  At the triple point of a repulsive screened Coulomb system, a
face-centered-cubic (fcc) crystal, a body-centered-cubic (bcc) crystal and a
fluid phase coexist. At their intersection, these three phases form a liquid
groove, the triple junction. Using confocal microscopy, we resolve the triple
junction on a single particle level in a model system of charged PMMA colloids
in a nonpolar solvent. The groove is found to be extremely deep and the
incommensurate solid-solid interface to be very broad. Thermal fluctuations
hence appear to dominate the solid-solid interface. This indicates a very low
interfacial energy. The fcc-bcc interfacial energy is quantitatively determined
based on Young's equation and, indeed, it is only about 1.3 times higher than
the fcc-fluid interfacial energy close to the triple point.
