Atomic Layer-by-Layer Engineering of High T c Materials and Heterostructure Devices
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gen that leads to relatively broad bands and delocalized states. Depending on the charge transfer that occurs, these bands can be partially or completely occupied. In other crystals the metal atom and oxygen states do not hybridize much at all; the bonds are predominantly ionic and the bands narrow. Sometimes both types of bonding occur at different sites within the same crystal. Furthermore, the symmetry of the crystalline order and the population of states in metal atoms determine local magnetic moments that underlie magnetic order. With such a wide variety of physical states, oxides provide a flexible system for studying interactions and transport at heterointerfaces. However, many of the phenomena of interest have very small characteristic length scales, called coherence lengths. Thus, heterojunctions made to study such phenomena must be fabricated, studied, and understood on an atomistic level, and the interfaces need to be nearly crystallographically perfect. In general, lattice mismatch between different materials makes this difficult. Fortunately, many oxides belong to broad structural classes that have similar atomic spacing. In such classes the lattice constant is predominantly determined by the size of the oxygen ion, with a smaller influence due to the particular metal atoms involved. The lattice spacing can be thought of as being governed by large touching oxygen anions, with the smaller metal cations fitting in the space left over. The circumstance that many different oxides exist with dramatically different physical properties, but all with similar atomic spacing, provides op-
portunities to investigate a wide spectrum of interface phenomena. A class of oxides exhibiting wide-ranging properties but having a limited range of lattice spacings is the perovskites.1 Simple perovskites have a chemical formula ABO3 and form a cubic (or nearly cubic) structure with the A ion in a body-centered position, the B ion at the corners, and the O ions on the edges. Typically, the states near the Fermi energy are derived from the atomic B and O states, while the states derived from the A atom are far apart. In particular, the A atoms can be thought of as electron donors to the BO, states. Often the A atoms are alkali metal, alkaline earth, or rare earth atoms, while the B atoms are transition metals with rich d-state properties. Besides the simple perovskites, there are related compounds that are "partially perovskite." Such materials, like some of the high critical temperature superconductors, e.g. Bi2Sr2CaCu2O8 + r, have complex unit cell structures in which some atoms form an oxygen deficient perovskite-like structure while others appear to be in a rocksalt structure. (This compound is often referred to as BSCCO-2212, where the letters denote the metal atoms and the numbers their stoichiometry.) Since the family of perovskites and related compounds contains materials with such diverse physical properties, it is a good choice to use for assembling heterostructures and studying and exploiting interfacial phenom
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