You’ve probably heard of “The Spy Who Came in from the Cold.” Well, now there’s something else that’s coming in from the cold: superconductivity.
Superconductivity is not something that orchestra directors aspire to; it refers to a discovery made 80 years ago at the University of Leiden (Holland) by Heike Kamerlingh Onnes, who was experimenting with extremely low temperatures–cryogenics. In 1908 he succeeded in liquifying helium, which has a boiling point just a little over 4 Kelvin (-269 Celsius). Reducing the vapor pressure can drop the temperature of liquid helium all the way to 1 degree Kelvin–one degree above absolute zero, the lowest temperature possible.
Kamerlingh Onnes used his liquid helium to study the effects of cryogenic temperatures on various materials, and in 1911 he noticed that electrical resistance in mercury virtually disappeared below 4.15 K–an effect he soon noted in other metals. An electrical current introduced into a circuit of such a supercold metal would flow for hours or even days without any additional electrical input–the electronic equivalent of perpetual motion.
Apparently at low temperatures the current’s electrons, which normally move independently, pair up, requiring almost no energy to keep moving.
Twenty-two years later, in 1933, German physicist Walther Meissner and his co-workers discovered that superconductors also repel magnetic fields. This means that a magnet placed over a superconductor, will “levitate,” held in mid-air by its own magnetic field, which is being repelled by the superconductor. This is called the Meissner effect.
Superconductivity offers exciting possibilities. Modern civilization uses a lot of electricity, and wastes a lot of it overcoming the resistance of various conductors. Ten to 20 percent is lost just in transmission. Superconducting power lines and electrical circuits would drastically reduce energy consumption, cost and environmental damage.
You can also turn a superconductor into a very powerful magnet by forming it into a loop and running a current through it. That generates a magnetic field in the centre of the loop, as much as 200,000 times stronger than the Earth’s. Such powerful magnets, in conjunction with the Meissner effect, could make possible high-speed, energy-efficient “levitating” trains. Superconducting magnets could also have applications in medical scanners and basic research instruments such as particle accelerators.
Finally, a third superconducting effect called the Josephson Effect, predicted by Brian D. Josephson in England, could revolutionize computers. If two superconductors are brought close toegther but not allowed to touch, electrons can jump the gap (or “Josephson junction”) and current can flow as if the two conductors were touching. Because the current across the gap is very sensitive to electric and magnetic fields, it can be used as a very accurate sensor or as an electronic on-off switch–the basic element of all computers.
Trouble is, it’s hard to make much use of materials that only work when they’re cooled to within a few degrees of absolute zero with expensive, hard-to-work-with liquid helium.
But in 1986 two IBM researchers in Zurich, Georg Bednorz and Alex Muller, stumbled on a ceramic made from lanthanum, copper, barium and oxygen that became super-conducting at 35 K. U.S. scientists then discovered one which became superconducting at 98 K–high enough that it could be cooled with cheap, easy-to-work-with liquid nitrogen.
Because they’re ceramics, the new materials can’t be easily formed into wires or other electrical components, but their very existence has prompted frantic work to find superconductors at ever-higher temperatures. They also prompted a new theory: that in these materials, superconductivity arises because, at a certain temperature, their oxygen atoms lose a few electrons, forming “quantum holes.” Adjacent copper atoms are pulled into line by the resulting attraction and drop some of their electrons into those holes, creating new holes which are filled by the next atoms in line, and so on and so on, allowing electrons to flow through the material indefinitely, or at least until the temperature rises.
This theory suggests an upper limit of about 200 K (-73 Celsius) for the new ceramic superconductors. (The current record is already above 120 K.) That’s high enough that ordinary dry ice could be used to keep things cool–high enough to spark a new revolution in electronics. The ultimate goal, though, is to create substances that are superconductive at room temperature. Only then will the full possibilities of superconductivity be realized.
Such substances may not exist. But the search goes on. After all, before 1986, there were no materials that could be made superconductive above 23 K.
And even if room-temperature superconductors are never found, who knows what else might be found in the search for them? After all, Heike Kamerlingh Onnes wasn’t expecting to find superconductivity when he liquified helium.
Sometimes in science the goal of the journey isn’t as important as what’s passed along the way.
