Locked-floating-solid to locked-smectic transition in colloidal systems.

We investigate two-dimensional melting of a colloidal system in the presence of a one-dimensional periodic substrate potential created by two interfering laser beams. We study the commensurability ratio p=sqrt[3]a/2d=2 with a the mean particle distance and d the period of the periodic potential. In contrast with the previously investigated case p=1, here we observe that melting of the locked-floating solid occurs via a novel locked-smectic phase, predicted by recent theoretical studies.