Nonpseudomorphic and surface-reconstructed ultrathin epitaxial fcc Fe films on Cu(100)

A high-precision low-energy electron-diffraction (LEED) structure analysis for an eight-monolayer ultrathin fcc Fe film epitaxially grown on Cu(100) is presented. The film is free of strain, i.e., its lattice coincides with that of equilibrium fcc iron rather than with that given by the Cu substrate. The top layer is reconstructed and its distance to the second layer is expanded. Similar features hold also for films in the range 5--11 monolayers. The theory-experiment fit is the best ever achieved in a LEED structure determination with a Pendry R factor of 0.09.