Discovery and properties
Electrical Resistance – the perfect conductor
Before the successful liquefaction of helium, scientists were unsure about the full temperature dependence of the electrical resistance of metals. It was known that in the region of room temperature, resistance dropped linearly with decreasing temperature. However, as the temperature was lowered this linear relationship failed and the reduction in resistance became smaller. Thus three possibilities were postulated
In fact an entirely different dependence was discovered. Using mercury, which could be easily be made very pure, Onnes discovered that, instead of a smooth transition down to zero resistance that had been proposed, at about 4.2K the resistance of the wire suddenly dropped to below the accuracy of his instruments. The resistance had indeed disappeared and he had discovered a new state, which he named the superconducting state. The temperature at which the transition to superconductivity occurs is known as the critical temperature, Tc.
In order to discover whether the resistance was in fact zero, or just very low, an experiment was designed to measure how long the current would flow in a ring where a current had been induced by a magnetic field. As any measurement of the current would inevitably alter it and introduce some resistance, the magnetic field produced by the flowing current was measured instead.
It has been found that there is no reduction in current over the time period that anyone has had the patience to measure it (the record is over 2 years!). This proved that the resistance was indeed zero.
Many other elements were subsequently found to exhibit a transition from normal to superconducting behaviour at some critical temperature and still more are found to be superconducting if high pressure is applied.
Magnetic Properties – perfect diamagnetism
In 1933 another breakthrough was made in the subject when Meissner and Ochsenfeld started to investigate the magnetic properties of materials as they transitioned from normal to superconducting behaviour. What they found was entirely unexpected and lead to the formulation of a theory that could explain superconductivity.
They observed that when a superconducting material was placed within a magnetic field, the field was completely expelled from the interior of the sample. The ability of a material to partially expel a magnetic field was known already and is called diamagnetism. Almost all materials exhibit some degree of diamagnetism although the effect is usually tiny. In the case of superconductors the effect is large and unexpected.
The phenomenon can be explained by considering a solid sphere of superconducting material. If a magnetic field is then applied, currents are induced in the surface of the sphere, which exactly oppose the applied field and cause no magnetic field to penetrate the sample.
This phenomenon is shown to dramatic effect when a section of superconducting material is placed above a magnetic track. The field from the base is excluded from the superconductor and it levitates. If the superconductor is tapped sideways, it will travel around the track with virtually no resistance to its motion. The video below shows this happening.
Until 1986 it was thought that superconducting behaviour was confined to certain materials at temperatures below ~30 K. A theory called “BCS theory” after its creators John Bardeen, Leon Cooper and Robert Schrieffer had been formulated to describe superconductivity. This theory, for which its creators received the Nobel Prize in Physics in 1972, appeared to back this up but put a limit on the critical temperature of around 30 K. However, in 1986 a new class of ceramics were discovered to have critical temperatures far in excess of this, much to the amazement of the scientific community. Research into this family of ceramics quickly yielded materials with critical temperatures in excess of 77 K. This breakthrough meant that superconducting behaviour could be observed using liquid nitrogen temperatures instead of the far more expensive liquid helium temperatures that had been used previously. The graph below charts the development of superconducting materials.