The theoretical understanding of the phenomena of superconductivity is extremely involved. It is far beyond the scope of this guide to attempt to delve into that subject. However, in this short section, we have emphasized some of the fundamental terms and phenomena that will make it possible for you to conduct the experiments suggested with our Kits.
Superconductors have the ability to carry an electrical current without loss of energy. Unlike normal conductors of electricity in which the current is carried by individual electrons, in superconductors the current is carried by pairs of electrons called Cooper Pairs, in honor of one of the formulators of the famous `BCS' theory of superconductivity. When the electrons move through a solid in Cooper Pairs, they are impervious to the energy absorbing interactions that normal electrons suffer. To form Cooper Pairs, a superconductor must operate below a certain temperature called the Critical Temperature, or Tc. Superconductors made from different materials have different values of Tc. For the new ceramic superconductors in these Kits, Tc is about 90 Kelvin for YBa2Cu3O7, and up to 110 Kelvin for Bi2Sr2Ca2Cu3O10. The Critical Temperature Kit, Complete Exploration Kit, Super Exploration Kit, and the new Magnetic Susceptibility Kit are designed to allow you to measure Tc in a simple and elegant manner.
It is not yet clear that these ceramic superconductors indeed do conduct electricity by means of Cooper Pairs as described by the `BCS' theory. In fact another theory called the `Resonant Valence Bond' theory has been advanced as being more effective. This theory may explain the gradual onset of superconductivity at a temperature around Tc in the ceramic materials.
Since there is no loss in electrical energy when superconductors carry an electrical current, relatively narrow wires made of superconducting material can be used to carry huge currents. However, there is a certain maximum current that these materials can be made to carry, above which they stop being superconductors. This maximum current flux is referred to as the Critical Current Density, or Jc. There has been a great deal of effort to increase the value of Jc in the new ceramic superconductors. For routine electrical measurements on the samples provided in these Kits, you must remember to use electrical currents that result in current densities that are smaller than Jc.
It has long been known that an electrical current in a wire creates a magnetic field around the wire. The strength of the magnetic field increases as the current in the wire is increased. Thus, on account of their ability to carry large electrical currents without loss of energy, superconductors are especially suited for making powerful electromagnets. Furthermore, if the electrical current travels only through a superconductor without having to pass through a normal conductor, then it will persist forever resulting in the formation of a powerful permanent electromagnet (see the Superconducting Energy Storage Kit). These permanent currents in a superconductor are referred to as persistent currents. The magnetic field generated by the superconductor in turn however, affects the ability of the superconductor to carry electrical currents. In fact, as the magnetic field increases, the values of both Tc and Jc decrease. When the magnetic field is greater than a certain amount, the superconductor is quenched, and can carry no superconducting current. This maximum magnetic field is called the maximum Critical Field, or Hc. Again, this is a large field, and even the powerful rare earth alloy magnets we will be using in our experiments will not significantly affect our superconductors. The Complete Exploration and Super Exploration Kits can be used to determine both Hc and Jc.
The experiments in these Kits delve into some of the basic physics of superconductivity. These phenomena are explained in greater detail in the Experiment Guide.