Superconductors
by Schuyler Corry
The Superconductor is a Grail of modern physics; a substance that could
revolutionize all electronic devices as we know them, and conserve electric
energy. Since their discovery in 1911, engineers have looked for a better
superconductor, one which will function under easily attainable conditions.
The first superconductors only displayed superconductivity at temperatures
near 0K, never above 23K. In recent years, a goal has been to find a
substance that becomes superconducting above the boiling point of liquid
nitrogen.
What is a Superconductor?
By definition, superconductors are substances which display zero
electrical resistance when a current is supplied, and are able to propagate
such a current in a circuit indefinitely. Another property of
superconductors, used to test a potential superconductor, is the inability for
a magnetic field to exist within the material when a current is applied. All
known superconductors become superconducting at very low temperatures -- none
above about 125K -- and this specific temperature is called the critical
temperature, abbreviated Tc.
Because they have no resistance, superconductors do not lose any
current in the form of heat, and are therefore totally energy efficient
(except for the energy required to cool them, at Earth temperatures).
Zero resistance allows for many strange phenomena, including the ability to
levitate a magnet in mid-air. This phenomenon made superconductors known to
the general public, from pictures on television.
In theory, high-temperature superconductors could improve all existing
electronics. By replacing copper with superconductors, no energy would be
wasted in electronic devices; released in the form of useless heat. Few
devices actually utilize the heat produced as a product of resistance --
the exceptions being heaters, toasters, hair-dryers, and incandescent light
bulbs -- and the rest try to use low-resistance materials in their circuits.
In addition to devices we use daily, large-scale instruments and apparatuses
are greatly hindered by resistance. The first example that comes to mind are
particle accelerators. Accelerators guide particles and accelerate them using
very strong magnetic fields. When intense current is applied to
electromagnets, they heat up, and with enough current melt or disintegrate.
In addition, excess power is necessary to produce so much heat energy.
Another application theoretically available using
superconductors is levitating trains. Again, the superconductors
magnetic potential would be used, and the trains would hover above a constant
field that requires no energy to sustain. Air resistance would be the source
of friction, and only minimal electricity would be used to accelerate the
train.
Some inventions already incorporate superconductors: the first
commercial success on Earth was Nuclear Magnetic Resonance Imaging (NMRI).
NMRI is the best way to obtain images from inside the body, without actually
cutting someone open. Using liquid-nitrogen-cooled superconductors, the NMRI is cheap to use,
and only exposes patients to harmless magnetic fields, instead of X-rays.
The BCS theory began in March, 1957, when John Bardeen, Leon Cooper,
and Robert Schrieffer published a paper. The three physicists at the
University of Illinois were responsible for the only accepted theory on
superconductivity, until the 1980s. BCS theory, like all the superconductor
theories, concerns the behavior of electrons at low temperatures. After all,
electrons are responsible for regular conductivity, being the carriers of
electric charge. BCS theory says that at low enough temperatures, the
electrons in a crystal lattice will behave cooperatively, a sea of electrons.
When temperatures drop even more, though, the electrons began acting
strangely. An electron passing between two nuclei (positively charged), will
create a ripple of non-uniform charge, as the nuclei are pulled slightly
toward the electron. Neighboring electrons then are attracted to the
positive charge, and follow along in the wake of the first electron, forming
an electron pair. This is so strange because electrons normally repel each
other, but apparently enough evidence exists to convince most scientists
that the pairings do exist.
Electron pairing is only the first part of BCS -- it gets even weirder.
The Pauli Exclusion Principle states that no two fermions (ie., electrons)
can have the same set of quantum numbers. This property of electrons is
very important to matter as we know it -- if electrons could all have the
same quantum numbers, they would all jump down to a ground state, a state of
lowest energy, and there would be no chemical interaction. Bosons, unlike
fermions, can have the same set of quantum numbers, and in phenomena like
lasers and masers (the microwave equivalent of a laser), bosons join together
and form amplified beams of resonating particles. Bosons actually induce
other bosons to have the same states. At low enough temperatures, the
electron doublets act like bosons, and move in synchronicity through the
conductor. No electrons are deflected in the lattice, and no energy is lost.
The BCS theory is definitely wrong about at least one thing: the temperature
range of superconductivity. BCS predicted a 23K limit on superconductors,
but certain types of material superconduct at temperatures greater than 125K.
