Superconductors

Trains that float, faster computers that can store more data, and electric power that zaps into your home wasting less energy are just a few of the benefits promised by superconductors—materials that offer little or no resistance to electricity. You're probably used to the idea that conductors (such as metals) carry electricity well, while insulators (such as plastics) barely let it pass through them at all. But how much do you know about superconductors that eliminate resistance almost entirely when you cool them down to very low temperatures? Let's take a closer look!

Photo: The Meissner effect: this tiny ceramic "floats" on a magnet because it's acting as a superconductor. We explain how this works in more detail below. Photo courtesy of US Department of Energy.

How resistance changes with temperature

It's a little bit misleading to divide materials into conductors and insulators. It's much more accurate to say that all materials conduct electricity, under the right conditions, but some conduct more easily than others. When we say a metal conducts electricity well, we really mean it offers little or no resistance when you try to make a current flow through it; when we say plastics insulate well, we're actually saying that they put up highresistance to electric currents. Resistance is often a much more useful concept than trying to divide materials into "conductors" and "insulators".

One of the interesting things about resistance is how it changes as you change the temperature. Suppose you have a piece of gold wire in an electrical circuit. Gold is one of the best conductors there is: it shows very little resistance to electricity. But increase its temperature and it puts up much more resistance. Why? Broadly speaking, the higher the temperature, the more thermal vibrations there are inside the gold's crystalline structure and the harder electrons (the negatively charged particles inside atoms that carry electric currents) will find it to flow through. Conversely, if you cool gold down, you reduce the vibrations and make it easier for electrons to flow.

Now this is a somewhat simplistic explanation because several different mechanisms cause resistance and they become more (or less) important at different temperatures. At low temperatures, for example, impurities and defects in the material cause most of the resistance. A fairly complex mathematical equation called Matthiessen's rule lets you figure out the total resistance of a material at any given temperature by summing the various effects. But that's much more detail than most of us want or need to know. I survived my physics degree without learning Matthiessen's rule—and you can almost certainly manage without it too.

Chart: The resistance of gold is directly related to temperature: the hotter it gets, the more it resists electricity. As we approach absolute zero (0K), the resistance falls almost to zero—which leads us to the idea of superconductivity. Chart plotted using data quoted in Resistivity of Gold, with original data from "Handbook of Chemistry and Physics" (75th Edition) by David R. Lide.New York: CRC Press, 1996–1997.

If resistance increases when you heat metals and falls off when you cool them, any scientist worth his or her salt is immediately going to wonder what happens if you really—and I mean seriously—cool a metal. Many have considered this, but the first person to really probe the issue, in 1911, was a Dutch physicist named Heike Kamerlingh Onnes (1853–1926). When he cooled a wire made of mercury to the toe-tingling temperature of −269°C (−452°F or 4K), Onnes found that its electrical resistance suddenly disappeared. In other words, he'd discovered superconductivity. But it was a fairly fleeting effect. Onnes found that if he applied a strong magnetic field to his mercury, the superconductivity vanished as quickly as it had arrived.

Look, no magnetism!

We call materials superconductors because of their "super" ability to "conduct"—but it's probably not the best name we could have given them—superconducting is not the only special thing they can do. Following Onnes' amazing discovery, it took another 20 years or so before two German physicists, Karl Meissner and Robert Ochsenfeld, found that superconductors have another amazing trick up their sleeves.

A superconductor is diamagnetic: it refuses to let magnetism penetrate inside it. How does that work? Stand a superconductor in a magnetic field and you'll make electric currents flow through its surface. These currents create a magnetic field that exactly cancels the original field trying to get inside the superconductor and repelling the magnetic field outside. This is known as the Meissner effect and it explains how you can make a superconductor levitate (float) in a magnetic field.

Photo: Another view of the Meissner effect. Photo courtesy of Brookhaven National Laboratory and US Department of Energy.

What causes superconductivity?

You have to be brilliant to win a Nobel Prize in Physics—it's the world's top science award. But imagine how utterly, stupendously, amazingly brilliant you need to be to scoop two of these prizes. That was the achievement of American physicist John Bardeen (1908–1991). He won his first prize in 1956 (with Walter Brattain and William Shockley) for inventing the transistor (a tiny amplifier and switch that revolutionized electronics and computing). But he won a second prize almost three decades later, in 1972 (with Leon Cooper and Robert Schrieffer), for developing the best theory we currently have of how superconductors work.

