Elastic Anomalies in Superlattices

The presence and origin of elastic anomalies in superlattices, interfaces, composites, and nanocrystalline materials has been a subject of much interest and controversy in recent years. In particular, superlattices are being used as model systems to study the effect of interfaces on the mechanical properties of novel materials. Early claims of anomalously large enhancements of the biaxial and flexural moduli of Au/Ni and Cu/Ni superlattices (“supermodulus effect”) created considerable controversy and contradictory reports in the experimental and theoretical literature. To understand the mechanical properties of superlattices and their implication for other types of nanofabricated materials, it is important to look critically at the field.Superlattices have been fabricated in different laboratories by a variety of preparation methods and have been characterized structurally and elastically to various degrees. Because of this, before addressing any sophisticated theoretical issues regarding elastic anomalies, it is important to understand in detail the experimental techniques and the possible pitfalls in the experimental determination of mechanical properties of thin films. Once the presence or absence of the effect is ascertained, the need arises to understand any possible experimental correlations with other structural and/or physical properties. These correlations can be used to address issues regarding the possible origins of the effect and their theoretical basis. It will be apparent from this article that slight changes in structural properties are correlated with relatively large elastic anomalies. Consequently, this points to the importance of precise, comprehensive, and detailed structural characterization. With our present understanding of the physics of superlattices, qualitative studies are no longer sufficient!