Soft Chemical Routes to Heterostructured High-T c Superconducting Materials
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Jin-Ho Choy, Soon-Jae Kwon, Seong-Ju Hwang, and Eue-Soon Jang Introduction Recently, inorganic/inorganic and organic/inorganic heterostructured materials have attracted considerable research interest, due to their unusual physicochemical properties, which cannot be achieved by conventional solid-state reactions. In order to develop new hybrid materials, various synthetic approaches, such as vacuum deposition,1 Langmuir–Blodgett films,2 selfassembly,3 and intercalation techniques, have been explored.4–11 Among them, the intercalation reaction technique—that is, the reversible insertion of guest species into the two-dimensional host lattice—is expected to be one of the most effective tools for preparing new layered heterostructures because this process can provide a soft chemical way of hybridizing inorganic/ inorganic,5,7–10 organic/inorganic,4,6,11 or biological/inorganic compounds.12 In fact, the intercalation/deintercalation process allows us to design high-performance materials in a solution at ambient temperature and pressure, just as “soft solution processing” provides a simple and economical route for advanced inorganic materials by means of an environmentally benign, lowenergy method.13–15 These unique advantages of the intercalation technique have led to its wide application to diverse fields of the solid-state sciences, namely, secondary (rechargeable) batteries, electrochromic systems, oxidation–reduction catalysts, separating agents, sorbents, and so on. Through these extensive studies, many kinds of low-dimensional compounds have
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been developed as host materials for the intercalation reaction, including graphite, transition-metal chalcogenides, transitionmetal oxides, aluminosilicates, metal phosphates, metal chalcogenohalides, and so on.16–18 Recently, the area of intercalation chemistry has been extended to high-Tc superconducting copper oxides, resulting in remarkable structural anisotropy. Since the discovery of high-temperature superconductivity in La-Ba-Cu-O systems, many different types of high-Tc superconductors have been synthesized.19 In view of their crystal structure, most of them have a unique layered structure where the superconductive CuO2 blocks are regularly interstratified with charge-reserving rocksalt or fluorite blocks. This common structural feature of high-Tc superconductors raises the question of how the interaction between two structural components influences superconductivity. Actually, it is rationalized that the superconductivity can be affected by the interlayer Josephson coupling, that is, by the interlayer distance of a CuO2 layer. In addition to the effect of Josephson coupling, several theoretical models on the interlayer interaction have been suggested, such as the WHA model (proposed by Wheatley, Hsu, and Anderson), which is based on a combination of the nearest CuO2 layer coupling and the next-nearest CuO2 one;20 and the IY model (proposed by Ihm and Yu), in which only the adjacent CuO2 layers within a basic block can interact with each other.21 How-
ever, the v
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