Called the BCS theory in honor of its three discovers, it explains that materials suddenly become superb conductors when the electrons inside them join forces to make what are called Cooper pairs (or BCS pairs). Normally, the electrons that carry electricity through a material are scattered about by impurities, defects, and vibrations of the material's crystal lattice (its scaffold-like inner structure). That's what we know as electrical resistance. But at low temperatures, when the electrons join together in pairs, they can move more freely without being scattered in the same way.

If you're a bit of a romantic, think of Cooper pairing as a kind of marriage. Just as marriage can help two people sail through life's ups and downs by joining forces, so Cooper pairing allows electrons to travel through a conductor without getting bogged down in lots of troublesome little obstacles. Quite why electrons choose to get married at sub-Antarctic temperatures, rather than room temperature like everyone else, is another story...

Artwork: Superconductivity happens when electrons work together in Cooper pairs.

Which materials superconduct?

Not all materials show superconductivity. Apart from mercury, the original superconductor, you can find the effect in about 25 other elements (mostly metals, semimetals, or semiconductors), though it's also been discovered in thousands of compounds and alloys. Each different material becomes a superconductor at a slightly different temperature (known as its critical temperature or Tc). The trouble with most of these materials is that they superconduct only within a few degrees of absolute zero (the lowest theoretically possible temperature: −273.15°C, −459.67°F, or 0K). That means whatever benefit you gain from their lack of resistance, you probably lose from having to cool them down in the first place. The idea of a power plant that gets electricity to your home down superconducting wires sounds brilliant: it would save huge amounts of wasted energy. But if you had to cool large parts of the plant and all the transmission wires to absolute zero, you'd probably waste far more energy doing that than you'd ever save from having no resistance in the cables. This is largely why superconductors have yet to make a really big impact on the world, despite being discovered almost a century ago.

High-temperature superconductors

For many years, scientists assumed superconductivity could happen only at very low temperatures. Then, in 1986, two European scientists working for IBM, German physicist J. Georg Bednorz (1950–) and Swiss physicist K. Alex M&uum;ller (1927–), discovered a ceramic cuprate (a material containing copper and oxygen) that could became a superconductor at much higher temperatures (−238°C, −396°F, or 35K). Other scientists have since found materials that show superconductivity at even higher temperatures and the record is currently held by a material called mercury thallium barium calcium copper oxide (Hg12Tl3Ba30Ca30Cu45O125), which superconducts at −135°C (−211°F or 138K) and was patented by Korean scientists in 1996.

The discovery of so-called high-temperature superconductors moved research on enormously. The original superconductors needed temperatures within a whisker of absolute zero—and you can reach those only by cooling materials using an expensive coolant gas such as liquid helium. But the high-temperature superconductors (that's relatively high, not absolutely high—remember we're still well below minus 100 Celsius and minus 200 Fahrenheit!) can be cooled using liquid nitrogen instead, which is about 10 times cheaper to produce. A lot of applications that weren't economic suddenly became a whole lot more practical when high-temperature superconductors were discovered.

Photo: You make materials into superconductors by cooling them to extremely low temperatures. Even so-called "high-temperature" superconductors operate at what are very low temperatures by normal, everyday standards. In this photo, a superconducting electric motor is being cooled to about −179°C (−290°F or 94K) with liquid nitrogen. Picture courtesy of US Department of Energy/Argonne National Laboratory.

Uses of superconductors

What's so good about superconductivity? Yes, you can make little bits of ceramics float if you make them really cold, but what else can you do? Imagine if we could make a material that was superconducting at room temperature. Our computers would work faster because they'd allow electric currents to flow more easily. We could make powerful electromagnets that turned electricity into magnetism without wasting anything like as much energy. That would mean electric appliances in our homes and offices would waste much less power. We could also make "Maglev" (magnetic levitation) trains that would float on rails using linear motorsand get us around with a fraction of the power used by current locomotives. Engineers are already trying to use superconductors in all these ways, but if they could find a really high-temperature superconductor (one that worked at about 0–20°C (32–68°F or 273–293K), their job would be an awful lot easier!