Superconductors

An extreme example of a diamagnet is a superconductor. These materials are known primarily through their electrical properties - at some relatively low temperature their electrical resistance is exactly zero. Thus, one can establish a current in a superconductor and it will never die away due to resistance, even when the source of potential difference that started the current is removed. Superconductors also have interesting magnetic properties; they are perfect diamagnets: when an applied magnetic field is applied, eddy currents in the superconductor induce a magnetic field which exactly cancels the applied magnetic field. This is the Meissner effect.

This effect is responsible for the magnetic levitation of a magnet when placed above a superconductor. Suppose, as in Fig. 9.17, we place a magnet above a superconductor. The induced magnetic field inside the superconductor is exactly equal and opposite in direction to the applied magnetic field, so that they cancel within the superconductor. What we then have are two magnets equal in strength with poles of the same type facing each other. These poles will repel each other, and the force of repulsion is enough to float the magnet. Such magnetic levitation devices are being tried on train tracks in Japan; if successful, this would make train travel much faster, smoother, and more efficient due to the lack of friction between the tracks and train (in some cases, rather than superconductors, strong electromagnets are used to provide the magnetic levitation).

Despite these interesting properties, superconductors are not widely used in today's world, outside of as electromagnets to generate strong magnetic fields in certain medical diagnostic devices and in particle accelerators. The reason for this is that superconductors exist only below a certain critical temperature, and above that temperature they behave like normal materials. When first discovered these critical temperatures were of the order of 10 K (about -260^o C), which was (and still is) fairly difficult to reach (this is about the temperature at which helium liquefies). However, recently high temperature superconductors have been discovered which have critical temperatures of the order of 100 K and above (about -170^o C). This is about the temperature that nitrogen liquefies, and is relatively easy to reach with today's technology - ``dry ice'' is liquid carbon dioxide at this temperature. These developments has spurred research into other uses of superconductors such as in magnetic levitation devices and as circuit elements in computers to increase speed by cutting down on resistance.