Superconductor is an element, inter-metallic alloy, or compound that will conduct electricity without resistance below a certain temperature. Resistance is undesirable because it produces losses in the energy flowing through the material.

 

Once set in motion, electrical current will flow forever in a closed loop of superconducting material ? making it the closest thing to perpetual motion in nature. Scientists refer to superconductivity as a ?macroscopic quantum phenomenon?.

 

Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by exactly zero electrical resistance and the exclusion of the interior magnetic field (the Meissner effect).

 

The electrical resistivity of a metallic conductor decreases gradually as the temperature is lowered. However, in ordinary conductors such as copper and silver, impurities and other defects impose a lower limit. Even near absolute zero a real sample of copper shows a non-zero resistance. The resistance of a superconductor, on the other hand, drops abruptly to zero when the material is cooled below its ?critical temperature?, typically 20 kelvins or less. An electrical current flowing in a loop of superconducting wire can persist indefinitely with no power source.

 

Superconducting Phase Transition
In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature T_C. The value of this critical temperature varies from material to material. Conventional superconductors usually have critical temperatures ranging from less than 1 K to around 20 K.

 

Meissner Effect
When a superconductor is placed in a weak external magnetic field H, the field penetrates the superconductor for only a short distance ?, called the penetration depth, after which it decays rapidly to zero. This is called the Meissner effect, and is a defining characteristic of superconductivity. For most superconductors, the penetration depth is on the order of 100 nm.

 

The History of Superconductors
In 1911 superconductivity was first observed in mercury by Dutch physicist Heike Kamerlingh Onnes of Leiden University. When he cooled it to the temperature of liquid helium, 4 degrees Kelvin (-452F, -269C), its resistance suddenly disappeared. The Kelvin scale represents an ?absolute? scale of temperature. Thus, it was necessary for Onnes to come within 4 degrees of the coldest temperature that is theoretically attainable to witness the phenomenon of superconductivity. Later, in 1913, he won a Nobel Prize in physics for his research in this area.

 

The next great milestone in understanding how matter behaves at extreme cold temperatures occurred in 1933. German researchers Walter Meissner and Robert Ochsenfeld discovered that a superconducting material will repel a magnetic field.

A magnet moving by a conductor induces currents in the conductor. This is the principle upon which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material - causing the magnet to be repulsed. This phenomenon is known as strong diamagnetism and is today often referred to as the ?Meissner effect?. The Meissner effect is so strong that a magnet can actually be levitated over a superconductive material.

 

In subsequent decades other superconducting metals, alloys and compounds were discovered. The first widely-accepted theoretical understanding of superconductivity was advanced in 1957 by American physicists John Bardeen, Leon Cooper, and John Schrieffer. Their Theories of Superconductivity became know as the BCS theory - derived from the first letter of each man's last name - and won them a Nobel prize in 1972. The mathematically-complex BCS theory explained superconductivity at temperatures close to absolute zero for elements and simple alloys. However, at higher temperatures and with different superconductor systems, the BCS theory has subsequently become inadequate to fully explain how superconductivity is occurring.

 

Superconductors can be divided into two classes according to how this breakdown occurs - Type 1 superconductors and Type 2 superconductors.

 

Type 1 Superconductors
In Type 1 superconductors, superconductivity is abruptly destroyed when the strength of the applied field rises above a critical value H_C. Depending on the geometry of the sample, one may obtain an intermediate state consisting of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field.

 

The Type 1 category of superconductors is mainly comprised of metals and metalloids that show some conductivity at room temperature. They require incredible cold to slow down molecular vibrations sufficiently to facilitate unimpeded electron flow in accordance with what is known as BCS theory. BCS theory suggests that electrons team up in ?Cooper pairs? in order to help each other overcome molecular obstacles - much like race cars on a track drafting each other in order to go faster. Scientists call this process phonon-mediated coupling because of the sound packets generated by the flexing of the crystal lattice.

 

Type 1 superconductors - characterized as the ?soft? superconductors - were discovered first and require the coldest temperatures to become superconductive. They exhibit a very sharp transition to a superconducting state and ?perfect? diamagnetism - the ability to repel a magnetic field completely.

 

Type 2 Superconductors
In Type 2 superconductors, raising the applied field past a critical value H_C1 leads to a mixed state in which an increasing amount of magnetic flux penetrates the material, but there remains no resistance to the flow of electrical current as long as the current is not too large. At a second critical field strength H_C2, superconductivity is destroyed. The mixed state is actually caused by vortices in the electronic superfluid, sometimes called fluxons because the flux carried by these vortices is quantized.

 

Except for the elements vanadium, technetium and niobium, the Type 2 category of superconductors is comprised of metallic compounds and alloys. The recently-discovered superconducting ?perovskites? (metal-oxide ceramics that normally have a ratio of 2 metal atoms to every 3 oxygen atoms) belong to this Type 2 group.

 

Type 2 superconductors - also known as the ?hard? superconductors - differ from Type 1 in that their transition from a normal to a superconducting state is gradual across a region of ?mixed state? behavior. Since a Type 2 will allow some penetration by an external magnetic field into its surface, this creates some rather novel mesoscopic phenomena like superconducting ?stripes? and ?flux-lattice vortices?.

 

The most ignominious military use of superconductors may come with the deployment of ?E-bombs?. These are devices that make use of strong, superconductor-derived magnetic fields to create a fast, high-intensity electro-magnetic pulse (EMP) to disable an enemy?s electronic equipment. Such a device saw its first use in wartime in March 2003 when US Forces attacked an Iraqi broadcast facility.