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There exists a large diversity of superconductors following different mechanisms to achieve the superconducting phase. Low-temperature superconductivity appears in metals and degenerate semiconductors; it is induced by the formation of electron pairs in a bipolaron state referred to as Cooper pair. The superconductive state is separated from the normal-conductivity state by an energy gap below the Fermi energy. This gap appears at the critical temperature and widens as the temperature decreases. In type I low-temperature superconductors an external magnetic field is expelled from the bulk up to an upper value, which eliminates superconductivity. In type II superconductors an array of flux lines penetrates into the bulk above a lower critical field, creating a mixed normal and superconductive phase up to the upper critical field.
High-temperature superconductivity of type II is observed mostly in layered compounds such as cuprates and iron pnictides, with critical temperatures exceeding 100 K. Superconductivity in these materials is usually carried by hole pairs and requires sufficient doping. The mechanism of pair formation differs from that in metals and involves an interaction with spin fluctuations. The symmetry of the layered superconductive system and of the superconductive gap is lower than in the basically isotropic metals; in cuprates pairs with a lateral d symmetry are found.
KeywordsAnderson RVB model BCS theory Ceramic superconductor Cooper pair Critical temperature Cuprates Flux-line lattice High-Tc superconductor Isotope effect Jospehson tunneling London penetration depth Magnetic ordering Meissner phase Meissner-Ochsenfeld effect Organic superconductor Pnictides Superconduction energy-gap SQUID Two-fluid model Type I and II superconductors Vortices
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