What is a Superconductor?

Michael Anissimov

Superconductivity is a property displayed by certain materials at very low temperatures.  Materials found to have this property include metals and their alloys (tin, aluminum, and others), some semiconductors, and certain ceramics known as cuprates that contain copper and oxygen atoms.  A superconductor conducts electricity without resistance, a unique property.  It also repels magnetic fields perfectly in a phenomenon known as the Meissner effect, losing any internal magnetic field it might have had before being cooled to a critical temperature.  Because of this effect, some can be made to float endlessly above a strong magnetic field.

Tin is a material that has displayed superconductivity.
Tin is a material that has displayed superconductivity.

For most superconducting materials, the critical temperature is below about 30 K (about -406°F or -243°C).  Some materials, called high-temperature superconductors, make the phase transition to this state at much higher critical temperatures, typically higher than 70 K (about -334°F or -203°C) and sometimes as high as 138 K (about -211°F or -135°C).  These materials are almost always cuprate-perovskite ceramics.  They display slightly different properties than other superconductors, and the way they transition has still not been entirely explained.  Sometimes they are called Type II superconductors to distinguish them from the more conventional Type I.

Aluminum has displayed superconductivity.
Aluminum has displayed superconductivity.

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The theory of conventional, low-temperature superconductors, however, is well understood.  In a conductor, electrons flow through an ionic lattice of atoms, releasing some of their energy into the lattice and heating up the material.  This flow is called electricity.  Because the electrons are continuously bumping up against the lattice, some of their energy is lost and the electrical current diminishes in intensity as it travels throughout the conductor.  This is what is meant by electric resistance in conduction.

In a superconductor, the flowing electrons bind to each other in arrangements called Cooper pairs, which must receive a substantial jolt of energy to be broken apart.  Electrons in Cooper pairs exhibit superfluidic properties, flowing endlessly without resistance.  The extreme cold means that its members atoms aren't vibrating intensely enough to break the Cooper pairs apart.  Consequently, the pairs remain indefinitely bonded to each other as long as the temperature stays below the critical value.

Electrons in Cooper pairs attract one another through the exchange of phonons, quantized units of vibration, within the vibrating lattice of the material.  Electrons cannot bond directly to each other in the way that nucleons do because they do not experience the so-called strong force, the "glue" that holds protons and neutrons together in the nucleus.  In addition, electrons are all negatively charged and consequently repel one another if they get too close together. Each electron slightly increases the charge of the atomic lattice surrounding it, however, creating a domain of net positive charge which in turn attracts other electrons.  The dynamics of Cooper pairing in conventional superconductors was described mathematically by the BCS theory of superconduction, developed in 1957 by John Bardeen, Leon Cooper, and Robert Schrieffer.

As scientists keep discovering new materials that superconduct at higher temperatures, they are approaching the discovery of a material that will integrate with our power grids and electronic designs without incurring huge refrigeration bills.  An important advance was made in 1986 when J.G. Bednorz and K.A. Müller discovered those that work at higher temperatures, raising the critical temperature enough that the necessary coldness could be achieved with liquid nitrogen rather than with expensive liquid helium. If researchers could discover additional materials that could be used in this way, perhaps it would become economically feasible to transmit electrical power for very long distances without any power loss. A variety of other applications also exist in particle accelerators, motors, transformers, power storage, magnetic filters, fMRI scanning, and magnetic levitation

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