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Msc4 High-Temp Superconductivity

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Msc4 High-Temp Superconductivity Solid State Physics-II (402S) High-temperature Superconductivity Dr. Asutosh Kumar P.G. Department of Physics, Gaya College Magadh University, Bodhgaya Introduction to High-temperature Superconductors The discovery of superconductivity at about 30 K in the Sr-doped La2CuO4 by Bednorz and Muller in 1986 ignited an explosion of interest in high temperature superconductivity. A class of rare earth copper oxides with various kinds of dopants exhibit high temperature superconductivity. Highest critical temperature, Tc is found at 164 K in the compound HgBa2Ca2Cu3O8 when subjected at pressure 30 GPa. The majority of high-temperature superconductors are ceramics materials. [A ceramic material is an inorganic, non-metallic, often crystalline oxide, nitride or carbide material. Some elements, such as carbon or silicon, may be considered ceramics. Ceramic materials are brittle, hard, strong in compression, weak in shearing and tension, heat and corrosion-resistant.] The major advantage of high-temperature ceramic superconductors is that they can be cooled by using liquid nitrogen (at a temperature 77 K). Unlimited quantities of nitrogen are available in the atmosphere, so that the cost of cooling is greatly reduced, since liquid nitrogen is overwhelmingly cheaper than liquid helium. Important non-ceramic superconducting systems are as follows: (i) The fullerene C60, which was discovered in 1985, is a carbon material in which carbon atoms are arranged in a shape resembling a soccer ball. The crystal structure of fullerene is fcc. Fullerene when doped with a small amount of one or more alkali metals becomes fulleride (A3C60 with A representing an alkali metal) and shows superconductivity with Tc ranging from 8 K in Na2Rb0.5Cs0.5C60 upto 40 K in Cs3C60. (ii) About fifty organic superconductors have been found with Tc’s extending from 0.4 K to near 12 K. One example is (TMTSF)2C1O4 (Tetra-methyl-tetra- selsna-fulvalene + acceptor). (iii) Borocarbides are superconducting system with critical temperature ranging upto 23 K in YPd2B2C. Some borocarbides containing elements of unusual magnetic properties (e.g., Holmium) show re-entrant behaviour, i.e., below Tc there is a discordant temperature where they should remain superconductive but they retreat to a non-superconductive state. (iv) Heavy fermions are a typical superconductors ranging upto 6 K in CeRu2. These compounds contain rare earth element such as Ce or Yb or actinide elements such as U. (v) Ruthenates show superconductivity at very low temperature. In June 1999 a ruthenium cuprate, RuSr2(Gd, Eu, Sm)Cu2O8 was discovered which shows both ferromagnetism and superconductivity at 58 K. (vi) In July 1999 the sodium doped tungsten-bronze Na0.05WO3 was discovered which shows superconductivity at 91 K. This would be the first known high temperature superconductor that is not a cuprate. (vii) The superconductor MgB2 (Magnesium-diBoride) is a simple intermetallic (inorganic) compound. It is a dark gray, water-insoluble solid. Its transition temperature approaches 39 K. High-Tc superconductors (HTS) denotes superconductivity in materials, mainly oxides, with high transition temperatures, accompanied by high critical currents and magnetic fields. The critical temperature ranges from 20-30 K in intermetallic compounds to halfway toward the room temperature (〜300K) in bulk superconducting oxides (see figure below). The superconductivity of mercury was discovered first by Kamerlingh-Onnes in 1911. Further discoveries of superconducting materials followed, and Tc rose slowly. A series of high-Tc cuprate superconductors, containing copper and oxygen, were first discovered in 1986. The metallic superconductor MgB2, with Tc = 39 K, was discovered in 2001. Iron arsenide superconductors, which are other candidates for high-Tc superconductors, were discovered in 2008 with Tc ≈ 56 K. The new superconducting materials passed the standard tests for superconductivity—the Meissner effect, ac Josephson effect, persistent currents of long duration, and substantially zero dc resistivity. Memorable steps in the advance include: BaPb0.75Bi0.25O3 [BPBO] Tc = 12 K La1.85Ba0.15CuO4 [LBCO] Tc = 36 K YBa2Cu3O7 [YBCO] Tc = 90 K Tl2Ba2Ca2Cu3O10 [TBCO] Tc = 120 K Hg0.8Tl0.2Ba2Ca2Cu3O8.33 Tc = 138 K The evolution of the critical temperature Tc of superconductors. .
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