superconductivity

What is Superconductivity?

I will keep this simple. Additional in-depth background on superconductivity is here.

The flow of electric charge through conductors is affected by the temperature of the conductor, among several other factors. The flow of charge is called the current. Current is measured in units of amperes, or amps for short.
For most conductors such as metal wire, as the temperature is cooled, the conductor offers less resistance to the flow of charge. Consequently, the cooled conductor will allow more amps of current to flow through it.

Some materials such as lead and mercury exhibit an abrupt new behavior when they are cooled to extremely low temperature by liquid Helium to about 4 degrees above absolute zero. Their resistance to electric current flow drops to zero. This superconductive property was discovered in 1911 by Heike Kamerlingh Onnes.

He studied the properties of materials at liquid helium temperatures, and discovered that metals such as lead and mercury lost all resistance when cooled to such temperatures, a phenomenon known as superconductivity (1911).

Kamerlingh-Onnes was elected to the Royal Academy of Sciences in Amsterdam and received the Nobel Prize in physics in 1913 "for his investigations on the properties of matter at low temperatures which led, inter alia, to the production of liquid helium."

Advances in Superconductivity

The production of liquid Helium is expensive and difficult. There has been an interesting search in the past 100 years for materials that would exhibit superconductivity at warmer temperatures. The holy grail would be to achieve it at room temperature requiring no expensive or technically difficult cooling.

Superconducting metals, alloys and compounds were discovered. In 1941 niobium-nitride was found to superconduct at 16 K. In 1953 vanadium-silicon displayed superconductive properties at 17.5 K. And, in 1962 scientists at Westinghouse developed the first commercial superconducting wire, an alloy of niobium and titanium (NbTi). High-energy, particle-accelerator electromagnets made of copper-clad niobium-titanium were then developed in the 1960s at the Rutherford-Appleton Laboratory in the UK, and were first employed in a superconducting accelerator at the Fermilab Tevatron in the US in 1987.

Then, in 1986, a truly breakthrough discovery was made in the field of superconductivity. Alex Müller and Georg Bednorz (above), researchers at the IBM Research Laboratory in Rüschlikon, Switzerland, created a brittle ceramic compound that superconducted at the highest temperature then known: 30 K. What made this discovery so remarkable was that ceramics are normally insulators. They don't conduct electricity well at all. So, researchers had not considered them as possible high-temperature superconductor candidates. The Lanthanum, Barium, Copper and Oxygen compound that Müller and Bednorz synthesized, behaved in a not-as-yet-understood way.

Researchers around the world began "cooking" up ceramics of every imaginable combination in a quest for higher and higher critical temperatures, Tc. In January of 1987 a research team at the University of Alabama-Huntsville substituted Yttrium for Lanthanum in the Müller and Bednorz molecule and achieved an incredible 92 K Tc. For the first time, a material had been found that would superconduct at temperatures warmer than liquid nitrogen - a commonly available coolant.

The Meissner Effect

A great step in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walther Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field. A magnet moving by an ordinary conductor induces currents in the conductor. This is the principle on which the electric generator operates. But, in a superconductor the induced currents exactly mirror the magnetic field that would have otherwise penetrated the superconducting material. That mirroring magnetic field causes the magnet to be repelled. This is known as the "Meissner effect". The Meissner effect is strong enough that a magnet can actually be levitated over a superconductive material. This short video will demonstrate the effect.