Strain Relief at Metal Interfaces with Square Symmetry

We report a novel mechanism, internal (111) faceting, of strain relief at heterointerfaces with square symmetry. The mechanism has been revealed for thin Cu films on Ni(100) by scanning tunneling microscopy. In the first monolayer monatomic chains of Cu atoms are shifting laterally by $1/\sqrt{8}$ lattice constant along $〈110〉$ and thereby protrude from the surface layer. With each Cu layer added, the protrusion stripes grow in width by one atom, forming internal $\{111\}$ facets in the Cu film. This picture is in marked contrast to the widely accepted continuum theory of epitaxial growth, which predicts a pseudomorphic film growth up to a critical thickness of 8 monolayers.