Breakdown of Poisson’s effect in Nb/Cu superlattices

The individual lattice spacings in Nb/Cu superlattices are determined from a crystallographic study using transmission and reflection x-ray diffraction. The Cu layers show a small in-plane expansion which increases to 0.6±{}0.1% as the modulation wavelength is reduced to 22 {}. Arguments based solely on Poisson's effect predict that the strains perpendicular to the layers must be smaller and of opposite sign; contrary to this expectation a large perpendicular expansion (4.2±{}0.7%) is measured. This finding brings into question the validity of models that rely only on interfacial forces to derive the perpendicular strains by using the bulk Poisson's ratio.