Superconducting Materials, physics of electrons, crystals, metals, ceramics
World expert on superconductivity
"Follow your intuition. In my own experience, this has always paid off."
Louis Taillefer is a materials scientist, a physicist who specializes in the behaviour of electrons in matter. He considers himself a modern-day alchemist — medieval chemists who worked at turning ordinary metals into gold or silver — because his research involves cooking up materials that have never been made before. He uses super-powerful furnaces in which metals and ceramics are melted by electric arcs (the same as a lightning bolt), intense radio waves or focused beams of super-hot light. Elements of unequalled purity are combined in new and precise ways. The ultimate goal: a superconductor, a material in which electrons can move happily with no resistance at all.
Many materials can be superconductors, but only at extremely low temperatures. Scientists have known for about 100 years that superconductivity can occur in aluminum, lead, mercury, tin and other metals, but it only happens below -250°C. This is near absolute zero, which is a temperature of -273°C (-460°F). Materials scientists use the Kelvin temperature scale, which uses the same units as Celsius but places “zero” at absolute zero, not as we register it in the Celsius or Fahrenheit scales. Nothing can be colder than absolute zero, which technically means the absolute lack of entropy, a physical property of matter sometimes called thermal agitation or disorder — the need for atoms to jiggle around. Think of it this way: when liquid water turns to ice, it loses entropy. The water molecules that were zooming around all over the place in the liquid are now locked into a solid crystal and they can’t move as much. They are also colder. By lowering the temperature, the water went through a phase change, from liquid phase to solid phase. It’s the same material, but a different physical phase of matter, hard and solid instead of a soft, flowing liquid.
The same thing happens with superconductivity. It’s just another phase change. When some metals are cooled down to near absolute zero, around one to 10K, they become superconductors. This temperature at which they change phase to superconductors is called the critical point. So, for instance, aluminum becomes superconducting at 1.2K and mercury at 4.1K. Pretty cold.
In 1986, superconductivity got hotter. Two Swiss physicists working at IBM Labs in Zurich, Karl Alexander Müller and J. Georg Bednorz, discovered that a certain type of copper oxide became a superconductor at temperatures around 40K — quite a lot warmer than ever before. Within a year, scientists around the world began creating similar materials and raised the temperature of superconductivity to 93K. It took the world of science by storm. Müller and Bednorz won the 1987 Nobel Prize for their discovery. Today the “warmest” superconductor works at about 133K.
Liquid nitrogen has a temperature of 77K and is cheaper than milk, so we can now easily create an environment where superconductivity works. Before this, superconductors had to be cooled with liquid helium, which costs about the same as whisky (30 times more than liquid nitrogen) and does not last very long.
The new so-called high-temperature superconductors (HTS) have led to new applications such as ultra-high-performance radio frequency filters for use in cellphone network base stations or high-current electricity transmission. In the summer of 2001 three 400-foot HTS cables were installed in Detroit, Michigan, capable of delivering 100 megawatts of power. Superconductors are also appearing in high-speed “maglev” (magnetic levitation) trains in Japan, Germany and Singapore. These trains have no wheels and ride on a frictionless magnetic “cushion.” Future HTS applications include ultra-fast computers capable of operating at petaflop speeds, much cheaper and smaller scanners for medical imaging, ultra-efficient electrical generators and new electric motors twice as efficient as and half the size of conventional motors.
The mathematical explanation of superconductivity was worked out in 1957 and is called BCS theory, after the three American scientists who discovered it: John Bardeen, Leon Cooper and Robert Schrieffer. In a normal conductor, flowing electrons collide with crystal impurities that slow them down and cause electrical resistance. In a superconductor this does not happen because the electrons pair up and form a coherent quantum state, making it impossible to deflect the motion of one pair without involving all the others. So collisions have no impact and there is no resistance. While BCS theory works for conventional superconductors, it does not explain the behaviour of the new high-temperature superconductors. Taillefer thinks the mechanism is a purely electronic interaction, possibly involving the magnetic spin of the electrons.
His work demonstrates a classic strength of the scientific technique: Scientists invent theories that predict and explain the behaviour of the physical world. However, they keep testing and retesting these theories, especially under extreme conditions, to see where they break down. In this way they discover newer and better theories that reveal the essential qualities of nature. Taillefer’s most recent experiments question a basic theory explaining why good electrical conductors are also good heat conductors. He showed that under certain conditions (extreme cold and pressure), a type of copper-oxide superconductor appears to conduct electricity and heat differently. Experiments like Taillefer’s inspire new, improved theories to explain more of the physical world.
1. Superconductivity is a phase of matter. This graph shows where phase changes occur in high-temperature copper-oxide superconductors, depending on the temperature (in Kelvin) and the number of electrons in its crystal structure. (Holes are the places left when atoms in the surrounding crystal matrix pull electrons away. Holes are like electrons, because they can move around and carry charge in conductors or superconductors.) Superconductivity occurs in the half-circle region at the bottom, when the material has from 5 to 25 percent holes and the temperature is below about 130K. At low-electron hole concentrations, the material becomes an insulator, while at high concentration it is a conventional metal — a good electrical and heat conductor. Remarkably, by changing electron concentration only 5 percent, the material goes from a perfect insulator (incapable of transporting any electricity) to the strongest known superconductor (a perfect conductor of electricity).
