'Superconductivity' is the state in which a material has literally no resistance to electric current. The phenomenon was discovered early in the 20th century, but for most of the following decades it remained little more than a curiosity. The materials that exhibited superconductive behavior only did so if they were cooled to within a few degrees of absolute zero, which limited their use to highly specialized applications. Superconductivity can perhaps lay claim to being the first child of low temperature physics, even though an adequate explanation of its physical origins had to wait for almost another half-century.
Superconductivity was discovered in 1911 by Dutch physicist Heike Kammerlingh Onnes. He discovered that when mercury was cooled by liquid helium to 4 degrees Kelvin, it lost all resistance to electric current. Onnes would later win the Nobel Prize for this work. Later research showed that many metals, such as tin, lead, and niobium, were also superconductive when cooled to extremely low temperatures. It is certainly a curious twist of fate that a phenomenon, which is such a striking manifestation of quantum mechanical behavior on the macroscopic scale, should have been discovered before the development of quantum theory.
Given the difficulties of working at such 'cryogenic' temperatures, superconductivity remained interesting but of little practical use, though materials were found that became superconductive at slightly higher temperatures. Theoreticians were fascinated by the phenomenon because nobody had any clear idea of why it occurred.
The theoretical principles of superconductivity were finally outlined in 1957, when John Bardeen, Leon N. Cooper, and J. Robert Schrieffer published a theory that would also win a Nobel Prize. The 'Bardeen-Cooper-Schrieffer (BCS)' theory suggested that cryogenic cooling of materials such as niobium suppressed the random thermal noise in their crystal structure. This allowed quantized mechanical vibrations ('phonons') to set up a weak electrical interaction that coupled electrons with opposite spin and momentum together in 'Cooper pairs', which had zero net spin and momentum.
Electrical resistance is caused by the scattering of electrons due to defects, impurities, and thermal vibrations in the crystal lattice of a conductor. However, the binding of electrons in Cooper pairs eliminates scattering, and so electrical resistance disappears. Above a specific 'Curie temperature (Tc)', thermal vibrations disrupt the Cooper pairs, and the material becomes resistive again. Intense magnetic fields and high currents can also disrupt the pairs and destroy superconductivity.
Despite the development of BCS theory, doing anything useful with superconductors remained an uphill struggle. What seemed to be a breakthrough finally occurred in the 1980s.
Interest in superconductivity skyrocketed in the late 1980s when materials were discovered that remained superconductive at relatively high temperatures, but after the initial excitement wore off, development of practical applications proved painfully slow. However, by the end of the century, work towards applications of superconductive materials in power electric systems, sensors, and digital electronics finally seemed to be on track.
In September 1986, Alexander Mueller and Georg Bednorz, two scientists at an IBM research center in Zurich, Switzerland, published a paper describing a copper-oxide compound that exhibited superconductivity at 35 degrees Kelvin, 12 degrees above the Curie temperature of any superconducting material known to that time. They published their paper in an obscure German physics journal in hopes that it wouldn't be noticed. This tactic allowed them to reinforce their preliminary research without interference, but still be able to prove the priority of their work if other reports were published.
After more studies, the two scientists became convinced that their findings were correct. Once their discovery became widely known, a flood of new 'high temperature superconductor (HTS)' materials were discovered. By December 1986, a material had been discovered with a Tc of 38 K. A year later, in early 1987, a team under physicist C.W. 'Paul' Chu discovered a compound, 'yttrium barium copper oxide' ('YBCO', pronounced "ibco"), that had a Tc of 93 K.
This moved the Curie temperatures of superconducting materials from the range of liquid helium temperatures to those of liquid nitrogen temperatures. The reduction in cooling requirements promised to greatly reduce the cost of superconducting technology and widen its range of applications.
The enthusiasm of researchers in the field was manifested that year by a special meeting of the American Physical Society in the Hilton Hotel in New York City, crammed with 3,000 physicists, many of whom stayed up all night discussing the new superconductors. The event became known as the 'Woodstock of Physics'.
Since 1986, over 100 HTS materials have been discovered. The record Tc now stands at 138 degrees Kelvin. This progress has been made even though nobody is exactly sure how high-temperature superconductivity works.
While there is clearly some electron pairing mechanism involved, as is the case with the old 'low-temperature superconductors (LTS)', the phonon-linkage mechanism associated with Cooper pairs in low-temperature superconductors can't work at high temperatures, since thermal vibrations would quickly break the phonon linkages. The most popular theory is that the pair coupling occurs due to subtle magnetic effects created by the HTS lattice, but nobody has been able explain how it happens. Understanding what causes the phenomenon will help researchers to address some of the problems they have encountered working with HTS.
For example, magnetic vortexes set up by the flow of electric current through an HTS have a tendency to drift through the material, and this drift dissipates energy, or in other words, causes resistance. The material needs to have strong 'flux pinning' to ensure the vortexes do not migrate.
More importantly, a better theoretical understanding may lead to raising the Curie temperature still further. Researchers believe this is perfectly possible, since a copper-oxide compound made with mercury has been shown to superconduct at 164 K when squeezed to extremely high pressure in a diamond anvil. As a result, one avenue of research has been to try to modify superconductive materials into configurations resembling those that they adopt under high pressure.
After the great expectations raised by the discovery of HTS materials, researchers found their enthusiasm gradually deflated when they found out the practical limitations of HTS.
The 'critical current density', or the maximum amount of current a superconductor can support before becoming resistive, of YBCO is very high, about a million amperes per square centimeter, and the material could remain superconductive in relatively high magnetic fields. Unfortunately, so far attempts to fabricate practical superconducting wires from YBCO have failed, since its irregular grains are difficult to work into strips and wires.
A better material for power electric applications eventually appeared in the form of 'bismuth, strontium, calcium, copper, and oxygen' ('BSCCO', pronounced 'bosco' or 'bisco') materials. BSCCO has flat, regular grains that can be more easily aligned, and proved much easier to fabricate, although it does not have nearly the current capacity of YBCO.
Researchers are now able to encase grainy, brittle BSCCO material in silver and extrude the assembly into long filaments. The filaments are then rolled and heated to align the BSCCO layers to form a continuous wire. BSCCO has a layered structure; rolling breaks up and spreads the layers, and heating merges them together. Unfortunately, the silver coating substantially increases the cost of BSCCO wires.
The current critical current density of BSCCO is about 70,000 amperes per square centimeter. This value is about three times greater than it was in the mid-1990s. Practical BSCCO wires are not made entirely of superconducting material, and so their actual critical current density is about 15,000 amperes per square centimeter.
A promising new HTS, 'magnesium diboride (MgB2)', was discovered in 2001 by a team of Japanese materials researchers. Although it has a Tc of only 39 degrees Kelvin, it is cheap, easy to fabricate, and much easier to work into wires than other HTS materials. The low Tc is a drawback, but it at least allows MgB2 to be chilled with a mechanical cryocooler system rather than liquid helium.
MgB2 suffers from a low critical current density of about 35,000 amperes per square centimeter and poor resistance to magnetic fields, but researchers are making progress in both these areas.
Practical applications of superconductors have focused in three areas: electric power systems and devices, such as power transmission lines, electric motors, and transformers; sensitive 'superconducting quantum interference device (SQUID)' sensors; and ultrafast superconducting digital logic components.
Electric power systems offer the greatest potential in the near term, and indeed are the primary application for HTS. As it turned out, HTS has some limitations that restrict its usefulness for SQUIDS and logic devices, but substantial improvements have also been made in traditional LTS technology to advance those fields.