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.
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.
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