Atomistic analysis of the enhanced-modulus effect in metallic superlattices

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Abstract
Experimental reports of large enhancements (severalfold) in certain elastic moduli of compositionally modulated transition-metal–noble-metal superlattices have appeared over the last decade. Two general classes of explanations have been proposed for this effect; one depends upon changes in the electronic structure resulting from the superlattice periodicity, whereas the other is based on nonlinear elastic effects driven by the coherency strain of the strained-layer superlattice structures. An atomistic analysis of the elastic properties of a Cu/Ni superlattice has been carried out to differentiate between these ideas. In the case of Cu/Ni superlattices with (100) interfaces and two, four, or six monolayers in each layer, we find roughly a 15% enhancement in the biaxial stretching modulus, compared to the reported severalfold enhancement. The observed enhancement is therefore not due to nonlinear elastic effects driven by coherency strain or interfacial energetics, but probably results from electronic effects associated with the superlattice periodicity.