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High-temperature superconductivity

High-temperature superconductors (abbreviated high-Tc or HTS) are materials that behave as superconductors at unusually[1] high temperatures. The first high-Tc superconductor was discovered in 1986 by IBM researchers Karl Muller and Johannes Bednorz,[2][3] who were awarded the 1987 Nobel Prize in Physics "for their important break-through in the discovery of superconductivity in ceramic materials".[4] Whereas "ordinary" or metallic superconductors usually have transition temperatures (temperatures below which they superconduct) below 30 K (?243.2 C), HTS superconductors have been observed with transition temperatures as high as 138 K (?135 C).[2] Until 2008, only certain compounds of copper and oxygen (so-called "cuprates") were believed to have HTS properties, and the term high-temperature superconductor was used interchangeably with cuprate superconductor for compounds such as bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). However, several iron-based compounds (the iron pnictides) are now known to be superconducting at high temperatures.The phenomenon of superconductivity was discovered by Kamerlingh Onnes in 1911, in metallic mercury below 4 K (?269.15 C). For seventy-five years after that, researchers attempted to observe superconductivity at higher and higher temperatures.[8] In the late 1970s, superconductivity was observed in certain metal oxides at temperatures as high as 13 K (?260.2 C), which were much higher than those for elemental metals. In 1987, K Alex Mueller and J. Georg Bednorz, working at the IBM research lab near Zurich, Switzerland were exploring a new class of ceramics for superconductivity. Bednorz encountered a compound of Lithium, Barium and Copper oxide whose resistance dropped down to zero at a temperature around 35 K (?238.2 C).[8] Their results were soon confirmed by two groups, Paul Chu at the University of Houston and Shoji Tanaka at the University of Tokyo.[9] Shortly after, P. W. Anderson, at Princeton University came up with the first theoretical description of these materials, using the resonating valence bond theory.[9] As of 20 8, the superconductor with the highest known transition temperature is mercury thallium barium calcium copper oxide (Hg12Tl3Ba30Ca30Cu45O125) at 138 K.[10] After more than twenty years of intensive research the origin of high-temperature superconductivity is still not clear, but it seems that instead of electron-phonon attraction mechanisms, as in conventional superconductivity, one is dealing with genuine electronic mechanisms (e.g. by antiferromagnetic correlations), and instead of s-wave pairing, d-waves are substantial. One goal of all this research is room-temperature superconductivity. "High-temperature" has two common definitions in the context of superconductivity: Above the temperature of 30 K that had historically been taken as the upper limit allowed by BCS theory.[citation needed] This is also above the 1973 record of 23 K that had lasted until copper-oxide materials were discovered in 1986. Having a transition temperature that is a larger fraction of the Fermi temperature than for conventional superconductors such as elemental mercury or lead. This definition encompasses a wider variety of unconventional superconductors and is used in the context of theoretical models. The label high-Tc may be reserved by some authors for those with critical temperature greater than the boiling point of liquid nitrogen (77 K or ?196 C). However, a number of materials including the original discovery and recently discovered pnictide superconductors had critical temperatures below 77 K but are commonly referred to in publication as being in the high-Tc class.[12][13] Technological applications benefit from both the higher critical temperature being above the boiling point of liquid nitrogen and also the higher critical magnetic field (and critical current density) at which superconductivity is destroyed. In magnet applications the high critical magnetic field may be more valuable than the high Tc itself. Some cuprates have an upper critical field of about 100 tesla. However, cuprate materials are brittle ceramics which are expensive to manufacture and not easily turned into wires or other useful shapes.

 
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