As its name suggests, about the only thing better than a semiconductor is a superconductor. You’ve probably heard this term before, and may have even learned about some of its interesting features. But just what is so super about it anyway? The name comes from the fact that these materials can carry something known as supercurrents, electrical currents that flow without electrical resistance.
We’ve already touched on current—it is the movement of electric charges in a conducting material. What about resistance? Resistance is a sort of electrical “friction” that leads to the unwanted loss of current by conversion into heat. In fact, when you hook up a battery to a plain old (nonsuperconducting) circuit, the resistance in the circuit determines just how much current can flow. If you raise the resistance, the current falls. The resistance in the circuit saps energy from the electric current and converts it to the much less useful form of heat.
Superconductors have the amazing property that as you cool them down, there is a certain, low temperature below which the resistance suddenly drops to zero. This is an enviable property, since it means that the power normally lost as heat in ordinary circuits can be preserved in their superconducting counterparts.
DEFINITION
Superconductors are materials within which resistance to electric currents drops to zero at low temperatures. In the superconducting state, they also oppose all penetration of external magnetic fields.
Supercurrents are the electric currents that flow within superconductors. These can persist for enormous lengths of time without dissipating.
Quantum physics provides us with an understanding of what is going on here. Electrical resistance appears in most normal materials because each flowing electron inevitably bumps into something during its journey. This could be a positive ion in the underlying material, an impurity atom, or an imperfection in the crystal structure, just to name a few. The bottom line is that each time an electron collides with one of these things, it loses some of its energy as heat.
Classical physics would predict that no matter how much you might cool your conducting material, these collisions will remain. It can therefore not explain superconductivity. Instead, the answer comes from considering not the individual particle properties of an electron, but rather the collective wavelike properties of many electrons in the solid. The long and short of it is that in the superconducting state, all the electrons in the material form a single, coherent wave function. Once in this state, the impurities and imperfections in the crystal become negligible impediments and the current can flow without resistance.
While zero-resistance conductors offer many advantages, in reality these come with a few major drawbacks. The first, of course, is the need to keep your circuit nice and cold—usually below -200°C or so. This is so cold that special cryogenic liquids are required, and these are both expensive and difficult to work with.
Fortunately a few “high temperature” superconductors have been discovered, though we must stress that “high” is a relative term. These still need to be cooled below about -100°C to enter the superconducting state. They also have the disadvantage of being brittle ceramic materials, which are not the best choice for electric wiring. It will therefore be some time before superconducting power lines carry electricity to our homes and lower our electricity bills.
QUANTUM LEAP
The field of superconductivity has been pretty super to its pioneers, who have been rewarded with no fewer than four Nobel Prizes in physics. Things began when Heike Kamerlingh Onnes was awarded the 1913 Nobel Prize for discovering the phenomenon, while John Bardeen, Leon Cooper, and Robert Schrieffer claimed the 1972 Nobel Prize for their theoretical, quantum mechanical explanation of it. In 1987, George Bednorz and K. Alexander Mueller shared the Nobel Prize for their discovery of high temperature superconductors the year before. Finally, the 2003 Nobel Prize went to Alexei Abrikosov, Vitaly Ginzburg, and Anthony Legget for their improved superconductivity theory.
Superconductors have another interesting property (among others) in that they expel external magnetic fields when in the superconducting state. This feature, known as the Meissner effect, is not just fascinating to physicists. It could also be very useful. Indeed, there are many technologies out there that strive to take advantage of this property.
One such technology is magnetic levitation. If a permanent magnet is placed just above a superconducting material, the force applied by the superconductor to expel the magnetic field from its innards will actually allow the magnet to float in the space above. This has long been a favorite laboratory trick for physics teachers. It has also gained traction in the development of a superfast, highly efficient train. The “maglev” train, as it is known, was designed to float over some form of superconducting “guide way” using magnets instead of wheels, axles, and bearings. Maglev has been under development for a few decades and a commercial train has been in service since 2004 in Shanghai, China. It can reach speeds exceeding 430 km/h (268 mph), making it the world’s fastest commercial train.
The search is on for new and improved superconducting materials, especially those that can operate at much more comfortable temperatures. Only time will tell if this technology will truly live up to its potential, or if its best known application will remain a nifty parlor trick.
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