Strain Driven fcc-bct Phase Transition of Pseudomorphic Cu Films on Pd(100)

The pseudomorphic growth of Cu on Pd(100) has been studied by tunneling microscopy. Cu films are found to grow layer by layer with a sharp structural transition from a pseudomorphic face-centered-cubic phase (fcc) to a pseudomorphic body-centered-tetragonal (bct, $a=2.75\mathrm{}$ Å, $c=3.10\mathrm{}$ Å) phase with increasing film thickness. The phase transition is driven by the total strain energy of the Cu film, which scales with the temperature dependent Cu-Pd misfit and the film thickness.