Iron in the Mix
Scientists look for the secret behind high-temperature superconductors
Physicist Johnpierre Paglione works in a kitchen of sorts: He precisely blends ingredients, heats his mixtures to just the right temperature and cools them to get the perfect product. But rather than only edible ingredients, his recipes call for toxic chemicals, such as arsenic, and metals — especially iron. His ovens, which line the shelves of his lab at the University of Maryland in College Park, reach 1,700Ë Celsius before he carefully cools his concoctions over days or weeks. When the timer finally dings, out pops a silvery-black pebble with one flat, shiny surface.
The newly made pebble is a superconductor, a material that shuttles electricity with essentially perfect efficiency, defying the resistance that typically slows electrons down. Because Paglione’s pebble incorporates iron into its molecular structure, it’s a member of a new class of materials known as iron-based superconductors. These materials, discovered in 2008, work at temperatures as high as −218Ë Celsius, or 55 kelvins (degrees above absolute zero). Though that sounds pretty cold, conventional superconductors must be cooled to within a few degrees of absolute zero. Only copper-oxide superconductors work at higher temperatures than the iron-based family, and together the two groups make up what are known as the high-temperature superconductors.
Scientists have been trying to figure out how high-temperature superconductivity works since copper oxides, or cuprates, were found to exhibit resistance-free flow in 1986. Right now, even the most promising cuprate must be cooled to about 138 kelvins. Though liquid nitrogen can get materials that cold fairly easily, the cuprates are hard to form into wires.
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With the new iron family on the scene, scientists may be able to identify what makes high-temperature superconductors tick, leading to new materials that are easier to work with or that operate at even higher temperatures. And the everyday potential of zero-resistance power lines and levitating magnetic trains may finally be realized.
“The field is feeling very liberated and very excited and very optimistic, because we’re not constrained to ‘it’s just copper oxide, it’s just iron arsenide,’ ” says physicist Paul Canfield of Iowa State University in Ames and the U.S. Department of Energy’s Ames Laboratory. “It may be that if we can figure out what’s similar between these two very different classes of materials, we may be able to generalize and find other materials that may even have more promising properties.”
Thousands of papers have been published about iron-based superconductors since their discovery. During 2009 papers came at an average of 2.5 per day. This flood of research has revealed differences in how electrons pair in the cuprates and in the iron-based compounds. But there are also striking similarities in the families’ magnetic properties. All together, the findings leave physicists wondering whether they have found a key to high-temperature superconductivity.
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A string of super discoveries
The very first superconductor was found in 1911, when Dutch physicist Heike Kamerlingh Onnes discovered that mercury lost its electrical resistance when cooled to 4 kelvins. Over the next 50 years more superconductors were uncovered, but all required temperatures below 25 kelvins.
In these old-school superconductors, electrons travel through the material in what are called Cooper pairs. A negatively charged electron passing through a crystal lattice of positively charged ions pulls nearby ions close, creating a region of positive charge. This region attracts another electron to come through, and it pairs with the first. This pairing prevents the electrons from bouncing around and losing energy. But at “high” temperatures (around 30 kelvins or above), the thinking went, heat energy would overwhelm the Cooper pairs and break them apart.
Cuprates were the first high-temperature superconductors discovered. Physicists K. Alex Müller and J. Georg Bednorz of the IBM Zürich Research Laboratory in Switzerland found a brittle, ceramic compound made of lanthanum, copper, oxygen and barium that superconducted at 35 kelvins, unprecedented at the time.
“The interesting thing about those materials was that the mechanism of superconductivity seemed new,” says David Singh of Oak Ridge National Laboratory in Tennessee.
There was a flurry of excitement in the late 1980s about cuprate superconductors. Scientists set about creating more copper-oxide compounds that could superconduct at high temperatures. But after 20 years, scientists still couldn’t agree on just how the cuprates worked.
“It was basically an unsolved problem,” Singh says. “People had ideas, but there wasn’t a general consensus on what were the right ideas.”
In the meantime, researchers looked for other types of high-temperature superconductors. A small breakthrough came in 2001, when a well-known compound called magnesium diboride was discovered to superconduct at 40 kelvins. But it appeared to work through traditional Cooper pairing and didn’t shed much light on high-temperature superconductivity more generally. After years of little success, scientists began thinking that the cuprates were somehow a warm exception to the cold rule.
Then in 2008, Japanese scientists led by Hideo Hosono of the Tokyo Institute of Technology reported in the Journal of the American Chemical Society that they had found an iron-arsenic mix that superconducted at 26 kelvins. Before long, scientists found related compounds working at temperatures up to 55 kelvins.
The iron-based compounds could help reveal how superconductivity worked at relatively high temperatures, researchers thought. They immediately started creating more iron mixes and comparing the cuprates with the iron-based family in search of a common explanation.
Cuprate and contrast
The silvery black pebble that pops from Paglione’s oven is a crystal that contains alternating sheets of atoms. Iron-arsenic layers are stacked on top of films of barium the way noodles and tomato sauce are layered in lasagna.
Not all iron-based superconductors combine arsenic with the iron to make the noodle layer. Some use phosphorus, selenium or tellurium instead. And not all have barium; some use other elements such as lithium or mixes of lanthanum and oxygen, for example. And some dispense with the sauce altogether.
