, the strong particles are more attracted to
the randomly distributed local minima of the optical potential, leaving a trail
of defects in the crystalline structure of the colloidal crystal. While, as
expected, the crystalline ordering initially decreases with increasing fraction
of strong particles, the crystalline order is surprisingly recovered for
sufficiently large fractions. We confirm our experimental results with
particle-based simulations, which permit us to elucidate how this non-monotonic
behavior results from the competition between the particle-potential and
particle-particle interactions.
