In this article, I will discuss the phenomenon of superconductivity, and in particular superfluid helium. This area of physics belongs historically to the branch of physics which is called low temperature physics. This is so because it was first discovered during research that was conducted on metals at temperatures close to zero kelvin.
The discovery of superconductivity traces back to 1911 and is attributed to the physicist Kamerlingh Onnes who did his research in the area of low temperature physics at the University of Leiden in the Netherland. During his research on electricity in metallic wires at low temperatures he observed that in certain materials electric current could flow without resistance and without the loss of heat.
Interest in electrical resistivity was prevalent due to its potential applications. The variation of electric resistance as a function of temperature was a potential use in producing thermometers from metal wires. Also it was interesting to search for materials that have zero electric resistance. This would make the use of electric conductance much cheaper due to the low heat dissipation and the more efficient current flow in the wires.
Scientists at that time thought that the conductance band in metals contains free moving electrons that behave like electrons gas and they thought that freezing these electrons gas by lowering the temperature of the metal would increase the electric resistance. However, this was not observed and the resistance decreased with decreasing the temperature of the metal.
Superconductivity is associated with several characteristics of the superconducting material. These include the expulsion of the magnetic field from the interior of the superconductor. It is known experimentally that magnetic fields can destroy the superconducting properties of a superconductor. Also the electric resistance of a superconducting metal has zero value. This can be used in designing materials for conducting electric power through superconducting wires.
Superconductivity was observed also in liquid helium. Helium is a unique atom since it does not solidify at 0K. There are two types of helium, He3 and He4. He3 behaves like fermions and therefore it obeys the Fermi-Dirac statistics. He4 is a bozon. Thus it obeys the statistics of Bose-Einestein. It is He4 that becomes superfluid below a certain temperature of 2.17K at which He4 becomes superfluid.
This transition gave He4 its characteristic two phases He1 and He2. He2 is the phase that behaves like a superfluid. The transition between He1 and He2 phases is a Bose-Einestein condensation. The heat conductance in He2 is remarkably high. The difference between He1 and He2 is that He1 contains helium atoms which are energetically excited. While He2 contains atoms in the ground state of zero energy E=0.