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Manufactured, reproducibly layered structures having layer thicknesses approaching interatomic distances. Modern thin-film techniques are at a stage at which it is possible to fabricate these structures, also known as artificial crystals or superlattices, opening up the possibility of engineering new desirable properties into materials. In addition, a variety of problems in solid-state physics can be studied which are otherwise inaccessible. The various possibilities include the application of negative pressure, that is, stretching of the crystalline lattice; the study of dimensional crossover, that is, the transition from a situation in which the layers are isolated and two-dimensional in character to a situation in which the layers couple together to form a threedimensional material; the study of collective behavior, that is, properties that depend on the cooperative behavior of the whole superlattice; and the effect and physics of multiple interfaces and surfaces. These structures serve as model systems and as a testing ground for theoretical models and for other naturally occurring materials that have similar structures. For instance, ceramic superconductors consist of a variable number of conducting copper oxide (CuO2) layers intercalated by various other oxide layers, and therefore artificially layered structures may be used to study predictions of the behavior of suitably manufactured materials of this class. A variety of applications have also been proposed or discovered. Of course, one of the most exciting prospects is the discovery of new, as yet unpredicted phenomena. For a discussion of semiconductor superlattices See also: Crystal structure; Semiconductor heterostructures