Control of Composition and Microstructure in High-Temperature Superconductors at the Atomic Level by Molecular Beam Epitaxy

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Abstract
The idea of engineering the detailed microstructure of materials by controlling the chemical synthesis process at an atomic level is particularly interesting for high-temperature superconductors (HTS) and many other oxide compounds. The prospect of integrating such materials as superconductors, ferroelectrics, and compounds with interesting magnetic properties into samples with atomic precision and perfect heterojunctions would seem to open the door to many novel possibilities. These materials, while complex, fundamentally have planar crystalline structure, and thus allow for the possibility of molecular and even atomic layer engineering. The “zoology” of phases that has been discovered in the study of naturally occurring HTS compounds is by now quite complex and illustrates this theme. While CuO2 planes are universally important for superconductivity, there is much more flexibility and variety in the other kinds of layers that act to complete the crystal structure. Examples of such “other” layers include alkaline earth oxides like SrO and BaO, copper oxide “chain” layers and rock-salt-like (BiO)2 bilayers, to name a few important ones. Moreover, families of very similar polytype phases have also been identified, in which members of each family differ principally in the number of copper oxygen planes within each formula unit.This great richness of structure sets a pattern that invites the imagination to devise new materials and heterostructures and has motivated the development of synthesis techniques that control growth at the level of the submolecular—atomic layers. For instance, with adequate control of the growth process, highly metastable phases, such as Bi2Sr2Ca7Cu8O20, can be synthesized and their properties measured.