Superconductors!

We have heard of electrical conductors, semiconductors, and insulators. Conductors are materials that allow electricity to pass through while insulators do not allow electricity to pass through ( well, they don’t allow direct current, but that is a story for another day; hoping that you are noting them down), and semiconductors allow current to pass through under very special circumstances.

All these three modes share one thing in common; the presence of ohmic resistance, that is, the resistance to the flow of current. But at the beginning of the 20th Century, a scientist by the name Onnes discovered a special type of material, a superconductor.

Let's put on our thinking caps as we delve into this mystery.

I will start by stating that a superconductor is a material that exhibits zero ohmic

electrical resistance. For us to fully appreciate this phenomenon, let us dive into what electrical

resistance is.

When electrons flow through a material, they may collide with impurities and imperfections in the lattice arrangement of the material (the shape formed when atoms bond with each other). This opposition to the flow of electrons is called electrical resistance. Once electrons encounter these impurities, they drop part of their energy, which is converted to heat in most cases.

This phenomenon is useful in incandescent lamps, electrical heating, toasters, electric

cookers...well, you get the drift. The higher the resistance, the more the heat released becomes. There are different materials with different resistivities (the tendency to resist the flow of current), with

nichrome having one of the highest resistivities among the conductors ( that is why it is used as

the heating element in most iron boxes) and silver having the lowest resistivity ---making it the

best conductor of electricity. One thing to note is that as the temperature of the conductor increases, its resistance increases too due to an increase in movement of the impurities.

Queue in superconductivity

In superconductors, all ohmic resistance is reduced to zero. None at all remains. Once the material is cooled to its critical temperature, all electrical resistance is lost, and superconductivity sets in. This should not happen in classical physics, since we can never attain zero kelvin (third law of thermodynamics), and secondly, even at zero degrees, according to classical physics, there should be some residual electrical resistance. Hence, superconductivity can only be explained through quantum mechanics, and the super cold metal being in another state of matter; the Bose-Einstein condensate (yes, there are more than three states of matter). This was shown by three scientists, John Bardeen, Leon Cooper, and John Robert Schrieffer, in their microscopic theory of superconductivity, where they showed how in extreme cold, electrons can pair up and attract (again, weird science) forming cooper pairs, which act as bosons (particles that can exist in the same space, compare to fermions). These bosons, being all in-ground states, allow for electrons to pass through without impeding their motion.

This brings on another effect, known as the Meissner effect. The Meissner effect occurs when a material expels all magnetic fields inside it. A cool effect is that superconductors are able to levitate in magnetic fields. This property is however destroyed once an excessive magnetic field is applied, and superconductivity is lost.

magnet levitating above superconductor. Credit: Mai-Linh Doan

So, you are probably wondering why superconductors are not the norm

Well, that’s because they are formed at very low temperatures. In fact, when Onnes discovered the first superconductor, he was working on liquid helium which was at 4.2 K, (-268.95°C). Most superconductors have a critical temperature below 30°C, but the discovery of superconductors above this temperature means that experimentation while working with liquid nitrogen is possible; making the process cheaper.

There are also other incidences of superconductors in fullerides, but factors such as anisotropy

(the quality of a material to have different properties when oriented in different ways) have to be

taken into consideration. Superconductors have many uses, such as in MRI machines where magnetic resonance is used. With superconductors, the superconducting magnet is smaller and more efficient than normal magnets. Meissner effect brings about a lot of possibilities in the field of magnetic levitation, which could be revolutionary in the transport sector.

Superconductors are also used in cyclotrons and particle accelerators to accelerate particles to speeds near the speed of light before smashing them together.

Modern research for high-temperature superconductors may revolutionize everything. We are talking about high-speed rails due to magnetic levitation ( superconductors have been found to support over 3000 times their own weight), a change in power transmission and distribution which could lower the cost of materials and reduce the amount of losses in the conductors (the reason

power is transmitted at very high voltages is because it reduces losses due to ohmic heating ).

Well, it's a wrap for today folks! More learning coming up next week.