Superconductivity part 2

High Temperature Superconductor

A while back I did an introductory post on superconductivity. In this second post I am going to dig a little deeper into this wonderful subject to describe the Meissner effect and its consequences.

Briefly, a superconducting material will exclude any magnetic field during its transition to the superconducting state. If the material is already superconducting when a magnetic field is applied it will set up an electric current near the surface. The magnetic field associated with this surface current will cancel the applied magnetic field. This is the Meissner effect and was discovered in 1933 by Walther Meissner and Robert Ochsenfeld. Meissner was the boss, so he got the credit!

Above it's critical temperature, T_c, the temperature at which it becomes a superconductor, a superconductor is just a regular material, for metals it behaves Ohms law for electric flow etc.
At  T_c, we have something called a phase change (water turning into ice is a phase change), at this point its resistance drops to zero. In addition it becomes a perfectly diamagnetic.

Diamagnetic materials are those that create a magnetic field in opposition to an externally applied magnetic field. For many materials this diamagnetic effect is very weak, and the magnetic field goes straight through the material. In superconductors the magnetic field is completely expelled from the interior of the superconductor. The magnetic field penetrates a very thin layer of the superconductor known as the London penetration depth. This is named after the two brothers who discovered it.

Furthermore, it was realised that the superconducting current does not travel through the body of the superconductor but in a thin layer of the material, which is the London penetration depth.

So imagine, we have a magnetic field penetrating our material. The temperature drops and we reach T_c. At this point things start to change. After the change and when things have settled we have an electric field expelling the magnetic field, the Meissner effect.

Above T_c we have magnetic field through the material, no current flowing. Below T_c we have no magnetic field in the material because we have a current flowing that cancels the field. So during the phase change we witness the creation of an electric current to cancel the magnetic field. 

Although we have now known about this effect for coming up to 80 years there is no dynamical explanation of the Meissner effect with the conventional understanding of superconductivity. We know what happens, the expulsion of the magnetic field. We cannot explain the stage when the super-current (this is the current that flows without resistance) goes from zero in the normal phase to the steady current required to exclude the field in the superconducting phase.

Many of the great minds in physics have had a go at this one including Einstein and Feynman and they have all failed to find a solution.

It may come as a surprise that we cannot explain what is probably the best known property of superconductivity. So let's have a little think and see if we can actually figure out just why it is so difficult.

Well it is difficult because we don't actually have a working theory for superconductivity, we have some good guesses and a real good theory called BCS (Bardeen, Cooper, Schrieffer - the authors), but none of them really hit the mark.

We do know an awful lot, but it just doesn't give us the clue we need to get a real break through. We may be just waiting for that single piece of information that opens the door and gives us a solution. Though the same argument could be applied to many areas of Physics.

I decided to cut this post short, I'll explain why later :-)