  We present experimental and theoretical studies of Pd/Cu(100) and Cu/Pd(100)
heterostructures in order to explore their structure and misfit strain
relaxation. Ultrathin Pd and Cu films are grown by pulsed laser deposition at
room temperature. For Pd/Cu, compressive strain is released by networks of
misfit dislocations running in the [100] and [010] directions, which appear
after a few monolayers already. In striking contrast, for Cu/Pd the tensile
overlayer remains coherent up to about 9 ML, after which multilayer growth
occurs. The strong asymmetry between tensile and compressive cases is in
contradiction with continuum elasticity theory, and is also evident in the
structural parameters of the strained films. Molecular Dynamics calculations
based on classical many-body potentials confirm the pronounced
tensile-compressive asymmetry and are in good agreement with the experimental
data.
