Superconductivity
Superconductivity is a phenomenon observed in several metals and ceramic
materials. When these materials are cooled to temperatures ranging from
near absolute zero ( 0 degrees Kelvin, -273 degrees Celsius) to liquid nitrogen
temperatures ( 77 K, -196 C), their electrical resistance drops with a jump
down to zero.
The temperature at which electrical resistance is zero is called the critical
temperature (Tc) and this temperature is a characteristic of the material as it
is shown in the following table:
Material Type Tc(K)
Zinc metal 0.88
Aluminum metal 1.19
Tin metal 3.72
Mercury metal 4.15
YBa2Cu3O7 ceramic 90
TlBaCaCuO ceramic 125
The value of the critical temperature is dependent on the current density and
the magnetic field as shown in this picture.
The cooling of the materials is achieved using liquid nitrogen or liquid helium
for even lower temperatures.There is already in this small table a clear
separation between the low and high temperature superconductors. While
superconductivity at low temperature is well understood, there is no clear
explanation as yet of this phenomena at "high temperatures".
The critical temperature is known to be inversely proportional to the square
root of the atomic mass. Take a look at the periodic table , to see which
elements have been found to have superconducting properties.
Some background .....
Electrical resistance in metals arises because electrons moving through the
metal are scattered due to deviations from translational symmetry. These are
produced either by impurities, giving raise to a temperature independent
contribution to the resistance, or by the vibrations of the lattice in the metal.
In a superconductor below its critical temperature, there is no resistance
because these scattering mechanisms are unable to impede the motion of the
current carriers. As a negatively-charged electron moves through the space
between two rows of positively-charged atoms, it pulls inward on the atoms of
the lattice. This distortion attracts a second electron to move in behind it.
An electron in the lattice can interact with another electron by exchanging an
acoustic quanta called phonon. Phonons in acoustics are analogous to
photons in electromagnetic. The energy of a phonon is usually less than 0.1
eV (electron-volt) and thus is one or two orders of magnitude less than that of
a photon.
The two electrons form a weak attraction, travel together in a pair and
encounter less resistance overall. In a superconductor, electron pairs are
constantly forming, breaking and reforming, but the overall effect is that
electrons flow with little or no resistance. The current is carried then by
electrons moving in pairs called Cooper pairs.
A Cooper Pair moving through the lattice
The second electron encounters less resistance, much like a passenger car
following a truck on the motorway encounters less air resistance.
Below the critical temperature these superconducting materials have no
electrical resistance and so they can carry large amounts of electrical current
for long periods of time without loosing energy as ohmic heat. For example,
loops of superconducting wire have been shown to carry electrical currents for
several years with no measurable loss. This property offers tremendous
challenges and opportunities in the modern world.
MEISSNER EFFECT
Another property of superconducting materials is the Meissner Effect. It was
observed that as a magnet is brought near a superconductor, the magnet
encounters a repulsive force. It can be said that the superconductor
completely expels the magnetic field and behaves as a perfect diamagnet.
The classic demonstration of the Meissner Effect.
A superconductive disk on the bottom, cooled by liquid nitrogen, causes the magnet above to
levitate. The floating magnet induces a current, and therefore a magnetic field, in the
superconductor, and the two magnetic fields repel to levitate the magnet.
This property has implications for making high speed, magnetically-levitated
trains, for making powerful, small, superconducting magnets for magnetic
resonance imaging, etc.
JOSEPHSON EFFECT
One other property of superconductors is that when two of them are joined by
a thin, insulating layer, it is easier for the electron pairs to pass from one
superconductor to another without resistance . This is called the Josephson
Effect. This effect has implications for superfast electrical switches that can be
used to make small, high-speed computers.
SPECIFIC HEAT
In a superconducting phase transition, the electric resistance changes with a
jump, while the energy undergoes a continuous variation. The specific heat,
or the amount of heat necessary to affect its temperature, also changes with a
jump. When a substance is cooled, its specific heat typically decreases but at
the critical temperature it increases suddenly.
SUPERFLUIDITY
This phenomenon was first observed in helium at a temperature below 2.17K.
Helium at these low temperatures was seen to flow quite freely, without any
friction, through any gaps and even through very thin capillary tubes. Once
set in circular motion, for example, it will keep on flowing forever - if there are
no external forces acting upon it. Unlike all other chemical elements helium
does not solidify when cooled down near absolute zero. Physicists explain this
phenomenon by extremely weak attractive forces between the almost
"perfectly round" atoms and by their rapid motion which is due to Heisenberg's
Uncertainty Principle
Bulk superfluid helium has many unusual properties - it can flow up walls
and through narrow pores without resistance. Helium-4 and Helium-3 become
superfluid below 2.12 and 0.003 Kelvin respectively. However, only a
proportion of the Helium becomes superfluid at the transition temperature.
This free movement of helium at a temperature below 2.17K looks very much
like the superconductivity behaviour mentioned above. To explain this
frictionless motion, we can imagine that all the particles in the liquid are
linked together and none of them can be separated, without violating the
whole state.
CONCLUSION
Both these two special properties were described by Ziman with a very apt
epigram: "The more of us gather, the merrier we are together".
The future of superconductivity research is to find materials that can become
superconductors at room temperature. Once this happens, the whole world of
electronics, power and transportation will be revolutionized
.