2. Physicists who work at low temperatures use the Kelvin (K) scale to measure temperature. Use the three thermometers to compare where common temperatures appear on more familiar Celsius and Fahrenheit scales.
3. The superconducting material YBa2Cu3O7, yttrium-barium-copper oxide, is part of the mineral family called “Perovskites.” This brittle ceramic material was one of the first high-temperature superconductors discovered, in 1987. Superconductivity occurs in the copper-oxide planes (speckled grey balls) as a result of electron interactions that are not entirely understood, but could involve the formation of Cooper pairs. Yttrium (dark blue), Barium (light blue).
4. Electrons have spin, which makes them into tiny little magnets. They also carry a negative charge. As charges repel, so do electrons normally strongly repel each other. However, in special circumstances they may be drawn to each other to form so-called Cooper pairs. This occurs when the surrounding crystal matrix, made of positively charged atoms, is locally deformed by the passage of a single electron, which in turn attracts a second electron in its wake. Think of the way two people can roll toward each other on a waterbed; it works something like that. In general, a Cooper pair of electrons “join” in such a way that their total spin is cancelled out (that is, the spin of one points up and the other points down, cancelling each other.) Because of this, a Cooper pair behaves like a single particle with zero spin and mass twice that of a single electron. But Cooper pairs do not behave independently of each other like single electrons in a normal metal conductor. They form a single coherent quantum state, which means that instead of having random behaviour, they all act in exactly the same way. In this sense, superconductivity is a large macroscopic quantum phenomenon.
5. Once a current gets going in a superconductor, it can be made to flow forever in a circular loop. This is the closest we come to perpetual motion in nature. Superconducting coils can become powerful lightweight electromagnets. Mounted on a Shanghai maglev train, with conventional magnets or electromagnets in the guideway, the train floats on magnetic fields, moving with no friction except air resistance. Such maglev trains cruise at 580 kilometres per hour. The train shown does not use high-temperature superconductors. It works with conventional superconductors requiring liquid helium, which is very expensive.
6. Electrons have two major features, charge and spin. Charge is responsible for the phenomenon of electricity — when electron charge flows, an electric current is created. Spin is responsible for magnetism — when all the electron spins in a material line up in the same direction, the material becomes a magnet. Scientists like Taillefer find new properties of materials as they discover how electron spin and charge behave in different materials. One new theory suggests that in high-temperature superconductors, electrons may lose their usual integrity, so that spin and charge are no longer carried together. If such spin/charge separation indeed occurs, then the fundamental particles in materials are no longer electrons but can be thought of as of two smaller particles, “chargeons” and “spinons.” In his research, Louis Taillefer is observing unusual phenomena that may be caused by such spin/charge separation.
So You Want to Be a Physicist
It was pure chance that Louis Taillefer became a physicist. He simply accepted a scholarship to study mining engineering at McGill University. “If some other place had given me an award in biology, I would have gone there,” he says. At McGill he soon switched to geophysics, but he enjoyed the fundamental science so much that he ultimately graduated with an honours degree in pure physics. Taillefer likes to tell young people, “Go in some direction, but don’t feel you need to be stuck there. Go with your intuition and change, readjust to what interests you. Feel free to switch to subjects where you feel more at home.”
As a graduate student, Taillefer’s quest for more relevant research brought out Gil Lonzarich’s passion for theoretical work on magnetism. This in turn inspired the young Taillefer to discover something essential in the natural world. Now that Taillefer himself supervises graduate students, he finds it to be the most satisfying part of his job as a university professor for the same reason. Seeing students find their own path is a wonderful feeling for him, and the only way this happens is if he gives them the freedom to do so. “The key point is that people must go where they feel their inspiration,” says Taillefer. “I give my students the freedom to develop as independent scientists but also to follow their destiny as individuals.”
Typical physics careers include specialties in electronics, communications, aerospace, remote sensing, biophysics, nuclear, optical, plasma or solid state physics, astrophysics and cosmology. Some physicists concentrate on experiments, while others prefer theory alone.
ACTIVITYhas 1 activity for you to try in the Activities section.
- The physics of the conduction of electricity is very similar to the conduction of heat. Learn how materials vary in their ability to conduct.
Is room-temperature superconductivity possible? So far, scientists have created materials that are superconductors at 133K, which is still -140°C — mighty cold. Taillefer wonders if we will ever find a material that is a superconductor at room temperature (293K). Other big questions in materials science: What makes some materials magnetic, and others not? And why does heat destroy the magnetic properties of some materials more rapidly than others?
Jean Matricon, Georges Waysand (translator) and Charles Glashausser, The Cold Wars: A History of Superconductivity, Rutgers University Press, 2003.
Michael Tinkham, Introduction to Superconductivity, second edition, Dover Books, 2004.
Explanation of maglev trains.
Scientific American magazine featured 12 Events That Will Change Everything in their June 2010 issue. Click on icon no.6 (out of 12), in the top right corner (after the introduction animation). Then click on "Interview with Dr. Louis Taillefer" and "The Canadian Institute for Advanced Research" to see some video clips of Louis Taillefer talking about his science.