Artificially Layered Superconductors
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ETIN/FEBRUARY1990
properties, i.e., the cohérence length and pénétration depth, are quite long in conventional low température superconductors. Existing techniques allow the préparation of layered structures which are perfect at length scales which détermine the physical properties. Consequently it is possible to prépare model Systems that approach idéal conditions and therefore become an excellent test ground for theoretical models. As in ail fields of materials science, unexpected interesting phenomena are also discovered which cannot be predicted a priori. This article will briefly describe préparation and characterization techniques for Artificially Layered Superconductors, the bulk of the article dedicated to describing the superconducting properties including critical température, critical current, pénétration depth, and critical field.
Préparation and Characterization With the récent developments in ultrahigh vacuum technology it is possible to prépare highly controlled thin films, and to probe the effects due to a very regular and contamination-free interface. The type of material, growth rates, substrate température and contaminant partial pressures are some of the parameters that affect the quality and properties of the multilayers. Currently two major techniques are used to produce multilayered Systems — sputtering and thermal evaporation. General reviews and références are available in the literature for each of thèse techniques.10"12 Because the détails of thèse techniques are generally known we only présent a comparison and show some of the advantages and disadvantages of each. Sputtering Systems hâve evolved to advanced variants of triode-supported
plasma, magnetron, and ion-beam sources which are characterized by high déposition rates (20-100 Â/s). A substantial problem related to sputtering is the interaction between the sample and the plasma, due to secondary électron impact, inert gas ion and atom bombardment as well as vacuum ultraviolet irradiation. Thèse interactions give rise to substrate heating and interface roughness because of intermixing and non-ideal growth modes. An interesting and useful feature of sputtering is the thermalization of atoms which enables control of the energy distribution of impinging atoms as well as easy control of the déposition rate. More recently high-quality multilayers hâve also been fabricated by Ultrahigh Vacuum (UHV) Molecular Beam Epitaxy (MBE) techniques using Knudsen cell and électron beam evaporation. The Knudsen cell allows a low, constant evaporation rate (0.1-10 jurt/h) enabling accurate control of the composition profile for materials with a melting température lower than 1600°C. Electron beam evaporation allows handling of high melting température materials, faster growth rates and avoidance of contamination by the crucible, so that a very pure material flux may be obtained. However, a major problem with électron beam evaporation is the control of the déposition rate due to the large température gradients in the crucible (up to 2000°C/cm). This requ
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