Superconductivity Totally Explained

Everything about Superconductors totally explained

Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect).
   The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. However, in ordinary conductors such as copper and silver, impurities and other defects impose a lower limit. Even near absolute zero a real sample of copper shows a non-zero resistance. The resistance of a superconductor, on the other hand, drops abruptly to zero when the material is cooled below its "critical temperature". An electric current flowing in a loop of superconducting wire can persist indefinitely with no power source. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It can't be understood simply as the idealization of "perfect conductivity" in classical physics.
   Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors. Superconductivity doesn't occur in noble metals like gold and silver, nor in most ferromagnetic metals.
   In 1986 the discovery of a family of cuprate-perovskite ceramic materials known as high-temperature superconductors, with critical temperatures in excess of 90 kelvin, spurred renewed interest and research in superconductivity for several reasons. As a topic of pure research, these materials represented a new phenomenon not explained by the current theory. And, because the superconducting state persists up to more manageable temperatures, past the economically-important boiling point of liquid nitrogen (77 kelvin), more commercial applications are feasible, especially if materials with even higher critical temperatures could be discovered.

Elementary properties of superconductors

Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature at which superconductivity is destroyed. On the other hand, there's a class of properties that are independent of the underlying material. For instance, all superconductors have exactly zero resistivity to low applied currents when there's no magnetic field present. The existence of these "universal" properties implies that superconductivity is a thermodynamic phase, and thus possess certain distinguishing properties which are largely independent of microscopic details.

Zero electrical "dc" resistance

The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source I and measure the resulting voltage V across the sample. The resistance of the sample is given by Ohm's law as

R = frac

, and thus (coupled with the quantum Hall resistivity) for Planck's constant h. Josephson was awarded the Nobel Prize for this work in 1973.
Until 1986, physicists had believed that BCS theory forbade superconductivity at temperatures above about 30 K. In that year, Bednorz and Müller discovered superconductivity in a lanthanum-based cuprate perovskite material, which had a transition temperature of 35 K (Nobel Prize in Physics, 1987). It was shortly found by M.K. Wu et al. that replacing the lanthanum with yttrium, for example making YBCO, raised the critical temperature to 92 K, which was important because liquid nitrogen could then be used as a refrigerant (at atmospheric pressure, the boiling point of nitrogen is 77 K.) This is important commercially because liquid nitrogen can be produced cheaply on-site with no raw materials, and isn't prone to some of the problems (solid air plugs, et cetera) of helium in piping. Many other cuprate superconductors have since been discovered, and the theory of superconductivity in these materials is one of the major outstanding challenges of theoretical condensed matter physics. As of October 2007, the highest temperature superconductor is a ceramic material consisting of thallium, mercury, copper, barium, calcium, and oxygen, with T_c=138 K.
In February, 2008, another different family of high temperature superconductors was discovered. Hideo Hosono of the Tokyo Institute of Technology and colleagues found that lanthanum oxygen fluorine iron arsenide (LaO_1-xF_xFeAs) becomes a superconductor at 26 kelvin. Other researchers quickly found other materials in the same family that have transition temperatures as high as 55K. Experts hope that having another family to study will simplify the task of explaining how these materials work.

Applications

Superconducting magnets are some of the most powerful electromagnets known. They are used in maglev trains, MRI and NMR machines and the beam-steering magnets used in particle accelerators. They can also be used for magnetic separation, where weakly magnetic particles are extracted from a background of less or non-magnetic particles, as in the pigment industries.
   Superconductors have also been used to make digital circuits (for example based on the Rapid Single Flux Quantum technology) and RF and microwave filters for mobile phone base stations.
   Superconductors are used to build Josephson junctions which are the building blocks of SQUIDs (superconducting quantum interference devices), the most sensitive magnetometers known. Series of Josephson devices are used to define the SI volt. Depending on the particular mode of operation, a Josephson junction can be used as photon detector or as mixer. The large resistance change at the transition from the normal- to the superconducting state is used to build thermometers in cryogenic micro-calorimeter photon detectors.
   Other early markets are arising where the relative efficiency, size and weight advantages of devices based on HTS outweigh the additional costs involved.
   Promising future applications include high-performance transformers, power storage devices, electric power transmission, electric motors (for example for vehicle propulsion, as in vactrains or maglev trains), magnetic levitation devices, and Fault Current Limiters. However superconductivity is sensitive to moving magnetic fields so applications that use alternating current (for example transformers) will be more difficult to develop than those that rely upon direct current.

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