The field of superconductivity is characterized by unexpected discoveries. Despite its discovery almost 100 years ago and its many applications, it is still not understood fully. For example, the mechanism behind high-temperature superconductivity continues to baffle scientists almost 15 years after its discovery.
Kamerlingh Omnes, a Dutch physicist dedicated to achieving ultracold refrigeration, opened this field in 1908 by liquefying helium at -452 F (4 K or -269 C).(1) This achievement enabled scientists to cool materials to very low temperatures and study their properties.
Scientists knew that a metal's resistance fell as the temperature was lowered, but did not know what the limiting value would be when 0 K (the absolute minimum temperature) was approached. In 1911, Omnes began investigating the electrical properties of metals at very low temperatures. Many contemporaries, including Lord Kelvin, believed that resistance eventually would level off to a nonzero value. While passing a current through a very pure mercury wire whose temperature was being steadily lowered, Omnes noticed that its resistance vanished at 4.2K. He remarked: "Mercury passed into a new state, which on account of its extraordinary electrical properties may be called the superconducting state." This marks the birth of superconductivity.
Scientific and commercial potentials were obvious. A resistance-free metal wire could carry current for a long time without any loss. Omnes tried to determine the amount of such a loss. After letting a superconducting loop run for a year, he determined that there was no significant current loss. He was awarded the Nobel Prize in 1913 for his discovery.
In 1933, Walter Meissner and Robert Ochsenfeld discovered that superconductors are both perfect conductors and perfect diamagnets, for magnetic fields cannot penetrate a superconductor's interior. When a material is superconducting and a field is applied, the current flowing on the superconductor's surface generates a magnetic field that cancels the applied field inside the superconductor (the Meissner effect). Since the magnetic field generated inside the superconductor opposes the applied field, superconductors are diamagnetic. This shielding of an applied magnetic field occurs only if the applied field is not very large. Superconductivity is destroyed at a certain point.
Theoretical progress was much slower, however, almost as if superconductivity had been discovered too early. The scientific community's incomplete understanding of quantum mechanics made it impossible to understand the mechanism behind superconductivity. Some phenomenological theories were developed during the 1930s and 1940s, but a clearer picture only began to emerge in 1957.
Three American physicists, John Bardeen, Leon Cooper, and Robert Schriffer, used quantum field theory and many-body physics to develop the BCS theory, which explains superconductivity for elements and some alloys.(2) In essence, the theory states that a superconductor's electrons condense into a quantum ground state and move together coherently. Pairs of electrons (Cooper pairs)-not single electrons-achieve current transfer. These physicists received the Noble Prize in 1972.
Another milestone came in 1962. Brian Josephson, a Cambridge University graduate student, predicted that an electrical current could flow between two superconductors separated by thin insulating barrier. He made a suitable device (the Josephson Junction) by inserting an insulating material between two superconductors. Sending current through one superconductor, he saw it pass through to the other one. Known as the Josephson effect, it is one of the most important components in superconducting electronics. Josephson was awarded a share of the Nobel Prize in 1973.
In 1986, Alex Miiller and Georg Bednorz of IBM Research Lab (Switzerland) synthesized a ceramic compound that superconducted at 30K (-243C), the highest superconductor temperature ever reached. This compound contained lanthanum, barium, copper, and oxygen. Scientists do not know why it super-conducts, for it insulates at high temperatures and conducts electricity very poorly before it superconducts. Bednorz and Miiller received the Noble Prize in 1987.
This discovery inspired many researchers to combine elements to achieve superconductivity at higher temperatures. In January 1987, researchers at the University of Alabama replaced the lanthanum in the Bednorz-Miiller compound with yttrium and reached a transition temperature (Tc) of 92K. As a result, the much cheaper liquid nitrogen could replace liquid helium as a coolant. By trial-and-error experimentation, a Tc of 138K was reached in a compound consisting of mercury, thallium, barium, calcium, copper, and oxygen.
These new materials all contain layered copper and oxygen planes with other elements in between the crystal structure. Superconductivity occurs on the planes, and the rest of the crystal serves as a charge reservoir. The magnetic field gradually penetrates these new materials (called High Tc superconductors), causing a mixed state between the normal state and the superconducting state.
In 1997, existing theories were shattered when an alloy of gold and indium was used as a superconductor and a magnet. This is expected to have an important effect on magnetic data storage.
The most recent surprise came in November 2000. About 10 years ago, scientists learned that carbon-60 could superconduct at near absolute zero. By expanding the lattice structure, a Tc of 52K was reached. About a month later, the same group reached a Tc of 90K. Many believe that this temperature could reach well over 100K. Carbon-60 is the only material that has reached such high Tcs without having copper and oxygen planes in its structure.
Superconductors do more than just conduct electricity. Other important functions are as follows:
• The Korean-developed SQUID (Superconducting QUantum Interference Device) can detect magnetic field changes that are 100 billion times smaller than Earth's minute magnetic field, and uses the most fundamental properties of superconductors and quantum mechanics. Medical researchers use SQUIDs to study the human brain. Systems in which hundreds of SQUIDs are arranged in a helmet-like configuration containing liquid helium are commercially available. These systems detect the magnetic field produced by thousands of neurons. Although neurons produce huge fields when compared to the SQUID's sensitivity, a magnetically shielded room is required to filter out fields produced by TVs, computers, cars, and so on. In these rooms, external stimulation applied to the patient's brain enables specialists to locate tumors or other ill-functioning areas by mapping the brain's functions. The human brain has two types of responses: stimulated and self-generated (spontaneous). By using the SQUID's fast temporal response, one can locate non-invasively the epileptic loci, which causes some diseases. Alzheimer's and Parkinson's research also use SQUIDs at the detection level.
• Magnetic levitation became possible after scientists built superconducting magnets. Since superconductors have no resistance, a small voltage can generate huge currents and, therefore, magnetic fields large enough to float vehicles on these superconducting magnets with almost no friction. In 1999, a train in Japan reached a speed of 343 miles per hour. Japanese researchers are studying the possibility of a mag-lev linear motor car.
• Superconducting magnets have been used extensively in particle accelerators since 1987. Particle physics requires the acceleration of subatomic particles to speeds very close to the speed of light. This necessitates high magnetic fields that, in turn, need high currents-something for which superconductors are ideal. One event that made superconducting better known is probably the American Congress' cancellation of the multi-billion Superconducting Super Collider (SSC) project in 1993. A European consortium is now pursuing this research field.
•Superconducting wires improve an electric generator's efficiency by more than 99 percent. In addition, such generators are about half the size of conventional ones. General Electric estimates that there is a potential $20-30 billion global market for superconducting generators. Unfortunately, the high costs of cooling systems rules out using this technology to supply cities with electricity. But the moment sufficiently high Tcs are reached, superconductivity's impact in this area will be immeasurable.
•Other applications are high-performance and high-capacity electronic filters (currently used in some cellular phone systems); a petaflop-computer (1,000 trillion floating point operations per second-1,000 times faster than today's computers); mine and submarine detection (the U.S. Navy); and storing energy to enhance power stability (American Superconductor Corp.); satellites; telescopes and other light detection instruments; and Internet routers.