But all iron-based superconductors share the layered structure. And it’s always the layer with the iron that does the electron shuttling. The other layers provide some structural support and keep unneeded electrons out of the way.
Cuprates are also layered, except that instead of containing iron, the cuprates’ superconducting layers are made of copper and oxygen. The copper and oxygen bond so that the layers lie flat, while iron-containing layers, say iron and arsenic, are a bit more three-dimensional, with arsenic atoms embracing iron atoms from above and below.
But a good lasagna requires more than noodles and sauce — it also needs cheese. Superconductors often add another ingredient too. In a process called doping, some atoms are swapped for others — a bit of cobalt replaces some iron in one of the iron-based superconductors, for instance.
The chemical substitution changes the number of electrons in the material, which helps superconductivity happen. Canfield and Iowa State colleague Sergey Bud’ko discussed the intricacies of doping in some iron-based superconductors in August in the Annual Review of Condensed Matter Physics.
Though the iron and cuprate families seem to share some structural features, the closer researchers have looked the more dissimilar the materials seem. To probe those differences, scientists have run a battery of tests, such as bombarding a crystal lattice with neutrons, hitting it with X-rays, applying magnetic fields and measuring current flow. And placing different pressures on differently doped crystals helps reveal the boundaries of superconductivity.
Such tests have revealed that electrons in the cuprates and iron-based compounds appear to travel in pairs while superconducting, but not in the way traditional Cooper pairs work.
In the high-temperature superconductors, paired electrons seem to repel each other. Like middle school dance partners who are afraid of cooties, the two travel around the gym floor together while still keeping a comfortable distance.
In cuprates, the linkage will break down if the pairs travel in one of four forbidden directions through the lattice.
In a paper published in Physical Review Letters in 2008, Singh and three other colleagues, including theorist Igor Mazin of the U.S. Naval Research Laboratory in Washington, D.C., suggested that pairing in iron-based superconductors is closer to pairing in cuprates than to that of traditional superconductors, yet is still different. Electrons maintain their distance, but there are no specific traveling directions that will break pairs apart. Such pairing differences could mean that the mechanism of superconductivity is not the same in the two families.
Papers published since 2008 provide some evidence for the proposal by Mazin, Singh and colleagues, but the idea hasn’t been confirmed experimentally yet. In a review published in Nature in March, Mazin says such confirmation remains the main experimental challenge and may be just one or two years away.
A magnetic surprise
Iron had always been an elemental ingredient in the kitchen cabinets of scientists. But because of its magnetic properties, no one suspected its usefulness for superconducting recipes.
An atom of iron is its own tiny magnet, with a north and south pole. If the magnetic fields created by individual atoms in a hunk of iron all point in the same direction, the hunk will pick up or repel a paper clip. That’s because the many small fields can give the iron a net magnetic field.
But traditional low-temperature superconductors are made of materials whose individual atoms don’t create strong magnetic fields. What’s more, these materials lose their superconductivity when magnetic impurities enter the compound. Any magnetic field that is introduced can tug on the charged electrons and break the pairs apart.
So it was long thought that magnetism and superconductivity were mutually exclusive.
“Nobody would have dreamed of seeing 55 kelvin superconductivity in an iron-based material,” Paglione says.
But in a review paper published in September in Nature Physics, Paglione and Richard Greene summarize some of the latest evidence suggesting that magnetism may in fact be important for superconductivity in iron-based materials.
When combined with arsenic, the tiny atomic magnets in the iron line up so that they alternate the direction of their magnetic fields in an orderly fashion — up, down, up, down and so on. This state is called antiferromagnetic. And though an antiferromagnetic material won’t pick up a paper clip, it’s still a type of magnet.
Cuprates also show antiferromagnetism. And in both groups, superconductivity appears when scientists destabilize the antiferromagnetic order — through doping or by applying pressure — and magnetic orientations within the crystal start to fluctuate, switching from up to down.
Most scientists believe that the similarity between the two families suggests that the breakdown of antiferromagnetism plays a role in helping electrons pair, though no one is yet sure exactly how. The magnetic fluctuations may be important for unlocking the mystery. In the review paper in Nature, Mazin suggests that magnetism is essential.
Further magnetism studies may help researchers find compounds that work as high as room temperature, doing away with the need for refrigeration that superconductors require today.
“There are many magnetic ions, there are many magnetic systems,” Mazin says. “There must be more superconductors and some of them might be higher-temperature or better to use.”
Canfield agrees: “One of the ways to look for other superconducting compounds is to try to modify ones that are antiferromagnetic.”
Though some scientists are still counting on one common factor underlying high-temperature superconductivity, others believe the differences that have been found between the two families leave open the idea that there may be more than one way to superconduct at high temperature. If that’s the case, many more types of high-temperature superconductors may still be found.
“We already know that there’s more than one way to get superconductivity,” Singh says. “The other point of view is that … maybe the cuprates and the iron superconductors take different paths.”
Until there’s a clear answer, physicists will continue to hunt for new materials and push the boundaries of currently known superconductors, to increase their working temperatures and come up with technologically useful materials.
“The trick is, figuring out what solutions might work,” says Canfield, who grows iron-arsenide crystals in his lab. “This is where the basic research and instinct comes in.”