Two superstructures of adsorbed sulfur coexisting on the stepped Cu(11 1 1) surface are identified by scanning tunneling microscopy. A p(2×2) and a c(4×2) structures are atomically resolved and their interaction with the steps investigated. The direction taken by a step edge is found to be strongly correlated with the sulfur arrangement on the adjacent terraces. The origin of this two-dimensional faceting phenomenon is discussed.

• Received 25 April 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1265