Phillip Anderson, of Princeton, and Sudip Chakravarty, of UCLA, have
come up with a new superconductor theory. Their theory is mainly used to
describe the behavior of high Tc superconductors, yttrium-barium-copper-oxides
also known as cuprates. The Anderson-Chakrvarty theory takes a
non-traditional stand on electron interactions with matter. Ordinarily, we
think of electrons as passing through conductors more or less unbound. The
nuclei and other electrons supposedly have little effect on electron movement.
If Anderson and Chakravarty are correct, the forces on these conducting
electrons may be much greater than was supposed. According to the theory, at
room temperature, the forces on electrons are sufficient to break some of them
into two particles, called holons and spinons. At lower temperatures, the
electrons again form pairs, but the strange stability of new superconductors
still has to be explained. The copper oxide layers serve as the "conductor"
part of the superconductor, and the yttrium and barium atoms are a sort of
barrier, through which the electron pairs must tunnel, to get to the next
conductive layer.
Electrons and other small particles can be accurately modeled as waves,
using Schrodinger's equation to predict probabilities of the particle being
at a certain location. Definites of the macro-world don't apply to such small
particles, and these particle-waves can do extraordinary things (to us), like
tunnel through barriers they shouldn't be able to surpass. In this tunneling,
the particles lose kinetic energy, or slow down. When an electron pair
tunnels through a yttrium or barium barrier, the pair loses energy, becoming
more stable. Where ordinary superconductors would break down, the special
cuprates retain their superconductivity.
Anderson and Chakravarty's theory explains several superconductor
phenomena. First, there is the observation that more layers in a
superconductor lead to a higher critical temperature. Since the layers give
more stability to the electron pairs, this model gives a perfect explanation.
Also, there is an experiment concerning the reflection of light. Reflected
light is just the emission of visible radiation by excited electrons, so a
higher degree of reflection in a given material means there are more electrons
in the material. Superconductors are found to have more reflectivity while
superconducting, and this theory says that there are more whole electrons at
superconducting temperatures -- some electrons are split up at room
temperature.
A third major theory has been proposed by Douglas Scalapino, of the
University of California at Santa Barbara, and David Pines, of the University
of Illinois. Pines and Scalapino's theory is not much different from the BCS
theory -- in the new theory, electrons again pair up, but this time due to
a different one of the four forces, the magnetic force. Whereas BCS
predicted incorrect limits on the highest possible Tc, Pines and Scalapino
predict stability at higher temperatures due to the different nature of the
force holding the pairs together.
In addition to the experiments that support the BCS theory, there is
one major experiment that reinforces Pines and Scalapino's ideas. Their
theory says that in a superconductor, if two perpendicular currents are
applied, the electron waves will cancel each other out, producing a net
current of zero. When this hypothesis was tested, it turned out to be
accurate.
Both new theories have plenty of evidence to support them, but many
physicists still think the true explanation for superconducting has yet to be
found. Perhaps the lack of a universally accepted theory doesn't matter much
to the scientists looking for new superconductors. Pioneer Bernd Matthias
never trusted theories that couldn't make predictions, and by fiddling around
in his lab, he discovered many new superconductors. Under similar conditions
today, but with better equipment, researchers scan the periodic table almost
randomly, but usually incorporating copper and oxygen somewhere, to come up
with new and improved high temperature superconductors.
Most "high temperature" superconductors being used today are yttrium-
barium-copper-oxide compounds, or at least contain copper-oxides. When the
first of this family of compounds was discovered by scientists in Houston,
Texas and Alabama, other superconductivity researchers rushed to improve upon
the original compound. One common technique was to replace one of the
elements of the compound with another element in the same group of the
periodic table. For example, silver or gold might be used instead of copper.
A problem with this technique is that the resulting compound must have
the right ratio of atoms -- in the case of the yttrium-barium-copper-oxide,
the magic ratio is 1-2-3-4. Another problem is the toxicity of some elements.
One superconducting compound broke all temperature records, but it contained
the highly poisonous element thallium, used in rat-poisons. Some researchers
were scared to try to make the compound, because of toxicity.
One of the biggest successes in the recent past is the production of a
flexible yttrium-barium-copper-oxide tape, created in April 1995 by a team
at Los Alamos National Laboratory. This tape can carry 1 000 times the
current of a traditional copper wire, and 100 times the current of the top
superconducting wires. If the current rate of discovery continues, we may
very well have levitating trains by the end of the century.
Sources:
"The Path of No Resistance" by Bruce Schecter. New York:
Simon and Schuster, 1989.
"Call Them Irresistible" by Tim Folger. Discover magazine, September
1995. Click here to read this