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A: A superconductor is a state of matter dominated by the strange laws of quantum mechanics. The electrons in the substance go into a very organized state when cooled below a certain temperature, giving the material a number of unusual properties. The first is that electric current can flow through the conductor with absolutely no loss of energy—in other words, the resistance goes to zero. Also, when a superconductor shaped like a ring is placed in a magnetic field, a current begins to flow. This by itself is not surprising, because Faraday’s law of induction states that changing the magnetic flux in a loop causes a voltage to arise, and this voltage can drive a current. However, because the resistance in a superconductor is zero, the current does not dissipate and die out; instead it keeps flowing around the loop in perpetual motion. Finally, the most dramatic demonstration of superconductivity can be seen if a magnet is put above a sheet of superconductor. The superconductor will push out the field from the magnet, causing it to levitate in the air.

The first superconductor was made in 1911 by H. Kamerlingh Onnes in the Netherlands. He did it by cooling mercury in liquid helium. When the temperature dropped below about 4 K above absolute zero (-452°F), he saw that the resistance abruptly fell to zero. Since then many other metals have been found which become superconductors at very cold temperatures, like lead, aluminum, tin, and niobium and titanium, which are mixed together and drawn into wires to make very strong magnets for magnetic resonance imaging (MRI).

In 1986 a breakthrough was made when scientists found that certain ceramic compounds became superconductors at relatively high temperatures. These compounds are layered structures, with the superconductivity usually occurring in a plane of copper and oxygen atoms. The world record holder for the highest-temperature superconductor is the compound Hg_0.8Tl_0.2Ba₂Ca₂Cu₃O_8.33, which becomes a superconductor at 138 K (-211°F). That is still quite cool for a picnic, but warm enough so that liquid nitrogen, which can be cheaply condensed from the air, can be used as the coolant. However, because ceramics are brittle, the superconductors cannot be easily drawn into wires like the metal ones. Instead, the ceramic is grown on top of a flexible tape in a thin layer so that it does not crack when bent. This tape can then be wrapped around a tube to carry the liquid nitrogen coolant, and surrounded with insulation to make a superconducting cable. These cables are starting to be used inside power plants to transmit electric power.

Timothy L. Robertson
Physics Department
University of California, Berkeley

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Magnet levitating above superconductor: Movies

Superconductivity Experiment