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Superconductors

So far, we have talked about electric charges and fields, and we have talked about conductors, real and ideal.

We started with "ideal" conductors, we defined these as materials in which charge is completely free to move, and in which there are arbitrarily large amounts of positive and negative charge available (charge is infinitely divisible). From these requirements, we derived a few useful facts - Any excess charge on a conductor must reside on its surfaces, no electric fields can exist within the conducting medium, and any conducting object must be at a single electric potential.

Next, we moved on to real conductors, we talked about the microscopic model of conduction in terms of free charges scattering within the conductor, and we characterized the material by its resistivity. We also discussed circuits, in which both real conductors (resistors) and ideal conductors (wires and capacitor plates) played a role.

Are these "ideal" conductors a useful concept without any physical analog, or do they really exist? To quote a popular commercial, "Not exactly." There are no materials which are simply perfect electrical conductors and have no other properties to speak of. However, there are materials which conduct electric current with zero resistance: superconductors.

Superconductivity is one of the strangest and most exciting discoveries of the twentieth century. Even though physicists had used the idea of perfect conductors for many years, they never really imagined that such a material would be found. However, in 1911, a Dutch physicist named Heike Kammerlingh Onnes discovered that mercury (Hg) lost any trace of resistivity if cooled to the temperature of liquid helium. Two years later, he won a Nobel prize for his discovery.

In the years since Onnes discovery, 25 other chemical elements were found to become superconductors at very low temperatures. The temperature at which a material becomes a superconductor is usually called T_c, which stands for "critical temperature." the highest critical temperature among the chemical elements is 9.5 K (for niobium, Nb). the lowest is 0.012 K (for tungsten, W). Until recently, the highest critical temperatures were to be found in niobium alloys such as NiTi (niobium titanium) and NbSn₃ (niobium tin).

The image at the right is the crystal structure of the first material known to be a superconductor at the temperature of liquid nitrogen (77 K),YBa₂Cu₃O₇. This is quite cold, but it is far warmer than the previous record, only 23 K. Furthermore, liquid nitrogen is fairly cheap and readily available in most places. This makes it a far more attractive coolant than liquid helium, which is far more expensive and is totally unavailable in many parts of the world. When we describe the resistance of a superconductor as zero, do we really just mean very small? No! According to the theory of superconductivity if a current is induced in a superconducting ring, the lifetime of that current is predicted to be ten to the ten to the ten years. This is to large a number to verify directly, but the best experiments thus far establish a lower bound of 10⁵ years.

For many years after Onnes discovery superconductivity was known to exist only near absolute zero, and only in a few pure elements. Later, superconductivity was discovered in several compounds and alloys. In some cases, these materials were found to superconduct at higher temperature than any of the known pure elements. In fact, for many years the "record" was held by an alloy Nb₃Ge which becomes superconducting at about 23.2 K.

In recent years, interest in superconductivity has blossomed with the discovery superconductivity at far higher temperatures. Two broad categories exist. Superconductivity was discovered in ceramic materials in 1986 by A. Muller and G. Bednorz, two scientists working at the IBM research lab in Zurich, Switzerland. Their samples showed signs of superconductivity at 35 K. Within a year, scientists all over the world were working to find new ceramic superconducting compounds. This activity culminated in the "Woodstock of Physics" an all night session at an American Physical Society meeting in March of 1987. At that meeting, several researchers showed data indicating that they had purified samples of YBa₂Cu₃O₇that showed superconductivity at about 93 K. Soon thereafter, Bednorz and Muller were also awarded the Nobel Prize.

Since 1987, physicists have discovered ceramic compounds that superconduct at even higher temperatures. In addition, they have found superconductivity in compounds based on "buckyballs" spherical carbon molecules whose shape is quite similar to that of a soccer ball.
In addition to their electronic properties, superconductors also have magnetic properties (see the picture at the top of this page!). In terms of the terminology in your text, superconductors are diamagnetic. That is, they have a negative susceptibility, and acquire a polarization opposite to an applied magnetic field. This is the reason that superconducting materials and magnets repel one another. Of course, unlike other diamagnets (such as copper) superconductors have a very LARGE negative susceptibility, so the repulsive force is LARGE as well.

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