|First evidence of a superconductor|
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
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|
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.
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|
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.
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.
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.
Link to previous post
Link to next post
This post was written while listening to Jimi Henrix. http://www.youtube.com/watch?v=G3VOLJPcldM. Thanks Jimi.