Superconductivity - an introduction

Meissner Effect

Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, a Dutch bloke who was very good at making fridges. He was awarded the 1913 Nobel Prize for Physics for this discovery.

While testing the "current best" theory on electricity he made a truly amazing discovery. He was cooling mercury and measuring it's resistance as a function of temperature. As the temperature of the mercury got less and less so did the electrical resistance of the mercury and it should have continued on this way until you got right down to absolute zero, which is the lowest temperature we can reach. Instead of this, when the temperature reached about 4.25 Kelvin, which is 4.25 degrees above absolute zero, a seriously cold temperature (outer space is 3 Kelvin) the resistance of mercury suddenly disappeared totally. 

It no longer had any measurable resistance whatsoever. Further research showed that this was not the only material to undergo this transition at low temperatures. Many metals have superconducting properties when cooled far enough. In recent times some ceramics have been found that demonstrate "high temperature" superconductivity. This is a relative term and often refers to materials that are superconducting just above the boiling point of liquid nitrogen, which is 77K (-196 Celcius).

For me there is nothing in physics that is more interesting, more puzzling and potentially more revealing about the universe than a superconductor. At the time of writing there is no accepted theory for high temperature superconductors.

Note: Low temperature superconductors can be described by the BCS theory (Bardeen Cooper Schrieffer - 1956), but it cannot account for high temperature materials.

One of the things that really amazes me is the flow of electric current in a superconducting material. Normally the world can be seperated into conductors at one end of the spectrum and insulators at the other, with every material fitting in somewhere between these two extremes. Metals are good conductors, most plastics are good insulators, Silicon is neither, it is a semiconductor. When a voltage is applied across a material a current will flow which is proportional to the voltage. The most common case of this are materials that are said to obey Ohm's law,

V = I R , V - voltage, I is current and R is resistance of the material. So if you have 240 V and the resistance is 120 Ohms, then

240 = I x 120, I =2 Amps.

But with a superconductor R = 0. This means that you can have a current flowing without a voltage. If the superconductor is like a doughnut shape and the current is circulating round the doughnut, it will continue to do so forever. It will never stop, it will just go on and on. Just like the Terminator!

So what do we know about them, besides them having no resistance? Well, when superconductors are placed in a magnetic field they create their own magnetic field opposite in magnitude to the field. So you get a South - South, or a North-North. This means that the magnetic field does not penetrate into the body of the superconductor. This can be seen most dramatically in the Meissner effect (see above).

The electrons in a superconductor pair up into what are known as Cooper pairs and it is these that move through the superconductor.

They can be used to produce extremely sensitive detectors of magnetic fields called SQUIDs or Superconducting Quantum Interference Devices.

They can often carry massive currents, way beyond the capacities of normal metals. 10 of thousands of Amps of current can be carried in superconducting wires no more than a few millimeters in diameter.

Should a room temperature superconductor be discovered the world will change as dramatically as it did in the last century because of oil and electricity. The uses that superconducting materials will be put to really is beyond the imagination. Already high speed maglev trains use the Meissner effect to float above the rail tracks. Speeds in excess of 500 km/h have been achieved by this train. There are also other "specialist" applications using superconductors, but this is small compared to what would be available if and when room temperature superconductors become available.

I truly believe that a theory of superconductivity will not only explain how high temperature superconductors work, but more fundamental properties of the universe will also be revealed. This subject will be covered in future posts.