A novel transition has been investigated within the stripe phase of a monomolecular organic (‘‘Langmuir’’) film. This transition mediates the transformation of an isotropic stripe liquid into an anisotropic state exhibiting twofold rotation symmetry. Stripe undulation and thickness fluctuations persist in the ordered state which also displays a high density of quenched topological defects. Stripe ‘‘melting’’ appears to coincide with the occurrence of thermal fluctuations which mediate the spontaneous breaking and rejoining of stripes, thereby altering the pattern connectivity.

• Received 15 June 1992

DOI:https://doi.org/10.1103/PhysRevLett.70.1658