Superconductivity is a property displayed by certain materials at very low temperatures. Materials found to be superconductive include metals and their alloys (tin, aluminum, and others), some semiconductors, and certain ceramics known as cuprates which contain copper and oxygen atoms. A superconductor conducts electricity without resistance, a unique property. It also repels magnetic fields perfectly in a phenomenon known as the Meissner effect, losing any internal magnetic field it might have had before being cooled to a critical temperature. Because of this effect, certain superconductors can be made to float endlessly above a strong magnetic field. For most superconducting materials, the critical temperature is below about 30K (30°C above absolute zero). But some materials, called high-temperature superconductors, make the phase transition to superconductivity at much higher critical temperatures, typically higher than 70K and sometimes as high as 138K. These materials are almost always cuprate-perovskite ceramics. They display slightly different properties than other superconductors, and the way they transition to superconductivity has still not been entirely explained. Sometimes they are called Type II superconductors to distinguish them from conventional Type I superconductors. The theory of conventional, low-temperature superconductors, however, is well understood. In a conductor, electrons flow through an ionic lattice of atoms, releasing some of their energy into the lattice and heating up the material. This flow is called electricity. Because the electrons are continuously bumping up against the lattice, some of their energy is lost and the electrical current diminishes in intensity as it travels throughout the conductor. This is what we mean by electric resistance in conduction. In a superconductor, the flowing electrons bind to each other in arrangements called Cooper pairs, which must receive a substantial jolt of energy to be broken apart. Electrons in Cooper pairs exhibit superfluidic properties, flowing endlessly without resistance. The extreme cold of the superconductor means that its members atoms aren't vibrating intensely enough to break the Cooper pairs apart. Consequently, the Cooper pairs remain indefinitely bonded to each other as long as the temperature stays below the critical value. Electrons in Cooper pairs attract one another through the exchange of phonons, quantized units of vibration, within the vibrating lattice of the superconducting material. Electrons cannot bond directly to each other in the way that nucleons do because they do not experience the so-called strong force, the "glue" that holds protons and neutrons together in the nucleus. In addition, electrons are all negatively charged and consequently repel one another if they get too close together. However, each electron slightly increases the charge of the atomic lattice surrounding it, creating a domain of net positive charge which in turn attracts other electrons. The dynamics of Cooper pairing in conventional superconductors was described mathematically by the BCS theory of superconduction, developed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer. As we keep discovering new materials which superconduct at higher temperatures, we are approaching the discovery of a material that will integrate with our power grids and electronic designs without incurring huge refrigeration bills. An important advance was made in 1986 when J.G. Bednorz and K.A. Müller discovered high-temperature superconductors, raising the critical temperature enough that the necessary coldness could be achieved with liquid nitrogen rather than with expensive liquid helium. If we could discover an even more impressive high-temperature superconductor, perhaps it would become economically feasible to transmit electrical power for very long distances without any power loss. A variety of other applications exist in particle accelerators, motors, transformers, power storage, magnetic filters, fMRI scanning, and magnetic levitation How does a Superconductor Work? In order to understand how a superconductor works, it may be helpful to examine how a regular conductor works first. Certain materials such as water and metal allow electrons to flow through them fairly easily, like water through a garden hose. Other materials, such as wood and plastic do not allow electrons to flow through, so they are considered non-conducting. Trying to run electricity through them would be like trying to run water through a brick. Even among the materials considered conductive, there can be vast differences in how much electricity can actually pass through. In electrical terms, this is called resistance. Almost all normal conductors of electricity have some resistance because they have atoms of their own, which block or absorb the electrons as they pass through the wire, water or other material. A little resistance may be useful to keep the electrical flow under control, but it can also be inefficient and wasteful. A superconductor takes the idea of resistance and turns it on its head. A superconductor is generally composed of synthetic materials or metals such as lead or niobiumtitanium which already have a low atomic count. When these materials are frozen to nearly absolute zero, what atoms they do have grind to a near-halt. Without all of this atomic activity, electricity can flow through the material with practically no resistance. In practical terms, a computer processor or electric train track equipped with a superconductor would use very little electricity to perform its functions. The most obvious problem with a superconductor is the temperature. There are few practical ways to supercool large supplies of superconductive material to the required transition point. Once a superconductor begins to warm up, the original atomic energy is restored and the material creates resistance again. The trick for creating a practical superconductor lies in finding a material which becomes superconductive at room temperature. So far, researchers have not discovered any metal or composite material which loses all of its electrical resistance at high temperatures. To illustrate this problem, imagine a standard copper wire as a river of water. A group of electrons are in a boat trying to arrive at their destination upstream. The power of the water flowing downstream creates resistance, which makes the boat have to work even harder to get through the entire river. By the time the boat reaches its destination, many of the electron passengers are too weak to continue. This is what happens with a regular conductor -- the natural resistance causes a loss of power. Now imagine if the river were completely frozen, and the electrons were in a sled. Since there would be no water flowing downstream, there would be no resistance. The sled would simply pass over the ice and deposit almost all of the electron passengers safely upstream. The electrons didn't change, but the river was altered by temperature to put up no resistance. Finding a way to freeze the river at a normal temperature is the ultimate goal of superconductor research.

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20th Jan 2015

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