August 21, 2007 > TechKnow Talk
What is a Superconductor?
By Todd Griffin
This is one of those scientific fields in which unknowns far outweigh certainties. The research into superconductivity - what it is and how it can serve humans - is brimming with opportunity. Students of physics who wish to make their mark have, in the study of superconductors, tremendous potential for discovery and invention in the years and decades ahead.
Before discussing superconductivity, we should briefly review conductivity as well as dip into the realm of the very cold. Common electrical conductors such as copper wire conduct electricity via movement of free electrons through an array or "lattice" of atoms or ions, in response to applied voltage. This flow of electrons (negatively charged particles) is called a "current." As electrons pass through the material, atoms act as obstacles, hindering the current or free flow of electrons. The degree to which the current is hindered is called "resistance." Most of us were exposed in high school to the equation V=IR, meaning voltage is the product of current and resistance. Another way to state this is that as resistance increases, current decreases, and vice versa.
Let's turn now to the concept of absolute zero. Though those of us who have spent a winter in Canada (or a summer in San Francisco) may dispute this, there is a limit as to how cold it can get. Nothing in the universe is colder than about 460 degrees below zero Fahrenheit, or -460 F. At this temperature all atomic motion ceases. This theoretical limit is known as absolute zero. Very low temperatures are measured with the Kelvin scale. Absolute zero is zero degrees Kelvin, or 0 K. Even in deep space, immense distances from any warming stars and galaxies, it is not quite as cold as absolute zero. Residual heat left over from the Big Bang warms space to a balmy 3 K, or 3 degrees above absolute zero.
If we cool our copper wire, we find that resistance decreases, thus increasing current flow. This is because the atomic lattice obstacle course becomes more stable, vibrating less violently, and offering easier passage to electrons. As the wire gets colder and colder, the current measurably increases, but the difference is not dramatic. Even as the wire is chilled to temperatures near absolute zero, resistance decreases in a gradual and predictable way.
In 1911, a Dutch physicist was working with some mercury at cryogenic (very cold) temperatures. As he cooled his sample to about 4 K, he observed a sudden, dramatic jump in electrical conductivity. In fact, resistance became zero; resistance was not merely reduced, it dropped abruptly to precisely zero. Something very different from normal conductivity was occurring in the mercury. He had discovered "superconductivity."
In a superconductor, an electrical current will flow essentially forever with no applied voltage. This is perhaps as close as science has come to producing a "perpetual motion machine," though must be energy input to the system to keep the material sufficiently cold. A superconductor can not only create a powerful magnetic field itself, it can also repel external magnetic fields, another behavior very different from a normal conductor. Interestingly, some of the best conductors, such as copper, gold, and silver, are not superconductors, even near absolute zero.
Throughout the 20th century, many more elements and compounds were found to exhibit superconductivity. But they all had to be cooled to a very low temperature before transitioning from a normal conductor to a superconductor. Since cooling and maintaining materials at such low temperatures is costly, research began to focus on creating materials that provided superconductivity at higher temperatures.
Not until 1987 were materials developed that were superconductors above 77 K. This was a critical achievement, since a relatively cheap and readily available commercial coolant, liquid nitrogen, provides a 77 K environment. Today, the highest documented temperature for a superconductor is 138 K (about -200 F), though this record will almost certainly be broken within a year or two. These modern "high temperature superconductors" do not occur in nature. They are very complex compounds concocted in laboratories, and patented by their creators.
The naturally occurring superconductors are mostly metals; for example: mercury, tin, and lead. These are called "Type 1" superconductors. Though the phenomenon is not completely understood, the mechanism by which current flows is very different than normal conductivity. It is thought that when the material transitions to a superconductor, individual electrons team up in pairs and travel through the lattice joined together. This creates quantum effects that result in the bizarre characteristics of superconductors. These effects are still being studied, nearly 100 years following the discovery of superconductivity.
In general, higher temperature, more complex superconductors are known as "Type 2" superconductors. While the nature of superconductivity may be similar in some ways in Type 1 and Type 2 materials, it is also very different. For example, the transition to superconductivity is not as abrupt in Type 2 materials, occurring instead over a range of temperatures. This behavior suggests a different mechanism is operating.
In addition to complex manmade compounds, some ceramic materials have also been found to exhibit Type 2 superconductivity. This is particularly strange, as ceramics are typically electrical insulators, exhibiting very high resistance, under normal conditions. Again, scientists do not yet fully understand Type 2 superconductors, or how they manage to exist at such relatively high temperatures. Current research is focused on the geometry of the lattice and its role in promoting superconductivity.
Why is all this effort and money being expended to create better (higher temperature) superconductors? What are they good for? The answers are still being formulated. It is likely many of the ultimate uses of superconductors have yet to be conceived. Room temperature superconductors have been a mainstay in science fiction for decades. If such a material were ever developed, it would revolutionize many industries. For example, room temperature superconducting wire could replace our existing electrical wires and cables, and carry far more current much more efficiently than traditional conductors.
But room temperature superconductors will probably remain solely in the domain of science fiction for the foreseeable future. In the meantime, many useful applications have been found for existing materials. For example, superconductors are used for high-powered electromagnets, including those used in medical devices such as MRI machines. They are also employed in particle beam accelerators - huge high-energy physics research facilities such as the Stanford Linear Accelerator (SLAC) in Palo Alto - as beam guidance magnets.
Because superconductors repel (or "push back against") external magnetic fields, a magnet can literally levitate or float above a superconductor. This is known as the Meissner Effect, and has led to several test installations of high-speed Magnetic Levitation (Maglev) trains based on this principle. Perhaps the most notable example is the Yamanashi Maglev train in Japan, which has clocked speeds in excess of 350 mph, floating on a nearly frictionless cushion of air over a superconducting rail. The primary deterrent to commercializing such transportation technology is the strength of the magnetic field required. A field sufficiently powerful to levitate a train can be dangerous to people and animals.
Many potential applications are under development, including superconducting electric generators and transformers, electronic frequency filters, microwave antennas, x-ray detectors, high-speed computer processors, and network routers. Industry experts have estimated a $5 billion market for superconductor products by 2010.
For all the progress, we remain on the frontier of this fascinating branch of science. New superconducting materials are created every year - we have barely begun to tap the potential applications - and a complete scientific explanation of the observed phenomena is still being sought. The Nobel Prize in Physics has been awarded five times for superconductivity research: in 1913, 1972, 1973, 1987, and 2003. How many more are yet to come?