Physics of Superconductors
Levitating under the Track!

A flow of charge is current, and hindrance to this flow is called as resistance. Resistance requires energy for the current to pass through a conductor and hence a power supply is needed to drive electrons across a conducting wire. A simple experiment in which both ends of a bulb are connected to a copper wire fail to glow it and this is because of the lack of a driving factor like battery. A battery provides the electrons in the copper wire energy to go against resistance. Heike Kamerlingh Onnes in an attempt to measure the resistance of mercury at liq. Helium temperature (4.2 Kelvin) noticed a sudden drop in its resistance. This discovery led to a new field of study called as superconductivity and the associated materials are called superconductors. Though not actively discussed during our physics tuition classes as it is out of the Cambridge A level syllabus, this field is very useful and interesting. To elucidate the repercussions of the discovery, let’s think of a situation where the two ends of a copper wire are connected to a battery and let’s say the battery is removed and the two wires are joined. There would be no current in the copper wire. A similar experiment done using a superconducting wire shows the opposite result and this property is called superconductivity. So, if this were the case an obvious question that pops up would be why don’t we use superconductors and save energy in the power transit?

The answer lies in the fact that there are no materials that can exist as superconductors at the room temperature. There is a quantity called critical temperature (Tc) beyond which a material loses its superconducting property. A wire conducting electricity generates magnetic field. Thus superconductors become the right choice for electromagnets, where the current need not be continuously triggered using a power source. In the Large Hadron Collider, using which the existence of Higgs bosons was confirmed; superconducting magnets are used to generate the necessary magnetic field to accelerate the particles.

An interesting phenomenon associated with superconductors that further helped in theorizing superconductivity is Meissner effect. The descent of a magnet along a pipe made of conducting material is slower compared to its free fall. This phenomenon can be explained by Lenz’s law, where the falling magnet generates current in the conducting pipe that in turn generates magnetic field that opposes the conduction of magnet along the pipe. Analogous to this, a superconductor opposes the flow of magnetic lines through it and this phenomenon is called as Meissner effect. The magnetic flux generates current and because of the absence of any resistance in the material this current can be of any value and is unopposed. Thus generated current in turn generates magnetic field in the opposite direction and cancels the net magnetic field inside the superconducting material. On these lines, superconductors are also called as perfect diamagnets.

Another interesting aspect is that the magnetic can penetrate up to a very tiny depth on the surface of the material. This depth is called London penetration depth and is a characteristic property of the superconducting material. A magnet placed on superconductor surface floats because of this effect. A superconductor need not be metallic. Although there are a lot of metallic superconductors like Mercury, Lead etc. there are also non-metallic organic superconductors like carbon nanotubes.