by Chris Woodford. Last updated: May 25, 2017.
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 high resistance 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.
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.
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.
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
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.
Photo: Another view of the Meissner effect. Photo courtesy of Brookhaven National Laboratory and
US Department of Energy.
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.
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.
Artwork: Superconductivity happens when electrons work together in Cooper pairs.
Called the BCS theory in honor of its three discovers, it explains that materials
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
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...
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.
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 Müller (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
which superconducts at −135°C (−211°F or 138K) and was patented
by Korean scientists in 1996.
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.
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.
Uses of superconductors
Photo: Floating Maglev trains would be much more practical if superconductors
worked at higher temperatures. Picture courtesy of US Department of Energy/Argonne National Laboratory.
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
motors and 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!