Thursday, 21 February 2013

Superconductors - what do we know

First evidence of a superconductor
The most famous thing about a superconducting material is its apparent lack or resistance. I say apparent because we will never know for sure that the resistance is absolutely zero. We only know that the most sensitive experiments so far tried have failed to detect any resistance.

Onnes, the discoverer of superconductivity, performed the best test for detecting resistance. He tried to detect any decay in an electric current flowing in a closed superconducting ring. If any resistance to electric current exists then the superconducting current would gradually be converted into Joule heat. No such decay was observed. Variations on this experiment designed to give even greater accuracy failed to detect any resistance whatsoever.

So the first thing we will say is that

superconductors offer no resistance to electric currents

This is true of all superconductors and is one of the tests used to define a superconductor.

Discontinuity in the heat capacity shown in blue
The second thing we will say is that this is not entirely true. If we use an alternating current rather than a direct current then we cannot say that the resistance is zero (more on this later).

If we continue to increase the current in a superconducting wire then we will reach a point where the superconductor stops being a superconductor, this point is known as the critical current. The critical current is temperature dependent and will be examined in further detail later.

Measurements of the specific heat capacity of a superconductor show a discontinuity at the critical temperature of the superconductor.

Any theory that explains superconductivity will have to explain how current can circulate in a superconducting ring forever. 

There was no theories around at the time that superconductivity was discovered that where adequate to explaining this fantastic behaviour. The development of quantum theory has offered a possible solution. Superconductivity is described as a "macroscopic quantum phenomenon".

The Meissner Effect
The second amazing property of a superconductor is the Meissner effect. In 1933 it was discovered that superconductors expel magnetic fields. What this means is that if a metal is placed inside a magnetic field, the magnetic field will penetrate the metal. When the material is cooled and becomes superconducting the magnetic field is completely expelled from the interior of the superconductor except for a very thin layer at the surface of the material. This is known as an ideal diamagnetic state and is another definitive test of superconductivity.

If the magnetic field is increased, much in the same way we discussed increasing the electric current, then another feature of superconductivity is discovered. Some superconductors will eventually stop being superconductors when the magnetic field reaches a critical value, BC. These are Type I superconductors. Other superconductors allow the magnetic field to start to penetrate the material at a value BC1, but continue to be superconducting until a second value BC2 is reached. These are known as Type II Superconductors.

superconductors are perfectly diamagnetic

The magnetic field that penetrates Type II superconductors is quantized. This means that it has very specific values, rather than just random magnetic values.

Not all elements are superconductors. Copper for example. Yet Yttrium Barium Copper Oxide is an example of a High Temperature Superconductor. Below is a list of all the elements that become superconducting. Note that some require high pressure.

Superconductors have different critical temperatures. The critical temperature of an element or material is the temperature that the element or material becomes superconducting. In 1987 the first high temperature superconductors were discovered. These ceramic materials had critical temperatures in excess of 100 K. No superconductivity theory can explain temperatures this high.

different isotopes of an element have different Tcs 

The final property of superconductors will be mentioned here is the Josephson effect, which earned its discoverer Brian Josephson the Nobel Prize. This will be covered in detail later, but is such an important property of  superconductivity that it gets a mention here.

The Josephson effect predicted how two superconductors should behave if they are separated by a thin insulating material. The idea depends on quantum mechanics and was found to be true.

So to round this all up. Any theory of superconductivity has to explain the following;

  • Zero resistance
  • Meissner effect
  • Discontinuity in the Specific heat capacity
  • Type I and Type II behaviour, ideally with some values for the magnetic field strengths
  • The isotope effect
  • The Josephson effect
  • The maximum critical temperatrue

That's enough for starters I reckon.

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This post was written while listening to Jimi Henrix. Thanks Jimi.

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